Introduction to Distributed Systems

Advanced Distributed Systems: CSEg 6108
Eta Fre
etafre@gmail.com
January 18, 2023
Eta Freetafre@gmail.com Distributed Systems. 1 / 31

How to achieve the Goals
▶ Read papers
▶ Practice/Labs

Objective of today’s class
▶ General discussion about distributed systems
▶ Define distributed systems
▶ Identify Major characteristics of distributed systems
▶ Understand the purpose of middlewares as a facilitator of
implementation of DSes
▶ State Design goals

Introduction
▶ They are common and they are in day to day use.
▶ Examples: Network, www, IoT, Grid systems, Cloud system, Mobile
system, etc.
▶ An often difficult area of research and difficult to implement
compared to a centralized systems.
▶ But what are distributed systems? What are their design goals?
which research challenges are still needs to be tackled?

Intro...History
▶ Computers were isolated from each other (not be able to connect
each other)
▶ Until mid of 1980: powerful processors and computer networks
▶ Processors
▶ 16, 32, 64 bit CPUs
▶ Multicore CPUs
▶ Networks
▶ LAN : two to thousands of machine within an enterprise, Gbps
▶ WAN: two to billions of computers connected to each other across the world
▶ Interconnection uses: wired or wireless or a hybrid
▶ Hence Distributed Systems(DSes) are emerged

Intro...Definition of DSes
▶ Various definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sufficient
A distributed system is a collection of autonomous computing
elements(nodes(HW, SW)) that appears to its users as a single coherent
system due to the collaboration that must exist between them.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
▶ How to establish such a collaboration without taking into
consideration the type of nodes(mainframe, plug computers), the
ways that nodes are connected is what we study in this course.

Intro...the two characteristics
▶ Collection of auto. Computing Elements (issues)
▶ act independently but must collaborate
▶ programmed to achieve common goals by exchanging messages. A good
analogy is a project work assigned to a team.
▶ At least the team needs to agree on division of work and deadlines(timing).
▶ Just consider the timing issue.
▶ What would happen if there is no synchronized time among the team
members?
▶ In DSes, very difficult to have a global clock.
▶ Managing group membership
▶ Admission control
▶ Authentication : a scalability bottleneck
▶ should communicate only with group members : trust issues
▶ Membership registration
▶ Organized as overlay networks: structured (well-defined set of nodes in a tree
or ring) vs unstructured (randomly selected nodes)
▶ How to organize nodes (architecture) in a DS requires a lot of effort
▶ Peer-to-peer(P2P) networks

Intro...the two characteristics
▶ Single Coherent System Issues
▶ Distribution transparency (invisibility)
▶ Analogy: Unix-like OSes where every resources (file, devices, sockets etc) are
treated as file. https://en.wikipedia.org/wiki/Everything_is_a_file
▶ Some things like partial failures are hard to hide
▶ There are many issues that makes development of DS difficult? help?

Intro...Middleware and distributed Systems
▶ To assist development of DS, have a separate layer of software that
addresses common DS issues
▶ Security
▶ Fault tolernace
▶ Auditing services
▶ Inter application communication
▶ Equivalent to what an OS gives to applications but in a networked
environment
▶ Obviously, this separate layer of software runs on top of OSes

Intro...Middleware...Communication
▶ Remote Procedure Call(RPC): allow to invoke a procedure on
remote computer. The RPC middleware handles the details of
generating code does remote invokations.
▶ Transactions on multiple services: executing a block of actions in
all-or-nothing style (atomicity). We only need to specify the services,
and the rest is handled by the middleware.
▶ Service Composition (Web services, Microservices): middleware
standardizes the way these services are accessed and providing a
means to generate their functions in specific order.
▶ Reliability: a middleware guarantees whether messages sent by a
process have been delivered to all or no other process.

Intro...Design goals
▶ 4 goals
▶ Supporting Resource sharing
▶ Making Distribution transparent
▶ Being Open
▶ Being Scalable

Intro...Design goals...Supporting Resource Sharing
▶
▶ make it easy to access and share remote resources (computational
power, devices, storage, code, data, files, services, networks,
documents ...)
▶ Printer: you can’t have a printer/person in an org. Instead a printer
client software at each node
▶ Code stored on Github, multi-user code development
▶ Video conferencing, group document editing
▶ Big data: difficult to download it to your PC
▶ File sharing: peer-to-peer torrents

Intro...Design goals...Making distribution transparent
▶ making scattered processes and resources hidden from users (
applications and people)
▶ Types of distribution transparency
▶ Access transparency: machine architectures, data representation, file-naming
conventions, object access
▶ Location transparency: hide the location, logical names like URL,
▶ Relocation transparency: relocate your files from one computer to another
▶ Migration transparency: relocation initiated by users without affecting
services. communication through mobile phones jumping through cell towers.
▶ Replication transparency: Duplicating data, increase availability, increase
performance. You don’t know that there are multiple copies. All copies
should be known by one name.
▶ Concurrency transparency: sharing same node, sharing a database. Must be
shared in a mutual exclusive manner and leave the resource in a consistent
state. Locking mechanism.
▶ Failure transparency: recovering from a partial failure without the user
noticing it. Masking failure is one of the difficult problem in DSes. How to
differentiate between dead processes and very slow processes??

Intro...Design Goals...Degree of Transparency
▶ When is hiding distribution bad? : Degree of distribution
transparency. Time Zone differences. Nature’s speed restriction.
Trade off b/n performance and transparency. Location services.
Further complication(rpc over fault lines, instead use message
communications).

Intro...Openness
▶ System that provide elements that can easily integrate into or used by
other systems
▶ Open systems are interoperable, composable and extensible
▶ How to achieve: adhere to standard rule, syntax(names of function
and parameters, types of parameters and return values) ,
semantics(what it does); implementation independence, easily add
new components or replace existing one
▶ Achieving flexibility in open distributed systems: separating policy
from mechanisms. Caching: mechanism for storing; policies for
eviction, sharing among users, refreshing rate.
▶ Still a lot of researches are needed to make DSes more open

Intro...Design Goals...Being Scalable
▶ A very good example for scalability is the Internet!
▶ Scalability dimensions: measures scalability along three dimensions
▶ Size scalability: add more resources and users without loss of performance.
CPU bound, Storage bound, Network bound.
▶ Geographical scalability: its users and resoruces are far apart but
communication delays hardly noticed. Synchronous communication in wide
area network where communication may 100ms (LAN applications become
non-perfroming); failure; bandwidth limitations; lack of broadcasting feature
for service discovery in wide-area setup, you need to setup naming and
directory services which becomes time consuming by them selves.
▶ Administrative scalability: many independent adminstrative organization;
major issue is solving conflicting policies about resources, management and
security. the Grid system. Peer-to-Peer.

Pitfalls when developing distributed systems
▶ False assumptions that everyone makes when developing a distributed
application for the first time:
▶ The network is reliable
▶ The network is secure
▶ The network is homogeneous
▶ The topology does not change
▶ Latency is zero
▶ Bandwidth is infinite
▶ Transport cost is zero
▶ There is one administrator

Tasks/Assignment/Homeworks
▶ Compare and contrast distributed systems communication
mechanisms. Also give examples that show how to do them in the
GO programming language.
▶ RPC
▶ gRPC
▶ RMI
▶ HTTP/HTTPS
▶ REST API
▶ GraphQL
▶ Total Order Broadcast
▶ WebSocket
▶ AMQP

Intro...Types of Distributed Systems
▶ High Performance Distributed Computing Systems
▶ Distributed Information Systems
▶ Pervasive Systems

Intro...Types of DS...High Performance Dist. Computing
▶ Started with the multiprocessor machines: CPUs with shared memory
(multiple threads sharing a memory which is protected by some kind
of synchronization mechanisms like semaphores). Got scalability
problem. Distributed-memory system. Message Passing. Turned
out to be more difficult and error-prone compared to shared memory.
▶ Multicomputer system connected with network.
▶ Types
▶ Cluster Computing
▶ Grid Computing
▶ Cloud Computing

Intro...Types of DS..HPC..Cluster Computing
▶ build supercomputer using commodity computers
▶ Similar hardware and OS
▶ high speed LAN
▶ history: became popular when the price/performance ratio of
computers improved
▶ Examples: Linux Beowulf clusters, MOSIX system(Single-System
image, providing strong distribution transparency. It allows processes
to dynamically and preemptively migrate b/n nodes)
Figure: An example of a cluster computing system

Intro...Types of DS..HPC..Grid Computing
▶ Diversity is assumed(not homogeneity like cluster computing): HW,
SW(OS), security policies, Administration
▶ Federation of systems : resources(DB, storage, compute servers etc)
from d/t organizations are brought together for the purpose of
collaboration, virtual organization (you are given rights, permissions
etc in this virtual organization)
▶ To realize a system like this, what do you think the software be able
to do?

Intro...Types of DS..HPC..Grid Computing
▶ Diversity is assumed(not homogeneity like cluster computing): HW,
SW(OS), security policies, Administration
▶ Federation of systems : resources(DB, storage, compute servers etc)
from d/t organizations are brought together for the purpose of
collaboration, virtual organization (you are given rights, permissions
etc in this virtual organization)
▶ To realize a system like this, what do you think the software be able
to do?
▶ Give access to resources from d/t institution, applications etc
▶ Architecture
Figure: A layered arch. for grid computing

Intro...Types of DS..HPC..Cloud Computing
▶ Sharing resources, which otherwise be wasted, to external users
▶ Utility computing, pay-per-use(SLA) scenario, use according to your
demands
Figure: Cloud organization
▶ Among other things, Virtualization tech is the main enabler of cloud
computing – access to a pool of Virtualized resources

Intro...Types of DS...Distributed Information Systems
▶ Another class of distributed systems found in an organization
▶ Networked applications in an organization – enterprise-wide
distributed information systems
▶ Solves the problem of interoperability between different applications
▶ Two Examples: Distributed Transaction processing and Enterprise
Application Integration
▶ Distributed Transaction Processing
▶ request/reply behaviour
▶ All-or-nothing
▶ Coordinating subtransactions – middleware(transaction processing monitor
which uses distributed commit)
▶ Travel handling: many reservations(Visas, Airline, hotel, buses ...)

▶ Enterprise Application Integration
▶ Integrating applications with each other without their databases
▶ Communication models: synchronous and tight coupling (RPC, RMI),
message-oriented middleware (publish/subscribe)
▶ Four ways to integrate applications
▶ File transfer
▶ Shared database
▶ Remote procedure call
▶ Messaging
▶

▶ Four ways to integrate applications. What are they?

▶ Four ways to integrate applications. What are they?
▶ File transfer: produce a file to be read by other applications; technically
simple but communicating parties need to agree on a lot of things
▶ File type(text, binary) and layout
▶ File Management: where to store, how to name, who to delete them?
▶ Update propagation
▶ Shared database
▶ Solve the problems with files
▶ Drawbacks: design a common data schema, performance
bottleneck(many reads and writes)
▶ Remote procedure call
▶ Sometimes it is better to offer a procedure to other applications to call
instead of data interfaces.
▶ Good for information hiding
▶ Messaging: Main drawback of RPC is the need for simultaneous availability
of communicating parties, which impossible to guarantee. Messagings are
better, but they also need data formatting and layout, where to send the
message, deal with lost messages, etc
▶ In general, many middleware producers targets enterprise application
integration

Intro...Types of DS..Pervasive Systems
▶ Mobile and embedded systems: where we don’t assume stability
▶ Distributed systems that blend into the environment
▶ Compared to the distributed computing and information systems,
they blur the separation line b/n users and system components
▶ Sensors(input information), Actuators(provide information)
▶ Devices are small, battery powered, mobile and with a wireless
connection
▶ Three types based on different aspects of pervasive systems:
▶ Ubiquitous computing systems
▶ Mobile Systems
▶ Sensor networks

Intro...Types of DS...Pervasive Systems...Ubiquitous
▶ Pervasive and continuously present
▶ Core requirements
▶ Distribution: Devices are networked, distributed, and accessible in a
transparent manner (the feeling a single coherent system, distribution
transparency)
▶ Interaction: Interaction between users and devices is highly unobtrusive
(implicit action - not aimed to interact - don’t know you are providing input
to a computer system)
▶ Context awareness: The system is aware of a user’s context in order to
optimize interaction(location, identity, time, activity)
▶ Autonomy: The system is aware of a user’s context in order to optimize
(reduced explicit mmgt, myriad of techniques: address allocation, adding
devices(UPnP), automatic updates) interaction
▶ Intelligence: The system as a whole can handle a wide range of dynamic
actions and interactions(methods and techniques from the field of AI-
incomplete input, unexpected events ... investigate the extent to which this
can be done in a distributed fashion)

Intro...Types of DS..Pervasive Systems..Mobile
▶ Mobility is key in pervasive systems
▶ Location change is key. Location change may mean service change
which need dynamic discovery of systems (letting services announce
their presence) from a distributed system view.
▶ Tracking and tracing applications
▶ Establishing routes among mobile devices

Intro...Types of DS..Pervasive Systems..Sensor networks
▶ Enabling technology for pervasiveness
▶ From a distributed system perspective, sensor netowrs are more than
just a collection of input devices
▶ They collaborate in an application-specific manner, makes them
different from traditional computer networks
▶ use wireless communications, battery powered, limited resources. All
these makes efficiency design a mandatory, hence a lot of research

Tasks/Assignment/Homeworks – 2
▶ Create example cluster computing system in Linux
▶ classify the last communication models (in assignment 1) as file
transfer, shared database, remote procedure call, messaging
▶ Write an essay to compare and contrast about different file formats
and structures (data serialization) used in distributed systems: JSON,
XML, YAML, ProtoBuf

Introduction to Distributed Systems

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