Introducing …

Distributed Systems

Paul Barry

Muhammed Cinsdikici

1. Definition of a Distributed System (1)“A distributed system is a collection of independent computers that appear tothe users of the system as a single computer.” [Tanenbaum]

“A system in which hardware or software components located at networkedcomputers communicate and coordinate their actions only by messagepassing.” [Coulouris]

“A system that consists of a collection of two or more independent computerswhich coordinate their processing through the exchange of synchronous orasynchronous message passing.”

“A distributed system is a collection of autonomous computers linked by anetwork with software designed to produce an integrated computing facility.”

1.2 Goals of Distributed Systems

• Easily Connect Users/Resources• Exhibit Transparency• Support Openness• Be Scalable

– in size– geographically– administratively

Looking at these goals helps use answer the question: “Is building a distributed system worth the effort?”

Definition of a Distributed System (2)

A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.

1.1

1.2.1 Connect Users & Resources

-Easy access of users to remote resources

-Geographically distributed users can be build groupware

-Exchange of various types of data (voice, data, video..)

-Share resources with other users in an appropriate way

-Limited resources can be assigned to multiple users

-Cost optimization is done

-BUT ! Security is important aspect that should be observed and implemented carefuly.

1.2.2 Transparency in a Distributed System

Different forms of transparency in a distributed system.

Transparency Description

AccessHide differences in data representation and how a resource is accessed

Location Hide where a resource is located

Migration Hide that a resource may move to another location

RelocationHide that a resource may be moved to another location while in use

ReplicationHide that a resource may be shared by several competitive users

ConcurrencyHide that a resource may be shared by several competitive users

Failure Hide the failure and recovery of a resource

PersistenceHide whether a (software) resource is in memory or on disk

Degree of Transparency

•Hide blindly all distribution aspects from users is not always a good idea.

•Ex1_Content: Morning paper.

•When you request your countrie’s morning newspaper at 7:00 am from other side of the world, time should not be transparent. If time is accepted as transparent, then requested paper should’nt be the expected one.

•Ex2_Performance: Replication of Failed Server

•When a data is searched, a failed server is tried several times. In order to getting rid of dealing with failed server, user can cancel the searching that server.

1.2.3 Openness

- Offers services according to standard rules that describe the syntax and semantics of those services.

- Interoperability

- Portability

- Flexibility of configuration by different developers

- IDL(Interface Definition Language) is used to describe standard interfaces.

Seperating Policy Mechanism(Flexibility)

- System is organized as a collection of relatively small and easily replaceable or adaptable components

- In Monolithic approach, system components are only logically seperated but implemented as huge program.

- In Monolithic approach, changing one of the logical components effects the whole.

- Ex: Web Caching. Browsers should provide basic facilities of storing documents and at the same time users can decide which docs are stored and how long etc…

1.2.4 Scalability

Distributed Systems Scalability can be summurized as;

- In Size: Add more users and resources

- Geographically: Users and resources lie far apart

- Administratively: Many Independent administrative organizations.

Scalability Problems (1)

Decentralization has the following specs;

1. No machine has complete info about system state

2. Machines make decisions based on local info

3. Failure of one machine does not ruin the algorithm

4. There is no implicit assumption that a global clock exists

Concept Example

Centralized servicesA single server for all users (single Windows Logon server)

Centralized dataA single on-line telephone book (single DNS server data)

Centralized algorithms

Doing routing based on complete information(Making all routing in a single routing process)

Scalability Problems (2)

BUT!!!

Decentralization has also consider the following limitations (out of the LAN);

1. Synchronization should be done in great deal for geographicaly far aparts.

2. Communication is should be guaranteed on unreliable connections.

3. Scaling distributed systems across multiple independent administrative domains.

4. System security issues should be provided carefully.

Scaling Techniques (Asynchronous Communication)

1.4

The difference between letting (asynchronous comm):

a) a server or

b) a client check forms as they are being filled

Scaling Techniques (Distribution)

1.5

An example of dividing the DNS name space into zones.

Scaling Techniques (Caching & Replication)

Considering that scalability problems often appear in the form of performance degradation. So,

- Replicate components

(it increases availability and also balance the load)

- Caching is a special form of replication

(in contrast to replication, caching is a decision made

by client of a resource and not by the owner of a resource)

BUT !!! CONSISTENCY OF THE DATA SHOULD BE PROVIDED

Modeling Distributed Systems

When building distributed applications, system builders have often looked to the non-distributed systems world for models to follow (… inspiration?)

Consequently, distributed systems tend to exhibit certain characteristics that are already familiar to us

This applies equally to hardware concepts as it does to software concepts

1.3 Modeling Hardware Concepts

1.6

Different basic organizations and memories in distributed systems

1.3.1 Multiprocessors (1)

A bus-based multiprocessor.

1.7

Multiprocessors (2)

A crossbar switch

An omega switching network

1.8

1.3.2 Homogeneous Multicomputer Systems

Grid

Hypercube

1-9

1.4 Modeling Software Concepts

An overview of • DOS (Distributed Operating Systems)• NOS (Network Operating Systems)• Middleware

System Description Main Goal

DOSTightly-coupled operating system for multi-processors and homogeneous multicomputers

Hide and manage hardware resources

NOSLoosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)

Offer local services to remote clients

MiddlewareAdditional layer atop of NOS implementing general-purpose services

Provide distribution transparency

1.4.1 Distributed Operating Systems (DOS)a) Uniprocessor Operating Systems

Separating applications from operating system code through a “microkernel” – can provide a good base upon which to build a distributed

operating system (DOS).

1.11

Distributed Operating Systems (DOS)b) Multiprocessor Operating Systems (1)

A monitor to protect an integer against concurrent access.

monitor Counter {

private:

int count = 0;

public:

int value() { return count;}

void incr () { count = count + 1;}

void decr() { count = count – 1;}

}

Distributed Operating Systems (DOS)Multiprocessor Operating Systems (2)

A monitor to protect an integer against concurrent access, but blocking a process.

monitor Counter {

private:

int count = 0;

int blocked_procs = 0;

condition unblocked;

public:

int value () { return count;}

void incr () {

if (blocked_procs == 0)

count = count + 1;

else

signal (unblocked);

}

void decr() {

if (count ==0) {

blocked_procs = blocked_procs + 1;

wait (unblocked);

blocked_procs = blocked_procs – 1;

}

else

count = count – 1;

}

}

Distributed Operating Systems (DOS)Multicomputer Operating Systems (1)

General structure of a (DOS) multicomputer operating system – all the systems are of the same type: homogeneous

1.14

Distributed Operating Systems (DOS) Multicomputer Operating Systems (2)

Alternatives for blocking and buffering in message passing.

1.15

Distributed Operating Systems (DOS) Multicomputer Operating Systems (3)

Relation between blocking, buffering, and reliable communications.

Synchronization point Send bufferReliable comm.

guaranteed?

Block sender until buffer not full Yes Not necessary

Block sender until message sent No Not necessary

Block sender until message received No Necessary

Block sender until message delivered No Necessary

Distributed Operating Systems (DOS) Distributed Shared Memory Systems (1)

Pages of address space distributed among four machines

Situation after CPU 1 references page 10

Situation if page 10 is read only and replication is used

Distributed Operating Systems (DOS) Distributed Shared Memory Systems (2)

False sharing of a page between two independent processes.

1.18

1.4.2 Network Operating System (1)

General structure of a network operating system – all the systems are of different types: heterogeneous

1-19

Network Operating System (2)

Two clients and a server in a network operating system – relatively primitive set of services provided.

1-20

Network Operating System (3)

Different clients may mount the servers in different places – difficult to maintain a consistent “view” of the system.

1.21

The Best of Both Worlds?

DOS: too inflexible (all systems of the same type)

NOS: too primitive (lowest common demoninator – too much diversity)

“Middleware” – best possible compromise?

Middleware = NOS + additional software layer

1.4.3 Positioning Middleware

General structure of a distributed system as middleware.

1-22

Middleware and Openness

In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications. This is a much higher level of abstraction than (for instance) the NOS Socket API.

1.23

Middleware Models/Paradigms

Distributed File Systems

The Remote Procedure Call (RPC)

Distributed Objects

Distributed Documents

[All of which we return to in detail later in this course … ]

Comparing DOS/NOS/Middleware

A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.

ItemDistributed OS

Network OS

Middleware-based OSMultiproc

.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 FilesModel

specific

Resource management

Global, central

Global, distributed

Per node Per node

Scalability No Moderately Yes Varies

Openness Closed Closed Open Open

The Classic DS Model

How are “processes” organised within a Distributed System?

General agreement/concensus:

“Client/Server” Model

Multi-tiering:

User Interface Level, Processing Level, Data Level.

1.5 Clients and Servers

General interaction between a client and a server.

1.25

An Example Client and Server (1)

The header.h file used by the client and server.

An Example Client and Server (2)

A sample server.

An Example Client and Server (3)

A client using the server to copy a file.

1-27 b

1.5.2 Application Layering

In the general organization of an Internet search engine into three different layers – often referred to as “tiers”.

1-28

1.5.3 Client-Server Multitiered Architectures (1)

Alternative client-server organizations (a) – (e).

1-29

Client-Server Multitiered Architectures (2)

An example of a server acting as a client – this is a very common vertical distribution model for distributed systems.

1-30

Client-Server Example Modern Architecture

An example of horizontal distribution of a Web service (often also referred to as “clustering”).

1-31

Examples of Distributed Systems

dsys.part1.pdf

Summary (Introduction)• Distributed Systems … autonomous computers working together to give the appearance of a single,

coherent system.• They are transparent, scalable and open.

• Unfortunately, they also tend to be complex.• Types of DS: DOS, NOS, Middleware.

• Processes within DSs conform to the “client/server model”.

• Architectures included vertical and horizontal arrangements, often into many levels/tiers.