The document discusses the history and concepts of distributed systems. It defines a distributed system as a collection of independent computers that appears as a single system to users. Distributed systems provide benefits like resource sharing, availability, scalability, and performance. However, they also introduce challenges around concurrency, security, partial failures, and heterogeneity. The document outlines common goals for distributed systems like transparency, openness, and scalability. It describes different approaches to scaling distributed systems through techniques like hiding latencies, distribution, and replication. Finally, it discusses key hardware concepts like multiprocessors and multicomputers as well as software approaches like distributed operating systems, network operating systems, and middleware.

1. Chapter 1 - Introduction

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1.1 Introduction and Definition
 before the mid-80s, computers were
 Very expensive (hundred of thousands or even millions
of dollars)
 Very slow
 Not connected among themselves
 after the mid-80s: two major developments
 cheap and powerful microprocessor-based computers
appeared
 computer networks
 LANs at speeds ranging from 10 to 1000 Mbps
 WANs at speed ranging from 64 Kbps to gigabits/sec

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 Definition of a Distributed System
 a distributed system is:
a collection of independent computers that appears to its
users as a single coherent system - computer (Tanenbaum
& Van Steen)
 this definition has two aspects:
1. hardware: autonomous(independent) machines
2. software: a single system view for the users

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 Why Distributed?
 Resource and Data Sharing
 printers, databases, multimedia servers, ...
 Availability, Reliability
 the loss of some instances can be hidden
 Scalability, Extensibility
 the system grows with demand (e.g., extra servers)
 Performance
 huge power (CPU, memory, ...) available
 Inherent distribution, communication
 organizational distribution, e-mail, video

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 Problems of Distribution
 Concurrency, Security
 clients must not disturb each other
 Privacy
 e.g., when building a preference profile
 unwanted communication such as spam
 Partial failure
 we often do not know where the error is (e.g., RPC)
 Location, Migration, Replication
 clients must be able to find their servers
 Heterogeneity
 hardware, platforms, languages, management

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1.2 Organization and Goals of a Distributed System
 to support heterogeneous computers and networks and
to provide a single-system view, a distributed system is
often organized by means of a layer of software called
middleware that extends over multiple machines
a distributed system organized as middleware; note that the middleware
layer extends over multiple machines

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 Goals of a distributed system: a distributed system should
 easily connect users with resources (printers, computers,
storage facilities, data, files, Web pages, ...)
 reasons: economics, to collaborate and exchange
information
 be transparent: hide the fact that the resources and
processes are distributed across multiple computers
 be open
 be scalable
 Transparency in a Distributed System
 a distributed system that is able to present itself to users
and applications as if it were only a single computer
system is said to be transparent

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 different forms of transparency in a distributed system
Transparency Description
Access Hide differences in data representation
(file naming, ...) and how a resource
is accessed
Location Hide where a resource is physically located; where
is http://www.google.com? (naming)
Migration Hide that a resource may move to another location
Relocation Hide that a resource may be moved to another
location while in use; e.g., mobile users using their
wireless laptops
Replication Hide that a resource is replicated
Concurrency Hide that a resource may be shared by several
competitive users; a resource must be left in a
consistent state
Failure Hide the failure and recovery of a resource
Persistence Hide whether a (software) resource is in memory
or on disk

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 Openness in a Distributed System
 A distributed system should be open
 we need well-defined interfaces
 Interoperability
 components of different origin can communicate
 Portability
 components work on different platforms
 Another goal of an open distributed system is that it
should be flexible and extensible;
 easy to configure the system out of different components;
 easy to add new components,
 easy to replace existing ones
 an Open Distributed System is a system that offers
services according to standard rules that describe the
syntax and semantics of those services; e.g., protocols in
networks

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 Scalability in Distributed Systems
 a distributed system should be scalable
 Size: adding more users and resources to the system
 Geographically: users and resources may be far apart
 Administratively: should be easy to manage even if it
spans many administrative organizations

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Concept Example
Centralized services
Single server for all users-mostly for security
reasons
Centralized data A single on-line telephone book
Centralized algorithms Doing routing based on complete information
examples of scalability limitations
 Scalability problems: performance problems caused by
limited capacity of servers and networks
 Scaling Techniques
 how to solve scaling problems
 For geographical scalability: the problem is mainly
performance, due to the limitations in the capacity of
servers and networks
 three possible solutions: hiding communication latencies,
distribution, and replication

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a. Hide Communication Latencies
 Try to avoid waiting for responses to remote service
requests
 let the requester do other useful job
 i.e., construct requesting applications that use only
asynchronous communication instead of synchronous
communication; when a reply arrives the application is
interrupted
 Good for batch processing and parallel applications but
not for interactive applications
 for interactive applications, move part of the job to the
client to reduce communication; e.g. filling a form and
checking the entries

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(a) a server checking the correctness of field entries
(b) a client doing the job

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b. Distribution
 e.g., DNS - Domain Name System
 divide the name space into zones
 for details, see later in Chapter 4 - Naming
an example of dividing the DNS name space into zones

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c. Replication
 Replicate components across a distributed system to
increase availability and for load balancing, leading to
better performance
 decided by the owner of a resource
 caching (a special form of replication) also reduces
communication latency; decided by the user
 but, caching and replication may lead to consistency
problems (see Chapter 6 - Consistency and Replication)

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1.3 Hardware and Software Concepts
 Hardware Concepts
 different classification schemes exist
 multiprocessors - with shared memory
 multicomputers - that do not share memory
 can be homogeneous or heterogeneous

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 a single
backbone
different basic organizations of processors and memories in distributed
systems

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a bus-based multiprocessor
 Multiprocessors - Shared Memory
 the shared memory has to be coherent –
 the same value written by one processor must be read by another
processor
 Performance problem for bus-based organization since the
bus will be overloaded as the number of processors
increases
 The solution is to add a high-speed cache memory
between the processors and the bus to hold the most
recently accessed words; may result in incoherent memory
 Bus-based multiprocessors are difficult to scale even with
caches
 Two possible solutions: crossbar switch and omega
network

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 Crossbar switch
 divide memory into modules and connect them to the
processors with a crossbar switch
 at every intersection, a crosspoint switch is opened and
closed to establish connection
 problem: expensive; with n CPUs and n memories, n2
switches are required

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 Omega network
 use switches with multiple input and output lines
 drawback: high latency because of several switching
stages between the CPU and memory

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 Homogeneous Multicomputer Systems
 also referred to as System Area Networks (SANs)
 the nodes are mounted on a big rack and connected
through a high-performance network
 could be bus-based or switch-based
 bus-based
 shared multiaccess network such as Fast Ethernet can be
used and messages are broadcasted
 Performance drops highly with more than 25-100 nodes
(contention)

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Grid
Hypercube
 switch-based
 messages are routed through an interconnection network
 two popular topologies: meshes (or grids) and
hypercubes

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 Heterogeneous Multi-Computer Systems (HMCS)
 most distributed systems are built on heterogeneous
multicomputer systems
 the computers could be different in processor type,
memory size, architecture, power, operating system, etc.
and
 I/O bandwidth and the interconnection network may be
highly are different
 the distributed system provides a software layer to hide
the heterogeneity at the hardware level; i.e., provides
transparency

24. Software Concepts
 Distributed OS (DOS)
 Network OS (NOS)
 Middleware
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 Operating systems for distributed computers can be
roughly divided into two categories:
 Tightly-coupled systems:
 The OS tries to maintain a single, global view of the resources
 Generally, referred to as distributed OSs (DOS)
 Used for multiprocessors and homogeneous multi-computers
 Each component dependent on the others
 Manages all the resources in a distributed system,
 Provides transparency (location, migration, concurrency,
replication)
 Loosely-coupled systems, referred to as network OSs
(NOS)
 In which, there are a collection of computers each running their
own operating system;
 However, these OS work together to make their services and
resources available to others
 Used for heterogeneous multi-computers
Software Concepts…….

26.  Middleware:
 To enhance the services of network operating
system (NOS)
 To provide distribution transparency
 To actually come to a distributed system
26
Software Concepts……

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System Description Main Goal
DOS
Tightly-coupled operating system for multi-
processors and homogeneous
multicomputers
Hide and manage
hardware
resources
NOS
Loosely-coupled operating system for
heterogeneous multicomputers (LAN and
WAN)
Offer local
services to remote
clients
Middleware
Additional layer atop of NOS implementing
general-purpose services
Provide
distribution
transparency
 Summary of main issues
an overview of DOSs, NOSs, and middleware

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 Distributed Operating Systems
 Two types of Distributed systems
 Multiprocessor operating system:
 Manages the resources of a multiprocessor
 Multicomputer operating system:
 An operating system that is developed for
homogeneous multi-computers

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Uniprocessor Operating Systems:
 One large kernel that handles everything (e.g., MS-
DOS, early UNIX).
 Microkernel architecture: The OS kernel is
decomposed into micro kernels,
 Separating applications from operating system code
through a microkernel

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 More on Multiprocessor Operating Systems
 Extended from uniprocessor operating systems to support
multiple processors having access to a shared memory
 Problem: Consistency
 Solution: using a protection mechanism (or two
synchronization mechanisms): semaphores and monitors
 Semaphore: an integer with two atomic operations down
and up
 Monitor: a programming language construct consisting
of procedures and variables that can be accessed only
by the procedures of the monitor;
 only a single process at a time is allowed to execute
a procedure

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 Multicomputer Operating Systems
general structure of a multicomputer operating system
 Processors can not share memory; instead communication
is through message passing
 Each node has its own
 kernel for managing local resources
 separate module for handling interprocessor
communication

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 Distributed Shared Memory Systems
 how to emulate shared memories on distributed systems to
provide a virtual shared memory
 page-based distributed shared memory (DSM) - use the
virtual memory capabilities of each individual node
pages of address space distributed among four machines

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situation if page 10 is read only and replication is used
situation after CPU 1 references page 10
 read-only pages can be easily replicated

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 Network Operating Systems
 possibly heterogeneous underlying hardware
 constructed from a collection of uniprocessor systems, each
with its own operating system and connected to each other
in a computer network
general structure of a network operating system

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 Services offered by network operating systems
 remote login (rlogin)
 remote file copy (rcp)
 shared file systems through file servers
two clients and a server in a network operating system

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 Middleware
 A distributed operating system is not intended to handle a
collection of independent computers but provides
transparency and ease of use
 A network operating system does not provide a view of a
single coherent system but is scalable and open
 Combine the scalability and openness of network operating
systems and the transparency and ease of use of distributed
operating systems
 this is achieved through a middleware, another layer of
software

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general structure of a distributed system as middleware

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Item
Distributed OS Network
OS
Middleware
-based OS
Multiproc Multicomp
Degree of
transparency
Very High High Low High
Same OS on all nodes Yes Yes No No
Number of copies of
OS
1 N N N
Basis for
communication
Shared
memory
Messages Files
Model
specific
Resource management
Global,
central
Global,
distributed
Per node Per node
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open
 a comparison between multiprocessor operating systems,
multicomputer operating systems, network operating
systems, and middleware-based distributed systems

39.  Networks
 Started in 1070s
 Ethernet and LAN were evented
 The internet continue to be the biggest examples of distributed
systems.
 Distributed systems have evolved from “LAN” based to
“Internet” based.
 Telecommunication networks
 Real-time systems
 Flight control systems
 Uber and Ride
 Manufacturing automation control systems
 Parallel Processing
 Distributed artificial intelligence
 Distributed Database Systems 39
Examples of Distributed Systems

40. The end
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