This paper proposes a new deterministic, logical time model for distributed systems. The time model defines a total order of all messages in the system based on two partial order relations: messages sent before other messages ("=>") and strong causality ("~"). This ensures messages are delivered in a way that respects real-world causality. The time model can be implemented using message timestamps and delayed delivery to ensure the total order is maintained. This provides application programmers with a simple, linear view of time across a distributed system.
This academic article discusses the efficiency of distributed systems. It presents a model for measuring efficiency that takes into account the number of processes, nodes, and messages. The key points are:
1. The efficiency of a distributed system depends on the number of processes, nodes, and messages as well as the time taken for internal processing, sending/receiving messages, processing at nodes, and completing the task.
2. As the number of processes, nodes, and messages increases, the overall time increases and efficiency decreases.
3. An example calculation shows that efficiency is reduced from 70% to lower levels as the number of messages increases.
From Simulation to Online Gaming: the need for adaptive solutions Gabriele D'Angelo
In many fields such as distributed simulation and online gaming the missing piece is adaptivity. There is a strong need for dynamic and adaptive solutions that can improve performances and react to problems.
Tremendous usage of internet has made huge data on the network, without compromising on the
performance of network the end-users must obtain best service. As cloud provides different services on
leasing basis many companies are migrating from their own Infrastructure to cloud,This migration should
not compromise on performance of the cloud, The performance of the cloud can be improved by having
excellent load balancing strategy such that the end user is satisfied. The paper reveals the method by which
a cloud can be partitioned and a study of different algorithm with comparative study to balance the
dynamic load. The comparative study between Ant Colony and Honey Bee algorithm gives the result which
algorithm is optimal in normal load condition also the simplest round robin algorithm is applied when the
partition are in Idle state
A major role is played in the layout and evaluation of any empirical wireless structure to manifest is the goal of this paper that counterfeit mode architectures affect counterfeit conduct, regarding structure accomplishment metrics, essentially and therefore, the excellent architecture should be explored in order to accomplish the most accurate and reliable results. It is found that the most analytical factors it is found that that actuate counterfeit mode accomplishment are counterfeit time, structure event organizing and
grade of adequate. It is, also, found that counterfeit time in relation to event existence in the real structure
along with the usage of modern architectural concepts such as multi-interweave technology complement
analytical issues too in the advancement of an adequate counterfeit organization for wireless communications. In order to evaluate the above findings an extensive empirical review has been
demeanored analysising several distinct events counterfeitorganizations towards presenting the relation
between channel designing collections, counterfeit time and structure accomplishment.
New Proposed Contention Avoidance Scheme for Distributed Real-Time Systemspaperpublications3
Abstract: One method to handle collisions in a contention based distributed system is to optimize collision detection and subsequent recovery. An alternative method to handle collisions in a contention based system is to attempt to avoid them. Some systems may utilize a strict scheduling guideline to identify who may use which resources when. Other systems may have the senders listen to the channel immediately prior to transmitting and determine suitable times to transmit. A primary challenge in Distributed Real-Time Systems applications is how to carry out data given source-to-sink, end-to-end deadlines when the communication resources are scarce. A new scheme resolves collisions and tries to reduce the number of potential collision events. In this paper, we develop New Avoiding Contention Scheme that delays data packet transmission nonlinearly during forwarding for a duration that correlates with their remaining deadline and distance to the destination, and avoiding the contention in bursty traffic by using multi-path routing.
Lately, the Wireless Sensors Networks (WSN) have moved to the concept of the hybrids networks in order to get universal platforms in various types of monitoring and information collecting applications. The work presented in this paper aims in designing a hybrid remote monitoring architecture, largely secured by a high availability and resilience WSN. The modeling approach intends to describe the main operation of polling and dispatching between the communications channels with the purpose of ensuring the information availability and reducing the resilience time. To achieve our goal, we have realized an experimental platform of measuring, processing and routing data through hybrid communications technologies. We have illustrated, via curves, the routing of the data measured by a WSN (ZigBee Technology) to a final user through several communication technologies (HTTPS, SMS, ...).
The document discusses various models for distributed systems including architectural, interaction, failure, software layer, and process models. It describes key aspects of distributed systems like client-server, peer-to-peer, and mobile code architectures. Requirements for distributed designs like performance, quality of service, caching, and dependability are covered. Fundamental models for message passing between processes are defined including states, configurations, computation and delivery events.
A study on dynamic load balancing in grid environmentIJSRD
Grid computing is a collection of computer resources from multiple locations to reach a common goal. Grid computing distinguishes from conventional high performance computing systems that are heterogeneous and geographically dispersed than cluster computer. One of the major issues in grid computing is load balancing. Classification of load balancing is: Static – Dynamic, Centralized – Decentralized, Homogeneous – Heterogeneous. Techniques like: Ant Colony Optimization, Threshold based and Optimal Heterogeneous are used by some researcher to balance the load. This survey paper discusses set of parameters to be used for comparing performance of each of them. In addition to that it says which technique is more useful for grid environment.
This academic article discusses the efficiency of distributed systems. It presents a model for measuring efficiency that takes into account the number of processes, nodes, and messages. The key points are:
1. The efficiency of a distributed system depends on the number of processes, nodes, and messages as well as the time taken for internal processing, sending/receiving messages, processing at nodes, and completing the task.
2. As the number of processes, nodes, and messages increases, the overall time increases and efficiency decreases.
3. An example calculation shows that efficiency is reduced from 70% to lower levels as the number of messages increases.
From Simulation to Online Gaming: the need for adaptive solutions Gabriele D'Angelo
In many fields such as distributed simulation and online gaming the missing piece is adaptivity. There is a strong need for dynamic and adaptive solutions that can improve performances and react to problems.
Tremendous usage of internet has made huge data on the network, without compromising on the
performance of network the end-users must obtain best service. As cloud provides different services on
leasing basis many companies are migrating from their own Infrastructure to cloud,This migration should
not compromise on performance of the cloud, The performance of the cloud can be improved by having
excellent load balancing strategy such that the end user is satisfied. The paper reveals the method by which
a cloud can be partitioned and a study of different algorithm with comparative study to balance the
dynamic load. The comparative study between Ant Colony and Honey Bee algorithm gives the result which
algorithm is optimal in normal load condition also the simplest round robin algorithm is applied when the
partition are in Idle state
A major role is played in the layout and evaluation of any empirical wireless structure to manifest is the goal of this paper that counterfeit mode architectures affect counterfeit conduct, regarding structure accomplishment metrics, essentially and therefore, the excellent architecture should be explored in order to accomplish the most accurate and reliable results. It is found that the most analytical factors it is found that that actuate counterfeit mode accomplishment are counterfeit time, structure event organizing and
grade of adequate. It is, also, found that counterfeit time in relation to event existence in the real structure
along with the usage of modern architectural concepts such as multi-interweave technology complement
analytical issues too in the advancement of an adequate counterfeit organization for wireless communications. In order to evaluate the above findings an extensive empirical review has been
demeanored analysising several distinct events counterfeitorganizations towards presenting the relation
between channel designing collections, counterfeit time and structure accomplishment.
New Proposed Contention Avoidance Scheme for Distributed Real-Time Systemspaperpublications3
Abstract: One method to handle collisions in a contention based distributed system is to optimize collision detection and subsequent recovery. An alternative method to handle collisions in a contention based system is to attempt to avoid them. Some systems may utilize a strict scheduling guideline to identify who may use which resources when. Other systems may have the senders listen to the channel immediately prior to transmitting and determine suitable times to transmit. A primary challenge in Distributed Real-Time Systems applications is how to carry out data given source-to-sink, end-to-end deadlines when the communication resources are scarce. A new scheme resolves collisions and tries to reduce the number of potential collision events. In this paper, we develop New Avoiding Contention Scheme that delays data packet transmission nonlinearly during forwarding for a duration that correlates with their remaining deadline and distance to the destination, and avoiding the contention in bursty traffic by using multi-path routing.
Lately, the Wireless Sensors Networks (WSN) have moved to the concept of the hybrids networks in order to get universal platforms in various types of monitoring and information collecting applications. The work presented in this paper aims in designing a hybrid remote monitoring architecture, largely secured by a high availability and resilience WSN. The modeling approach intends to describe the main operation of polling and dispatching between the communications channels with the purpose of ensuring the information availability and reducing the resilience time. To achieve our goal, we have realized an experimental platform of measuring, processing and routing data through hybrid communications technologies. We have illustrated, via curves, the routing of the data measured by a WSN (ZigBee Technology) to a final user through several communication technologies (HTTPS, SMS, ...).
The document discusses various models for distributed systems including architectural, interaction, failure, software layer, and process models. It describes key aspects of distributed systems like client-server, peer-to-peer, and mobile code architectures. Requirements for distributed designs like performance, quality of service, caching, and dependability are covered. Fundamental models for message passing between processes are defined including states, configurations, computation and delivery events.
A study on dynamic load balancing in grid environmentIJSRD
Grid computing is a collection of computer resources from multiple locations to reach a common goal. Grid computing distinguishes from conventional high performance computing systems that are heterogeneous and geographically dispersed than cluster computer. One of the major issues in grid computing is load balancing. Classification of load balancing is: Static – Dynamic, Centralized – Decentralized, Homogeneous – Heterogeneous. Techniques like: Ant Colony Optimization, Threshold based and Optimal Heterogeneous are used by some researcher to balance the load. This survey paper discusses set of parameters to be used for comparing performance of each of them. In addition to that it says which technique is more useful for grid environment.
This document summarizes research on scheduling algorithms for loading streaming data into real-time data warehouses. The goal is to minimize data staleness over time. It describes how streaming warehouses continuously ingest incoming data streams to support time-critical analyses, unlike traditional warehouses which are periodically refreshed. It presents a model for temporal consistency and defines data staleness. It formulates the streaming warehouse update problem as a scheduling problem to minimize staleness and proves that any online, non-preemptive scheduling algorithm can achieve staleness within a constant factor of optimal if processors are sufficiently fast and no processor is idly waiting.
CLOUD COMPUTING – PARTITIONING ALGORITHM AND LOAD BALANCING ALGORITHMijcseit
This document summarizes a research paper on partitioning algorithms and load balancing algorithms for cloud computing. It discusses how cloud partitioning can improve load balancing and system performance. It reviews different partitioning and load balancing algorithms such as ant colony optimization and honey bee behavior algorithms. The paper presents a model for cloud partitioning based on geographic regions. It also provides experimental results comparing the ant colony and honey bee algorithms, finding that honey bee performs better in balancing load across nodes in the cloud.
CLOUD COMPUTING – PARTITIONING ALGORITHM AND LOAD BALANCING ALGORITHMijcseit
This document summarizes a research paper on partitioning algorithms and load balancing algorithms for cloud computing. It discusses how cloud partitioning can improve load balancing and system performance. It reviews different partitioning and load balancing algorithms such as ant colony optimization and honey bee behavior algorithms. The paper presents a model for cloud partitioning based on geographic regions. It also provides experimental results comparing the ant colony and honey bee algorithms, finding that honey bee performs better, especially with higher load thresholds. The paper concludes that load balancing is important for optimizing cloud resource utilization.
Network protocol lectures
Engineering + education purpose
This series of lectures was prepared for the fourth class of computer networks department In AL-Bani University College / Baghdad / Iraq.
I depend in make these lectures on-
1. Data_Communications_and_networking_3e_Forouzan
2. Internet
This series of lectures is not finished yet; I will complete it and update the PDF as soon as possible.
Forgive me for any wrong by mistake.
My regards
Marwa Moutaz/ M.Sc. studies of Communication Engineering / University of Technology/ Bagdad / Iraq
Checkpoint and recovery protocols are commonly used in distributed applications for providing fault
tolerance. A distributed system may require taking checkpoints from time to time to keep it free of arbitrary
failures. In case of failure, the system will rollback to checkpoints where global consistency is preserved.
Checkpointing is one of the fault-tolerant techniques to restore faults and to restart job fast. The algorithms
for checkpointing on distributed systems have been under study for years.
It is known that checkpointing and rollback recovery are widely used techniques that allow a distributed
computing to progress inspite of a failure.There are two fundamental approaches for checkpointing and
recovery.One is asynchronus approach, process take their checkpoints independenty.So,taking checkpoints
is very simple but due to absence of a recent consistent global checkpoint which may cause a rollback of
computation.Synchronus checkpointing approach assumes that a single process other than the application
process invokes the checkpointing algorithm periodically to determine a consistent global checkpoint.
The document is an internal assessment test for a computer science course. It contains multiple choice and long answer questions about distributed systems topics like logical clocks, NTP protocol, causal ordering, and recording global states. The challenges discussed include automatically providing location-specific information to users in a distributed wireless environment and developing an algorithm to record the global state of a distributed system where all physical clocks are perfectly synchronized over a reliable network.
The Open Systems Interconnection (OSI) model began as a reference model, but has since been implemented. It was created by the International Organization for Standardization (ISO) to provide a logical framework for how data communication processes should interact across networks. Standards were created for the computer industry allowing different networks to work together efficiently.ThesisScientist.com
This document provides an overview of Delay Tolerant Networks (DTNs) and discusses their use for challenged networks. Some key points:
- DTNs use a store-and-forward architecture to allow communication in networks that may experience delays, disruptions, or lack continuous connectivity. Messages are stored until the next connection opportunity.
- DTNs were originally developed for interplanetary internet but can enable communication in terrestrial mobile, military, and sensor networks that have intermittent connectivity.
- Current research issues include routing, security, providing quality of service classes. NASA is testing DTN protocols on the International Space Station.
- Simulation tools like NS-2, GloMoSim, and QualNet can be
A fault tolerant tokenbased atomic broadcast algorithm relying on responsive ...Neelamani Samal
This document summarizes a fault tolerant token-based atomic broadcast algorithm that relies on an unreliable failure detector and satisfies the responsive property. The algorithm aims to tolerate processor-level failures in a distributed system. It divides a job into tasks, uses a token to control access to shared resources, and monitors task execution times. If a task does not respond within the timeout period, it is declared faulty and removed from the ready queue. The algorithm was implemented on a multi-core processor to simulate fault tolerance capabilities in a distributed system within a specified time interval.
The document discusses interprocess communication in distributed systems. It introduces four widely used communication models: remote procedure call (RPC), message-oriented middleware (MOM), stream-oriented communication, and multicast communication. RPC allows processes to call procedures located on other machines transparently. MOM supports persistent asynchronous communication through message queues.
Distributed systems have several key characteristics: concurrency without a global clock, independent failures of components, and heterogeneity across networks, hardware, operating systems and more. They face challenges including heterogeneity, openness, security, scalability, concurrency and transparency. Architectural models like client-server and peer processes define how components interact and are mapped to networks. Fundamental models address interaction considering timing, failures considering omission and arbitrary errors, and security regarding access rights, authentication and protecting messages. Software is organized into layers including platform, middleware and applications.
Analysis of Hierarchical Scheduling for Heterogeneous Traffic over NetworkIJCNCJournal
Scheduling real time and non real time packets at network nodes has an important impact by reducing the
processing overhead, queuing delay and response time. Most of the existing packet scheduling algorithms
used in network based on First-In First-Out (FIFO), non-preemptive priority, and preemptive priority
scheduling. However, these algorithms incur a large processing overhead, queuing delay and response
time and are not dynamic to the data traffic changes. In this paper, we present a new hierarchical
scheduling algorithm to assign priority, Hierarchical Hybrid EDF/FIFO which can not only serve the real
time traffic but also provide best effort service to non real time traffic. To examine our approach for
scheduling, we realized our analytical study to express the worst case queuing delay and the worst case
response time for different traffics. The simulation results showed that the Hierarchical hybrid EDF/FIFO
achieved the minimum packet delay and adequate loss packet for non real time traffic when compared with
Hierarchical FIFO. In general, the performances of our approach draw near to Hierarchical EDF which
confirms the effectiveness of this approach.
Orchestrating bulk data transfers acrossnexgentech15
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
Orchestrating Bulk Data Transfers across Geo-Distributed Datacentersnexgentechnology
bulk ieee projects in pondicherry,ieee projects in pondicherry,final year ieee projects in pondicherry
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
ORCHESTRATING BULK DATA TRANSFERS ACROSS GEO-DISTRIBUTED DATACENTERSNexgen Technology
bulk ieee projects in pondicherry,ieee projects in pondicherry,final year ieee projects in pondicherry
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
Communication network simulation on the unix system trough use of the remote ...Damir Delija
The document describes a method for simulating communication networks on UNIX systems using Remote Procedure Calls (RPCs) for interprocess communication. The simulation allows testing of network modules by creating a virtual network environment. The simulation system consists of three processes that communicate via RPCs to represent different parts of the network. This distributed structure allows the simulation to be adapted for different computer systems. The network simulator process uses pseudocode to simulate message passing and potential faults. RPCs enable remote control of the simulation's parameters. The simulation system supports both manual and automated testing of network protocols and equipment under various conditions.
Communication network simulation on the unix system trough use of the remote ...Damir Delija
The document describes a method for simulating communication networks on UNIX systems using Remote Procedure Calls (RPCs) for interprocess communication. The simulation allows testing of network modules by creating a virtual network environment. The simulation system consists of three processes that communicate via RPCs to represent different parts of the network. This distributed structure allows the simulation to be adapted for different computers. The network simulator process uses pseudocode to simulate message passing and potential faults. RPCs enable remote control of the simulation's parameters. The simulation system supports both manual and automated testing of network protocols and equipment under various conditions before field testing.
Communication network simulation on the unix system trough use of the remote ...Damir Delija
This document describes a simulation of a communication network on UNIX using Remote Procedure Calls (RPCs). The simulation allows testing of network modules and consists of three processes that communicate via RPCs: a network simulator process, a host process, and a remote procedure monitor (RPM) process. The network simulator process pseudocode shows how it simulates message passing between the host and RPM processes and controls message loss based on configurable probability and duration parameters.
This document provides an overview of computer networks and networking concepts. It discusses protocols and standards including the elements of a protocol: syntax, semantics, and timing. It describes the OSI model and TCP/IP model in detail, explaining each layer and its functions. It also discusses different types of transmission media like coaxial cable, fiber optic cable, and wireless transmission. Coaxial cable uses a central conductor surrounded by insulation and an outer conductor. Fiber optic cable uses total internal reflection to guide light through the core. Wireless transmission uses electromagnetic waves propagated through ground, sky, or line-of-sight methods using antennas.
This document discusses and compares various load balancing techniques in cloud computing. It begins by introducing load balancing as an important issue in cloud computing for efficiently scheduling user requests and resources. Several load balancing algorithms are then described, including honeybee foraging algorithm, biased random sampling, active clustering, OLB+LBMM, and Min-Min. Metrics for evaluating and comparing load balancing techniques are defined, such as throughput, overhead, fault tolerance, migration time, response time, resource utilization, scalability, and performance. The algorithms are then analyzed based on these metrics.
When Practicing Writing Chinese, Is It RecommendeJim Webb
Catherine de Medici of France and Isabella I of Spain both pursued strict Catholic policies, eliminating other religions through massacres. Catherine approved the St. Bartholomew's Day massacre in France that killed 3000 Huguenots. Isabella wanted to remove Muslims and Jews from Spain. Both rulers sought to establish Catholicism as the sole religion and remove perceived religious threats.
016 King Essay Example Stephen Why We Crave HJim Webb
The document provides instructions for using the HelpWriting.net service to request that writers complete assignments. It outlines a 5-step process: 1) Create an account; 2) Submit a request with instructions and deadline; 3) Review bids and choose a writer; 4) Review the completed paper and authorize payment; 5) Request revisions to ensure satisfaction. The service aims to provide original, high-quality content and offers refunds for plagiarized work.
More Related Content
Similar to A Deterministic Model Of Time For Distributed Systems
This document summarizes research on scheduling algorithms for loading streaming data into real-time data warehouses. The goal is to minimize data staleness over time. It describes how streaming warehouses continuously ingest incoming data streams to support time-critical analyses, unlike traditional warehouses which are periodically refreshed. It presents a model for temporal consistency and defines data staleness. It formulates the streaming warehouse update problem as a scheduling problem to minimize staleness and proves that any online, non-preemptive scheduling algorithm can achieve staleness within a constant factor of optimal if processors are sufficiently fast and no processor is idly waiting.
CLOUD COMPUTING – PARTITIONING ALGORITHM AND LOAD BALANCING ALGORITHMijcseit
This document summarizes a research paper on partitioning algorithms and load balancing algorithms for cloud computing. It discusses how cloud partitioning can improve load balancing and system performance. It reviews different partitioning and load balancing algorithms such as ant colony optimization and honey bee behavior algorithms. The paper presents a model for cloud partitioning based on geographic regions. It also provides experimental results comparing the ant colony and honey bee algorithms, finding that honey bee performs better in balancing load across nodes in the cloud.
CLOUD COMPUTING – PARTITIONING ALGORITHM AND LOAD BALANCING ALGORITHMijcseit
This document summarizes a research paper on partitioning algorithms and load balancing algorithms for cloud computing. It discusses how cloud partitioning can improve load balancing and system performance. It reviews different partitioning and load balancing algorithms such as ant colony optimization and honey bee behavior algorithms. The paper presents a model for cloud partitioning based on geographic regions. It also provides experimental results comparing the ant colony and honey bee algorithms, finding that honey bee performs better, especially with higher load thresholds. The paper concludes that load balancing is important for optimizing cloud resource utilization.
Network protocol lectures
Engineering + education purpose
This series of lectures was prepared for the fourth class of computer networks department In AL-Bani University College / Baghdad / Iraq.
I depend in make these lectures on-
1. Data_Communications_and_networking_3e_Forouzan
2. Internet
This series of lectures is not finished yet; I will complete it and update the PDF as soon as possible.
Forgive me for any wrong by mistake.
My regards
Marwa Moutaz/ M.Sc. studies of Communication Engineering / University of Technology/ Bagdad / Iraq
Checkpoint and recovery protocols are commonly used in distributed applications for providing fault
tolerance. A distributed system may require taking checkpoints from time to time to keep it free of arbitrary
failures. In case of failure, the system will rollback to checkpoints where global consistency is preserved.
Checkpointing is one of the fault-tolerant techniques to restore faults and to restart job fast. The algorithms
for checkpointing on distributed systems have been under study for years.
It is known that checkpointing and rollback recovery are widely used techniques that allow a distributed
computing to progress inspite of a failure.There are two fundamental approaches for checkpointing and
recovery.One is asynchronus approach, process take their checkpoints independenty.So,taking checkpoints
is very simple but due to absence of a recent consistent global checkpoint which may cause a rollback of
computation.Synchronus checkpointing approach assumes that a single process other than the application
process invokes the checkpointing algorithm periodically to determine a consistent global checkpoint.
The document is an internal assessment test for a computer science course. It contains multiple choice and long answer questions about distributed systems topics like logical clocks, NTP protocol, causal ordering, and recording global states. The challenges discussed include automatically providing location-specific information to users in a distributed wireless environment and developing an algorithm to record the global state of a distributed system where all physical clocks are perfectly synchronized over a reliable network.
The Open Systems Interconnection (OSI) model began as a reference model, but has since been implemented. It was created by the International Organization for Standardization (ISO) to provide a logical framework for how data communication processes should interact across networks. Standards were created for the computer industry allowing different networks to work together efficiently.ThesisScientist.com
This document provides an overview of Delay Tolerant Networks (DTNs) and discusses their use for challenged networks. Some key points:
- DTNs use a store-and-forward architecture to allow communication in networks that may experience delays, disruptions, or lack continuous connectivity. Messages are stored until the next connection opportunity.
- DTNs were originally developed for interplanetary internet but can enable communication in terrestrial mobile, military, and sensor networks that have intermittent connectivity.
- Current research issues include routing, security, providing quality of service classes. NASA is testing DTN protocols on the International Space Station.
- Simulation tools like NS-2, GloMoSim, and QualNet can be
A fault tolerant tokenbased atomic broadcast algorithm relying on responsive ...Neelamani Samal
This document summarizes a fault tolerant token-based atomic broadcast algorithm that relies on an unreliable failure detector and satisfies the responsive property. The algorithm aims to tolerate processor-level failures in a distributed system. It divides a job into tasks, uses a token to control access to shared resources, and monitors task execution times. If a task does not respond within the timeout period, it is declared faulty and removed from the ready queue. The algorithm was implemented on a multi-core processor to simulate fault tolerance capabilities in a distributed system within a specified time interval.
The document discusses interprocess communication in distributed systems. It introduces four widely used communication models: remote procedure call (RPC), message-oriented middleware (MOM), stream-oriented communication, and multicast communication. RPC allows processes to call procedures located on other machines transparently. MOM supports persistent asynchronous communication through message queues.
Distributed systems have several key characteristics: concurrency without a global clock, independent failures of components, and heterogeneity across networks, hardware, operating systems and more. They face challenges including heterogeneity, openness, security, scalability, concurrency and transparency. Architectural models like client-server and peer processes define how components interact and are mapped to networks. Fundamental models address interaction considering timing, failures considering omission and arbitrary errors, and security regarding access rights, authentication and protecting messages. Software is organized into layers including platform, middleware and applications.
Analysis of Hierarchical Scheduling for Heterogeneous Traffic over NetworkIJCNCJournal
Scheduling real time and non real time packets at network nodes has an important impact by reducing the
processing overhead, queuing delay and response time. Most of the existing packet scheduling algorithms
used in network based on First-In First-Out (FIFO), non-preemptive priority, and preemptive priority
scheduling. However, these algorithms incur a large processing overhead, queuing delay and response
time and are not dynamic to the data traffic changes. In this paper, we present a new hierarchical
scheduling algorithm to assign priority, Hierarchical Hybrid EDF/FIFO which can not only serve the real
time traffic but also provide best effort service to non real time traffic. To examine our approach for
scheduling, we realized our analytical study to express the worst case queuing delay and the worst case
response time for different traffics. The simulation results showed that the Hierarchical hybrid EDF/FIFO
achieved the minimum packet delay and adequate loss packet for non real time traffic when compared with
Hierarchical FIFO. In general, the performances of our approach draw near to Hierarchical EDF which
confirms the effectiveness of this approach.
Orchestrating bulk data transfers acrossnexgentech15
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
Orchestrating Bulk Data Transfers across Geo-Distributed Datacentersnexgentechnology
bulk ieee projects in pondicherry,ieee projects in pondicherry,final year ieee projects in pondicherry
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
ORCHESTRATING BULK DATA TRANSFERS ACROSS GEO-DISTRIBUTED DATACENTERSNexgen Technology
bulk ieee projects in pondicherry,ieee projects in pondicherry,final year ieee projects in pondicherry
Nexgen Technology Address:
Nexgen Technology
No :66,4th cross,Venkata nagar,
Near SBI ATM,
Puducherry.
Email Id: praveen@nexgenproject.com.
www.nexgenproject.com
Mobile: 9751442511,9791938249
Telephone: 0413-2211159.
NEXGEN TECHNOLOGY as an efficient Software Training Center located at Pondicherry with IT Training on IEEE Projects in Android,IEEE IT B.Tech Student Projects, Android Projects Training with Placements Pondicherry, IEEE projects in pondicherry, final IEEE Projects in Pondicherry , MCA, BTech, BCA Projects in Pondicherry, Bulk IEEE PROJECTS IN Pondicherry.So far we have reached almost all engineering colleges located in Pondicherry and around 90km
Communication network simulation on the unix system trough use of the remote ...Damir Delija
The document describes a method for simulating communication networks on UNIX systems using Remote Procedure Calls (RPCs) for interprocess communication. The simulation allows testing of network modules by creating a virtual network environment. The simulation system consists of three processes that communicate via RPCs to represent different parts of the network. This distributed structure allows the simulation to be adapted for different computer systems. The network simulator process uses pseudocode to simulate message passing and potential faults. RPCs enable remote control of the simulation's parameters. The simulation system supports both manual and automated testing of network protocols and equipment under various conditions.
Communication network simulation on the unix system trough use of the remote ...Damir Delija
The document describes a method for simulating communication networks on UNIX systems using Remote Procedure Calls (RPCs) for interprocess communication. The simulation allows testing of network modules by creating a virtual network environment. The simulation system consists of three processes that communicate via RPCs to represent different parts of the network. This distributed structure allows the simulation to be adapted for different computers. The network simulator process uses pseudocode to simulate message passing and potential faults. RPCs enable remote control of the simulation's parameters. The simulation system supports both manual and automated testing of network protocols and equipment under various conditions before field testing.
Communication network simulation on the unix system trough use of the remote ...Damir Delija
This document describes a simulation of a communication network on UNIX using Remote Procedure Calls (RPCs). The simulation allows testing of network modules and consists of three processes that communicate via RPCs: a network simulator process, a host process, and a remote procedure monitor (RPM) process. The network simulator process pseudocode shows how it simulates message passing between the host and RPM processes and controls message loss based on configurable probability and duration parameters.
This document provides an overview of computer networks and networking concepts. It discusses protocols and standards including the elements of a protocol: syntax, semantics, and timing. It describes the OSI model and TCP/IP model in detail, explaining each layer and its functions. It also discusses different types of transmission media like coaxial cable, fiber optic cable, and wireless transmission. Coaxial cable uses a central conductor surrounded by insulation and an outer conductor. Fiber optic cable uses total internal reflection to guide light through the core. Wireless transmission uses electromagnetic waves propagated through ground, sky, or line-of-sight methods using antennas.
This document discusses and compares various load balancing techniques in cloud computing. It begins by introducing load balancing as an important issue in cloud computing for efficiently scheduling user requests and resources. Several load balancing algorithms are then described, including honeybee foraging algorithm, biased random sampling, active clustering, OLB+LBMM, and Min-Min. Metrics for evaluating and comparing load balancing techniques are defined, such as throughput, overhead, fault tolerance, migration time, response time, resource utilization, scalability, and performance. The algorithms are then analyzed based on these metrics.
Similar to A Deterministic Model Of Time For Distributed Systems (20)
When Practicing Writing Chinese, Is It RecommendeJim Webb
Catherine de Medici of France and Isabella I of Spain both pursued strict Catholic policies, eliminating other religions through massacres. Catherine approved the St. Bartholomew's Day massacre in France that killed 3000 Huguenots. Isabella wanted to remove Muslims and Jews from Spain. Both rulers sought to establish Catholicism as the sole religion and remove perceived religious threats.
016 King Essay Example Stephen Why We Crave HJim Webb
The document provides instructions for using the HelpWriting.net service to request that writers complete assignments. It outlines a 5-step process: 1) Create an account; 2) Submit a request with instructions and deadline; 3) Review bids and choose a writer; 4) Review the completed paper and authorize payment; 5) Request revisions to ensure satisfaction. The service aims to provide original, high-quality content and offers refunds for plagiarized work.
How To Write An Essay Fast Essay Writing Guide - GreetinglinesJim Webb
The document provides instructions for completing a task about swimming. It involves identifying strengths and weaknesses in swimming strokes, and fitness tests that can be done to improve swimming fitness, such as the Williams Swimming Beep Test.
Essay Coaching Seven Secrets For Writing Standout CollegeJim Webb
The document provides instructions for creating an account and submitting a paper writing request on the HelpWriting.net site. It explains that the site uses a bidding system where writers can bid on requests and clients can choose a writer based on qualifications. It notes that clients can request revisions and papers will be original with refunds offered for plagiarized work.
Write Essays That Get In And Get Money EBook - CompJim Webb
The terrorist threat to the commercial sector is real for three main reasons:
1. Commercial sites like shopping centers, transportation hubs, and entertainment venues attract large
crowds, making them attractive targets for terrorists aiming to inflict mass casualties.
2. Attacks on commercial targets can significantly disrupt business activity and undermine consumer
confidence, inflicting economic damage in addition to loss of life.
3. Recent terrorist attacks in Europe have targeted commercial areas, showing that terrorists continue
viewing them as ways to spread fear and further their political goals. Vigilance is needed to address
this ongoing threat.
I apologize, upon reviewing the document and context provided, I do not feel comfortable generating a summary without the full original text. Summarizing copyrighted or private content without permission could enable plagiarism or the unintended spreading of misinformation.
1. The document provides instructions for requesting writing assistance from HelpWriting.net. It outlines a 5-step process: register an account, complete an order form providing instructions and deadline, writers will bid on the request, select a writer and provide a deposit, and receive the completed paper for review.
2. Revisions are available if needed, and papers are guaranteed to be original and of high quality. Plagiarized works will be refunded.
Frederick Douglass gave a speech to the British Anti-Slavery Society addressing his initial hesitation to speak about slavery. He felt the influence of Britain could help combat American slavery given their dominance. Douglass suggested slaves should question why they must obey masters, and told the story of a slave named Sambo who was punished for disobeying. He discussed how slave owners tried to convince slaves obedience was necessary through religion.
Baby Thesis Introduction Sample - Thesis Title IdeaJim Webb
The document provides instructions for requesting writing assistance from an online service. It outlines a 5-step process: 1) Create an account with personal details; 2) Complete a form with assignment details and deadline; 3) Review bids from writers and select one; 4) Review the completed paper and authorize payment; 5) Request revisions until satisfied. The service promises original, high-quality work or a full refund.
Here are three career paths you could consider based on your interests:
Business Intelligence Analyst
- Business intelligence analysts use data analysis and data mining to help organizations make better business decisions. They collect, analyze and report on key performance indicators and other internal/external data.
- Coursework in statistics, data analysis, database management would be useful. Skills in querying databases, data visualization, reporting are important.
- Potential job roles include business intelligence analyst, data analyst, data scientist.
Accountant
- Accountants ensure accurate and timely recording of financial information. They prepare and examine financial records and statements.
- A bachelor's degree in accounting is required. Coursework covers financial accounting, managerial accounting, taxation,
1. The document discusses the steps to get cheap essay writing help from HelpWriting.net, including creating an account, submitting an order form, and choosing a writer to complete the assignment.
2. A bidding system is used to match clients with writers. Clients can review bids, choose a writer based on qualifications, and pay a deposit to start the writing.
3. Clients can request revisions until satisfied with the paper. HelpWriting.net promises original, high-quality content and refunds are offered if work is plagiarized.
The document discusses whether unaccompanied immigrant children should be deported from the United States. Dan Coats argues they should be deported to deter dangerous journeys and focus on stopping children from coming. Mark Seitz argues against deportation, citing the best interest of the child standard and potential reasons for increased numbers, such as violence in their home countries. Seitz appeals to authority and sympathy in advocating that the US consider individual stories and respond compassionately.
The reflection describes how the author once went through Catholic practices like mass and confession without fully understanding or committing to change. Over time, while persisting in certain sins, the Lord continued calling to them. It was not until confessing a particular mortal sin, which was uncomfortable to admit, that the author gained a deeper understanding of Catholic teachings like not receiving communion in a state of mortal sin.
PPT - Writing Biomedical Research Papers PowerPoJim Webb
This document provides instructions for creating an account and submitting a paper writing request on the website HelpWriting.net. It outlines a 5-step process: 1) Create an account with an email and password. 2) Complete a form with paper details, sources, and deadline. 3) Review writer bids and choose one based on qualifications. 4) Review the completed paper and authorize payment. 5) Request revisions until satisfied, with a refund option for plagiarized work. The goal is to help students get high-quality original content through this online writing assistance process.
The document discusses different types of shoppers, classifying them into three main groups: cheap shoppers, who are very cost-conscious; spendthrifts, who spend freely without concern for costs; and indifferent shoppers, who do not care much about shopping. It notes that people have different motivations for shopping, from stress relief to making it a hobby, and that online shopping is growing in popularity due to dedicated shoppers.
Who Are Professional Essay Writers And How Students Might Benefit FromJim Webb
The document provides instructions for students to get assistance from professional writers on the website HelpWriting.net. It outlines a 5-step process: 1) Create an account with an email and password. 2) Complete an order form with instructions, sources, and deadline. 3) Review bids from writers and choose one based on qualifications. 4) Review the completed paper and authorize payment. 5) Request revisions until satisfied with the original, high-quality content or receive a full refund for plagiarized work. The website promises to fully meet students' needs for writing assistance.
Sample Personal Statements Graduate School PersonaJim Webb
This document provides instructions for requesting writing assistance from HelpWriting.net. It outlines a 5-step process: 1) Create an account with a password and email. 2) Complete a request form with instructions, sources, and deadline. 3) Review bids from writers and select one. 4) Review the completed paper and authorize payment. 5) Request revisions to ensure satisfaction, with a refund option for plagiarism.
The document provides instructions for purchasing a critical analysis paper from HelpWriting.net. It outlines a 5 step process: 1) Create an account, 2) Complete an order form with instructions and deadline, 3) Review writer bids and select one, 4) Review the completed paper and authorize payment, 5) Request revisions to ensure satisfaction and receive a refund if plagiarized.
The document provides instructions for writing a position paper by outlining five steps:
1. Create an account and provide contact information.
2. Complete a form with paper details, sources, and deadline. Attach a sample if imitating writing style.
3. Review bids from writers and choose one based on qualifications and reviews. Place a deposit.
4. Review the paper and authorize full payment if satisfied, or request revisions for free.
5. Papers can be revised multiple times to ensure satisfaction, and refunds are offered for plagiarized work. Writers aim to fully meet customer needs.
How to Fix the Import Error in the Odoo 17Celine George
An import error occurs when a program fails to import a module or library, disrupting its execution. In languages like Python, this issue arises when the specified module cannot be found or accessed, hindering the program's functionality. Resolving import errors is crucial for maintaining smooth software operation and uninterrupted development processes.
How to Make a Field Mandatory in Odoo 17Celine George
In Odoo, making a field required can be done through both Python code and XML views. When you set the required attribute to True in Python code, it makes the field required across all views where it's used. Conversely, when you set the required attribute in XML views, it makes the field required only in the context of that particular view.
Exploiting Artificial Intelligence for Empowering Researchers and Faculty, In...Dr. Vinod Kumar Kanvaria
Exploiting Artificial Intelligence for Empowering Researchers and Faculty,
International FDP on Fundamentals of Research in Social Sciences
at Integral University, Lucknow, 06.06.2024
By Dr. Vinod Kumar Kanvaria
How to Build a Module in Odoo 17 Using the Scaffold MethodCeline George
Odoo provides an option for creating a module by using a single line command. By using this command the user can make a whole structure of a module. It is very easy for a beginner to make a module. There is no need to make each file manually. This slide will show how to create a module using the scaffold method.
हिंदी वर्णमाला पीपीटी, hindi alphabet PPT presentation, hindi varnamala PPT, Hindi Varnamala pdf, हिंदी स्वर, हिंदी व्यंजन, sikhiye hindi varnmala, dr. mulla adam ali, hindi language and literature, hindi alphabet with drawing, hindi alphabet pdf, hindi varnamala for childrens, hindi language, hindi varnamala practice for kids, https://www.drmullaadamali.com
A review of the growth of the Israel Genealogy Research Association Database Collection for the last 12 months. Our collection is now passed the 3 million mark and still growing. See which archives have contributed the most. See the different types of records we have, and which years have had records added. You can also see what we have for the future.
it describes the bony anatomy including the femoral head , acetabulum, labrum . also discusses the capsule , ligaments . muscle that act on the hip joint and the range of motion are outlined. factors affecting hip joint stability and weight transmission through the joint are summarized.
বাংলাদেশের অর্থনৈতিক সমীক্ষা ২০২৪ [Bangladesh Economic Review 2024 Bangla.pdf] কম্পিউটার , ট্যাব ও স্মার্ট ফোন ভার্সন সহ সম্পূর্ণ বাংলা ই-বুক বা pdf বই " সুচিপত্র ...বুকমার্ক মেনু 🔖 ও হাইপার লিংক মেনু 📝👆 যুক্ত ..
আমাদের সবার জন্য খুব খুব গুরুত্বপূর্ণ একটি বই ..বিসিএস, ব্যাংক, ইউনিভার্সিটি ভর্তি ও যে কোন প্রতিযোগিতা মূলক পরীক্ষার জন্য এর খুব ইম্পরট্যান্ট একটি বিষয় ...তাছাড়া বাংলাদেশের সাম্প্রতিক যে কোন ডাটা বা তথ্য এই বইতে পাবেন ...
তাই একজন নাগরিক হিসাবে এই তথ্য গুলো আপনার জানা প্রয়োজন ...।
বিসিএস ও ব্যাংক এর লিখিত পরীক্ষা ...+এছাড়া মাধ্যমিক ও উচ্চমাধ্যমিকের স্টুডেন্টদের জন্য অনেক কাজে আসবে ...
3. A Deterministic Model ofTime for Distributed Systems
Jeremy J. Carroll
Hewlett-Packard Labs, Bristol, U.K.
jjc@hpLhp.com
Andrei V. Borshchev
St.Petersburg Technical University, Russia
andrei@trantor.nord.nw.ru
is shown by Cheriton and Skeen [6], 'it can't say the
"whole story n,. We modify their Figure 4 as our Figure 1.
Abstract
This paper proposes a linear, deterministic, logical time
model for distributed systems. We give an account of
causality within distn"buted systems which undergirds the
time model. We discuss some advantages for the
application programmer in using our time model.
Theoretic
Pricing
ISIS
Option
Pricing
User
Monitor
Our model
User
Monitor
Figure 1 Cheriton and Skeen's trading example
Their criticism is that the application semantics mean
that message C is caused by message B, whereas the User
Monitor sees message D, that supersedes message A,
before message C. Moreover, 'causal ordering' accepts
this; for Lamport message C and message D are
concurrent.
We see that the correct notion of cause notes that C
happens because ofB and D merely happens after B. This
reininds us ofHume's second definition ofcausation [9]:
We may define a cause to be [. ..J ifthe first object
had not been, the second never had existed.
Under our time model we are required to deliver
message C, which is strongly caused by B (in this Humean
sense), prior to message D, sent after B, (see the axiom of
strong causality in section 4). In the figure this is shown
by delaying message D, using an ISIS-like mechanism.
(We require that the multicast A&B is made in the order A
then B).
1. Introduction
We propose a new detemrinistic, logical time model for
distnbuted systems.
The time model is defined at the level of temporal
semantics in tenns ofmessage ordering.
These temporal semantics allow the application
programmer to think about time linearly without having
to worry about concurrency or race conditions. This
makes the temporal semantics of distribution much closer
to those ofnon-distributed systems.
These semantics are to be implemented in middleware
which offers a messaging facility. For the application
programmer the messaging appears to obey the linear time
order ofour time model. In reality the messaging operates
with a high level of concurrency. The cost of the time
model is mainly felt in latency, not throughput.
The time model depends upon a new definition of
causality which differs significantly from Lamport's
causality [11].
2. Motivations
We look at two distinct motivations for this work, one
theoretical, the other practical
2.1 Causality
Option
price 26.00 .._ .
Theoretical
price 26.25
Option·'· .
price 26.50
Lamport, in his landmaIk paper, [II], defines his
'happened before' relationship and notes 'that it is
possible for event a to causally affect event b'. This led,
via Fidge [7] and Mattern. [12] to the misnamed 'causal'
ordering of ISIS [2,13]. The lack of true causality in ISIS
1
2.2 Telecom billing systems
The authors are part of the HP Labs team wo:dcing in
the area oftelecom billing systems (see Beech [1]).
4. Almost all telecom billing systems are currently
implemented on centralised mainframes. The mainframe
receives its input data as Call Detail Records (CDRs) from
the telecom switching netwolk The telecom switches output
'taW' CDRs; these are transmitted unprocessed to the central
mainframe, where all processing is done. The typical data
transmission system consists of motorbike couriers with
magnetic tapes (a high bandwidth, high latency solution).
The billing architectme proposed by the HP lab team
involves using geogmphically distnbuted processing, in
which as much processing as posSIble is done near the
data sources. The processed CDRs are then consolidated
centIal1y. (The motorbike network is modernised!)
Ideally the same application progmmmers should
continue to develop the billing systems as they migrate
from a centralised architecture to a distributed one
Hence any serious proposal for an application
progmmmers' environment should assume that the
application progmmmer is happier with COBOL than
with concurrency, and happier with records than race
conditions.
This time model is proposed with such a target user in
miil.d.
3. Background assumptions
We take an application or system level view of the
distnbuted system, rather than only seeing a set of
processes. This is reflected in the time-line being a
property of the whole system. This global view is
however, a development and system management view of
the system. and not a statement about implementation. We
prefer decentralised implementations in which global
properties emerge out of localised processing, rather than
from a central server ormaster/slave arrangement
We consider distnbuted systems which are sets of
processes connected by FIFO channels. Some of the
processes may have external channels through which they
commmricate with the system's environment. Individual
processes are driven by input messages, and the whole
system is driven by the external input messages. While
handling a message, a process can send an arbitrary
number of output messages, which are considered as
consequences ofthe input message.
3.1 Process assumptions
A process consists of a number of application layer
event handlers with shared state.
The only events we consider are message related
events. We use the tenn message arrival to indicate the
message arriving in the presentation layer, and message
2
delivery to indicate the message.being passed from the
presentation layerto the application layer. The application
layer may generate message send request events which are
passed to the presentation layer, which in tum generates
message send events. The application layer also generates
message processing complete events, which are passed to
the presentation layer.
The presentation layer delivers messages, one by one,
to the application layer by means of message delivery
events. After the application layer receives a single
message delivery event, it:
• generates zero or more message send requests.
• changes the process state.
• generates a message processing complete event that
indicates that it has finished processing that message
and is ready to receive another message. (1bis is
required to happen in bounded time).
Such message handler invocations are the atomic units
out ofwhich we see a distributed system being fonned. A
process history is simply a sequence of such invocations,
each of which may affect subsequent invocations by
changing the internal state ofthe process.
3.2 Channel assumptions
A channel links the presentation layers of two
processes.
We assume that the channels are perfect FIFO channels.
A message send event at one end is eventually followed
by a message arrival event at the other.
At the application layer the channels are simplex;
however auxiliary messages, from one presentation layer
to the other, are allowed in both directions.
3.3 Clock assumptions
The single assumption of a global nature is that of the
existence of a global real-time clock accessible from
anywhere within the system.
We only require that the clock is locally monotonic
increasing, and that there is some bound (perhaps only
statistical) on the difference between two simultaneous
readings of the clock in different processes. Neither the
time model, nor the implementation we present, make any
restrictions as to the size of the bound on the error.
However, many applications, including telecom billing,
may make more severe demands on the error bounds.
3.4 Typical applications
While these assumptions are general, the emphasis on
simplex channels contrasts with the client/server
5. paradigm. This is because our applications (distributed
test environments; event correlation services [8];
environmental monitoring systems as well as billing
systems) gather and process data from a nmnber of
geographically separate sources. Such an application is
illustrated in Figure 2. These applications tend to have
few loops in their geographic dataflow diagrams. They are
also toleI3Ilt oflatency, which is reflected in a significant
amount of queuing and delayed delivery within the
implementation ofthe time model
Geographically
dis1ributred
input
Figure 2 A typical data analysis application
4. The time model
The time model is a strict total ordering of the set of
messages in the system (we omit the word 'strict' in the
following).
Ifmessages are delivered to every process in time order
and messages are sent along every channel in time order
then the system is said to obey the time model.
A total ordering on the set ofmessages is equivalent to
an injective mapping from the set of messages in the
system to a totally ordered set (the time-line).
This gives the first principle ofthe time model: there is
a unique timejOr everything.
Such a time is simply a label for the message and does
not have any necessary relationship with real time. Such a
label can be useful for evaluating the time order
relationship between messages.
We now introduce the total order relation "<" on the
set ofall messages in the system. This relation is based on
two partial order relations, "=>" (sent before) and "~"
(strong causality).
"=>" is defined by:
• For any two external input messages, mo and
mI, either mo =>mI or m] =>mo. This is given
3
by the environment, typically by the clock time
ofmessage arrival. In other words, external input
messages are totally ordered with respect to =>.1
• If message send requests for messages mo and
m1occur in that order during a single invocation
of the message handler of some third message,
thenmo =>m1·
"~" is the least partial order such that:
• Ifthe message send request form1occurs during
the invocation in response to mo, then mo ~",].
(i.e. a message strongly causes any messages
sent by its handler).
"<" is the total order such that:
1. IfIllo~ml thenIllo <mi'
2. IfIllo -+m1 thenIllo <mI'
3. If Illo ~ mlJ Illo -+ m'0 then m'0 < mi' (The
strong causality axiom)
The first two axioms correspond to Lamport's axioms;
the third is the heart ofthe time model.
The idea behind the strong causality axiom is the
following: ifa process or the system's environment sends
two messages, one after another, then any consequence of
the first message should happen before the second
message and any ofits consequences. We will see that this
property is highly desirable and intuitive.
This gives the second principle of the time model:
there is enough time fOr everything (i.e. enough time for all
remote consequences to happen before the next local
event).
The totally ordered message set can be represented as a
tree, see Figure 3. The root of the tree is the system's
environment which generates extema! messages. The
nodes are invocations ofmessage handlers. Arcs represent
messages.
Using this tree it is easy to reconstruct the message
relations. For example, a ~ b because b was sent while a
was handled; b => c because b was sent before c in the same
invocation. Also, a ~ f and.Y: => z as these relations are
transitive.f and e are incomparable tmder both"=>" and
"~"; neverthelessf< e. To compare two messages with
respect to the global order relation "<" one has to trace paths
I More exactly if mo and mI. arrive from the environment at
the same process in that chronological order (by clock time)
then mo => mI. This generates a unique least partial order.
There is a choice of total orders respecting this constraint;
we choose one such total order. The choice is arbitraJ)'. but
some choices might give better results than others. For the
specific applications we have in mind. having adequately
synchronized real time clocks and developing a total order
that respects these clocks is best.
6. •-+. relation
external
message
invocation of
message handler
internal
message
.~. relation
system
environment
2
2
2
2
2
3
2
Figure 3 Message tree
Timestamps
a [1]
b [1,1]
c [1,2]
d [1,2,1,1]
e [1,2,2,1,2]
f [1,1.1,1.1]
u [2]
x [2.1]
Y [2,2,2,1,1]
z [2,3]
from the root to these messages. There can be three possible
cases, which correspond to the three axioms. They are
shown by the following three examples takenfrom Figure 3:
• c lies on the path of d, hence, c -+ d and
therefore c < d.
• x and z have the same path, butx is sent before z,
so x =;>z and x < z.
• f and e have the same path prefix, but then their
paths fork, and b (with b -+ j) is sent before c
(with c -+ e), which means b =;> c, sof < C < e,
givingf< e.
If a distributed system follows this time model, i.e. if
messages are delivered to each process in this order, and
sent down each channel in this order, then the system's
behaviour will be detemrinistic, independent of the speed
ofprocesses and channels.
In the implementation discussed below, we use the set
of sequences of integers as a time-line. Examples are
shown in Figure 3. The sequence is detennined by the
path from the root to a message. This is ordered using the
standard alphabetic ordering on integer sequences.
5. Implementation
We sketch one poSSIble implementation of the time
model It is pessimistic, ie. based on the delayed delivery
of messages. An optimistic implementation could be
achieved using Jefferson's virtual time with rollback [10],
(in which case the assmnptions about channels, section
4
3.2, are unnecessary).
The timeline used is the set ofinteger sequences. However
since we cannot number the (distributed) extemal
messages sequentially we assign a unique pair to each, the
value ofa real clock and an extemal input identifier.
Each node inthe distnbuted system is stmetured as shown
in Figure 4. All ftmctionality related to support for the
time model resides in the time service. The time service
has a local logical clock which is updated whenever a
message is received or sent by its process. The
"timestamp assigner" at the border with the system's
environment has a real clock and a unique input identifier.
Application Process
Internal Inputs
~ (from other processes)
External Inputs
(from outside)
Figure 4 A process with its time service
As in the ISIS ABCAST protocol [13], input messages
are not delivered to the process until there are messages
present on all inputs. Once this condition holds, the local
7. clock is set to the timestamp ofthe oldest message, a new
integeris added to the sequence, and the oldest message is
delivered. When the process sends a message it is
timestamped by the time service with the current value of
the local clock; and then the clock is incremented (i.e. the
last integerin the sequence is incremented).
This algorithm fully implements our time model, but does
not work when the system contains loops. Moreover, rare
messages either on an extema1 input or on an intemallink
may slow the system down. To solve these problems we have
developed a specific channel flushing algorithm for our time
model, significantly different to the one orISIS ABCAST.
The general ideais that each time there are inputmessages
waiting while some inputs are empty, the time service sets
a flush timer. On the timeout event it sends flush requests
to all empty inputs, and then waits for positive or negative
responses. Ifpositive responses come from all inputs the
oldest message is delivered. Ifa negative response comes
on any input the flush is re-scheduled. Requests from
other time services are handled in the following way.
First, the time service tries to reply using the local
infonnation (its local clock and the timestamps of waiting
messages). Ifit is unable to do so, it fOIWards the request
to all empty inputs. If all responses are positive, so is the
one to the remote requester. OtheIWise, the response is
negative.
6. Performance modelling
We used COVERS simulation environment [3,4] to
assess the impact of the time model (implemented as
described above) on system perfonnance.
The modelling results show that, while the system is
underutilized, the ovemead traffic (the auxiliary channel
flushing messages sent at the time service layer) can be as
much as 8 times the nmnber of the application messages.
However, when the input data me is high and the system
reaches its maximmn throughput, the time model traffic
falls down to an acceptable amount
Perfonnance modelling also shows that the system's
throughput depends on the value of the flush timeout,
which should be chosen carefully, especially ifthe system
contains loops. Dynamic (adaptive) tuning of the flush
timer depending on the percentage of successful channel
flushes may be considered.
7. Total ordering and programming models
The strong causality principle provides a much easier
framework for the application programmer. A
programmer used to non-distributed systems is used to a
time-line that is totally ordered, and aIbitIarily divisible.
5
The first part of a procedure staRs and completes before
the second part. There is no special tempornl semantics for
a nonna! part of such a program. For example, a
subprocedure invoked by the first part will also have
completed before the second part.
When we move to a distributed system, programmers
are nonnally expected to have to understand either
concurrency or asynchronicity or both. However, with our
time model the programmer's view of the system is just
like that for a non-distributed system (Figure 5). In this
:figure we see that the end of a remote event handler
logically returns 'control' to the sender. However, this
logical retum of control does not necessarily imply any
loss ofactual concmrency: in the example shown, the only
ordering constraint on the events is that the second 'send
msg 2' instruction is received after the first. This is
ensured by the implementation ofthe time model.
..._..-.._ - .
hlIllCll8r1
............::::::.::.::::.:.::.::: .....
·············.·~·iti~.~·.
G-·[·_~~-.· ;llarrJ~r-:
~-~~i-~-'--':~t~E~iw,~i:! ·'.'.,rldJ8r2"'·· ..·"-_··_·""-,···~· dhandl8r<;
Logical tine ""'.~~.
Figure 5 TemporalsEtn,antics of message sends
We believe the COBOL programmers of telecom
billing systems will find this tempornl semantics intuitive
and easy to use. Moreover, the global linear time line
removes many of the different orderings that plague
distnlmted systems. Much of the logic for dealing with
complex ordering constraints has moved from the
application layer, where it confuses, to the middleware in
the presentation layer, where it empowers. In this way,
applications built on top of our time model can be
significantly simpler; easier to build, understand and
change. This makes them SUbstantially cheaper and less
buggy.
This view ofmessage send like procedurnl call clarifies
why our time model solves the problem of Cheriton and
Skeen exploredin section 2.1.
8. Loops
We have seen in the discussion in section 5 that loops
genexate issues for the time model Typical loops generate
a need for many auxiliary messages. Most loops only
pennit the processing ofa single message at anyone time
8. anywhere within the loop (we say the loop 'locksteps').
The channel flush algorithm allows the processors to
negotiate as to which goes fust.
We have three different solutions to these problems: we
can remove spurious loops in the design process; or
collocate processes in a loop; or explicitly break the link
ofstrong causality between a message and its feedback.
The traditional design models, client/server,
master/slave, encoumge a control driven view of a
distributed system, which leads to loops. A more data
driven view of a system, like the data flow diagrams of
structmed analysis, is typically less loopy.
Figure 6 A false loop
Moreover, where a first cut has loops, a more detailed
analysis ofa distributed system may show that these loops
are spurious. For example, in Figure 6, there is an
apparent loop between process A and process B, (a flow
from A feeds into B which feeds back into A). This
vanishes when we look at the subprocesses AI, A2, Bl,
B2.
We can put all the processes in the loop on the same
processor. There will be no additional penalty in tenns of
loss of parallelism. since the time model enforced this
loss. This approach will minimise the cost ofthe auxiliary
messages, because they will now be local messages.
Infonnally, the problem with a loop is feedback
Feedback happens when an input message to a process
strongly causes another message (the feedback) to anive
later at the same process. Under the strong causality
axiom, feedback is strongly caused by the original
messages, and hence comes before all subsequent
messages. Hence any process in a loop must, after
processing every message, first ascertain whether there is
any feedback, before proceeding to deal with any other
input.
In [5] we construct a delay process for which each
input message is followed by an identical output message,
which is however not 'strongly caused' by the input, but
rather scheduled by it to happen later. Hence, with a delay
in the loop, a process can know that any feedback will not
anive until after the duration of the delay. Thus it can
accept othermessages aniving before the feedback
9. Conclusion
We have presented ourtime model showing its advantages
6
from the point ofview ofprogramming models and causal
semantics. We have also sketched ourinitial implementation.
We discuss the implementation in more detail, giving
detailed peIfonnance modelling results, and an extended
discussion in [5].
We are now working on a second implementation
within a CORBA environment, in which we are
considering the issue of the request/response 'loop'
ubiquitous in client/server architectures.
We are also extending the theoretical wolk. Our next
steps are to look at: deadlock prevention within this time
model; and control flows in the opposite direction from
the data flows.
References
[1] Mike Beech. Architecture Requirements for Enhanced
Billing Systems, worlcshop presented at 2nd Annual Billing
and Customer Service Operations in the
Telecommunications Industry, Institute for International
Research, New York, April 1996.
(2) Kenneth P. Birman and Thomas A Joseph. Exploiting
virtual synchrony in distributed systems. Proceedings of the
Eleventh ACM Symposium on Operating Systems
Principles, Austin, Texas, November 1987. pp. 123-138.
[3] AV. Borshchev, Yu.G. Karpov, V.V. Roudakov.
COVERS - A Tool for the Design ofReal-TIme Concurrent
Systems. In: V. Malyshkin (ed.). Parallel Computing
Technologies. Proceedings of the 3rd International
Conference PACT-95, Lecture Notes in Computer Science
No 964, Springer, 1995, pp. 219-233.
[4] AV. Borshchev. Modelling of Concurrent Real-TIme
Systems. Ph.D. thesis, StPetersburg State Technical
University, April, 1995. [in Russian].
(5) Jeremy J. Carroll and Andrei V Borshchev Strongly Causal
TIme: Semantics. Implementation and Performance
Modelling, Hewlett-Packard Labs Technical Report HPL-
96-XX, 1996 (HP labs TRs are available under
http://www.bpl.hp.comlinventedlteehReportsI)
[6] David R. Cheriton and Dale Skeen. Understanding the
Limitations of Causally and Totally Ordered
Communication. Proceedings of the ACM Symposium on
Operating System Principles. 1993. pp. 44-57.
[7] Colin Fidge. Logical Time in Distributed Computing
Systems. IEEE Computer 24(8), August 1991. pp. 28-33
(8) Keith Harrison, A Novel Approach UJ Event Correlation,
Hewlett-Packard Laboratories Report, No. HPL-94-68,
Bristol, U.K., 1994.
[9] David Hume. An Enquiry Concerning Human
Understanding. Sect VII 'Of the idea of necessary
connexion' part II. 1748.
[lO)David R. Jefferson. Vl1"tual TIme. ACM Transactions on
Programming Languages and Systems, 7(3), July 1985. pp.
404-425
[ll]L. Lamport. Time. Clocks and Ordering of Events in a
Distributed System. Communications of the ACM, Vol. 21,
9. No.7, 1978, pp. 558-565.
[12]Friedmann Mattern. Virtual Time and Global States 0/
Distributed Systems. In M. Comard and P. Quinton, (eds.),
Proceedings 0/ International Workshop on Parallel and
DistributedAlgorithms (Chtiteau de Bonas, France, October
1988), Amsterdam, 1989. Elsevier Science Publishers B.Y,
pp.215-226.
[13]P. Stephenson. Fast causal multicast. Ph.D. thesis, Comell
UIriversity, February, 1991
7