Definition of a Distributed System (1) A distributed system is: A collection of independent computers that appears to its users as a single coherent system.

Definition of a Distributed System (1)

A distributed system is:

A collection of independent computers that appears to its users

as a single coherent system.

Definition of a Distributed System (2)

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

1.1?

users

Standard OS

Goals

• Access and share remote resources

• Interoperability

• Portability

• Flexibility

• Transparency

• Scalability

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

Replication Hide that a resource may be replicated

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

Scalability problems in a distributed system

Concept Example

Centralized services A single server for all users

Centralized data A single on-line telephone book

Centralized algorithmsDoing routing based on complete information

Scalability •Size•Geography•Administrative organizations

Decentralized algorithms characteristics

•None has complete information about the system state•Machines take decisions on local info•Failure of one machine doesn’t affect the algorithm•There is no assumption about a global clock

Scalability

How to solve scalability problems?

a) Hiding communication latencies

• Asynchronous communications (but not only, not always)

b) Distribution

c) Replication (with care for consistency)

Scaling Techniques (1)

1.4

Moving part of the computation to the client…(hiding comm. latency)

The difference between letting:

a) a server or

b) a client

check forms as they are being filled

Scaling Techniques (2)

1.5

Distribution… Replication…

DNS name space divided into zones, with possibility of slave server

Software ConceptsOperating systems for distributed computers

An overview between • 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

Uniprocessor Operating Systems

Separating applications from operating system code through a microkernel.

1.11

Multicomputer Operating Systems (1)

General structure of a multicomputer operating system

Communication Message passing

1.14

Multicomputer Operating Systems (2)

Alternatives for blocking and buffering in message passing

• Buffering: sender and receiver side

• 4 possible synchronization points

Reliable communication?

1.15

Multicomputer Operating Systems (3)

Relation between blocking, buffering, and reliable communications.

Synchronization point Send bufferReliable comm.

guaranteed?

Block sender until buffer not full (S1) Yes Not necessary

Block sender until message sent (S2) No Not necessary

Block sender until message received (S3)

No Necessary

Block sender until message delivered (S4)

No Necessary

Network Operating System (1)

General structure of a network (not distributed) operating system.Users see the structure

1-19

Network Operating System (2)Evolution : file server

Two clients and a file server in a network operating system.

File servers generally maintain hierarchical file system.

1-20

Network Operating System (3)

Different clients may mount the servers in different places.

Different clients may have different views of the file systems

1.21

With a basic network operating system we loose in transparency

•Explicit remote login

•Independent accounting

•Independent access permission

•Low protection

Positioning Middleware

General structure of a distributed system as middleware.

Middleware does not manage an individual node

1-22

Middleware Models

•Plan 9 – file oriented paradigm (Unix like)

•Distributed file system – transparency supported only for traditional files

•Models based on RPCs – hide network communications

•Distributed objects – each object can be located on a single machine, each object interface hides internal details including communications

•Distributed documents - WWW

Middleware Services

•Naming allows entities to be shared and looked up.

•Persistence for storage, i.e. databases or facilities to connect to databases

•Distributed transactions allow multiple read and write operations to occur atomically.

•Security

•Access transparency by communication facilities hiding message passing through network

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.

1.23

Interfaces definition has to be complete. Incompleteness lead to bad portability and bad interoperability

Comparison between Systems

ItemDistributed OS

Network OS

Middleware-based OSMultipro

c.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

MessagesMessages/

FilesModel

specific

Resource management

Global, central

Global, distributed

Per node Per node

Scalability No Moderately Yes Varies

Openness Closed Closed Open Open…

In green open problems are emphasized

Clients and Servers

General interaction between a client and a server.

Request-reply behavior What kind of protocol for communication?

1.25

Client-server model is widely used to understand the complexity of distributed systems processes

Clients and Servers

• Connectionless protocol

– Efficient

– Not reliable

• Retransmission on “failure” can be dangerous

• Connection oriented protocol

– Lower performance

– reliable

The three Levelsfor client server model

The general organization of an Internet search engine into three different layers

(we can have this organization on two or more physically distributed machines)

1-28

How can we make a distinction between client and server ?

Multitiered Architectures (1)

Alternative client-server organizations (two-tiered architecture)

1-29

Multitiered Architectures (2)

An example of a server acting as a client( vertical distribution). Three-tiered architecture

1-30

Modern Architectures

An example of horizontal distribution of a Web service.

1-31

Often, clients and servers distribution counts more then different logical components

Request from client