Introduction Chapter 1

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 A DS is a collection of independent computers that appears as a single system. It is expected to have BOB. This is actually a middleware as a layer between the applications and the network. It gives the same interface to all the applications. All applications communicate with its components and with other applications.

Goals For a distributed system to be worthwhile it should have some reasonable useful goals:  Connection of users and resources: Accessibility  Transparency  Openness: syntax and semantics  Scalability: asynchronous comm, distribution, replications, etc.

Accessibility  Almost every component available over the network can be shared, and it is useful to share: computers, devices, data, communication medium.  But, how efficient and secure is all this sharing.  DS must work to provide enough security and efficiency.

Type of transparency Different forms of transparency in a distributed system: Hide the multiplicity, with best of the both TransparencyDescription Access Hide differences in data representation and how a resource is accessed LocationHide where a resource is located Migration Hide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use Replication Hide that a resource may be shared by several competitive users Concurrency Hide that a resource may be shared by several competitive users FailureHide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk

Clarification of Transparency Hide difference in data representation Resources have logical names, independent of whre they are located Resources can be migrated without changing how they are accessed logically Hide the fact that several copies of the same resource exist Concurrent access should leave the resource in a consistent state The user of DS is not aware of a failure and the recovery from the failure

Degree of Transparency High degree of transparency may be impractical: For example, you travel to Tokyo from Istanbul, you have time difference that may make electronic morning paper irrelevant. Certain things are impossible to high: signal transfer time, delay caused by high traffic on the shared servers. Replication is costly in time, because of consistency requirement in all copies.

Openness An open system offers services according to the standard rules that allow interoperability and portability. It should be possible to built systems out of different components. It should be easy to add new components replace the existing ones. So, the system should be extensible

Scalability With respect to size: more or less users or resources Geographical: users and resources may be far apart Administratively: DS may span over more than one administrative bodies

Scalability Problems Examples of scalability limitations. ConceptExample Centralized servicesA single server for all users Centralized dataA single on-line telephone book Centralized algorithmsDoing routing based on complete information

Decentralized Algorithms for Scalability No machine has complete inf about the system Decision are made on locally available inf Failure of one machine does not invalidate the system There is no global clock Geographic centralization hinders geographic scalability

Scaling techniques Asynchronous communication Moving the processing from server to the client, if it is more efficient. In this case instead of (a) it is more efficient as in (b) Spreading the work across the system, as in DNS

Scaling Techniques (1) 1.4 The difference between letting: a)a server or b)a client check forms as they are being filled

Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.

Scaling techniques Replication Caching, as a special form of replication: –consistency is an important issue in caching Combination of distribution, replication and caching with appropriate consistencyoften provide sufficient scalability.

False assumptions on developing DS Network is reliable Network is secure Network is homogeneous Topology does not change Latency is zero Bandwidth is infinite Transport cost is zero There is one administrator

Type of Distributed Systems Network is reliable Network is secure Network is homogeneous Topology does not change Latency is zero Bandwidth is infinite Transport cost is zero There is one administrator

Hardware Concepts

1.6 Different basic organizations and memories in distributed computer systems

Multiprocessors (1) A bus-based multiprocessor. 1.7

Multiprocessors (2) a)A crossbar switch b)An omega switching network 1.8

Homogeneous Multicomputer Systems a)Grid b)Hypercube 1-9

Software Concepts SystemDescriptionMain Goal DOS (Distribut ed Operating Systems) Tightly-coupled operating system for multi-processors and homogeneous multicomputers Hide and manage hardware resources NOS (Network Operating Systems) 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

Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

Multiprocessor Operating Systems (1):MONITOR A simple monitor, allowing one access at time: one processes finishes what it is doing, before the other can start Monitor protects integer count against concurrent access. A monitor is implemented using semaphores monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count – 1;} }

Multiprocessor Operating Systems (2):MONITOR A evolved monitor to protect an integer against concurrent access: count is incremented or decremented concurrently and correctly. Monitor is implemented using semaphores, a process is blocked using condition variables in the Monitor 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; }

Multicomputer Operating Systems (1) General structure of a multicomputer operating system 1.14

Multicomputer Operating Systems (2) Alternatives for blocking and buffering in message passing. 1.15

Multicomputer Operating Systems (3) Relation between blocking, buffering, and reliable communications. Synchronization pointSend buffer Reliable comm. guaranteed? Block sender until buffer not fullYesNot necessary Block sender until message sentNoNot necessary Block sender until message receivedNoNecessary Block sender until message deliveredNoNecessary

Distributed Shared Memory Systems (1) remote RAM is used as backup storage a)Pages of address space distributed among four machines b)Situation after CPU 1 references page 10 c)Situation if page 10 is read only and replication is used

Distributed Shared Memory Systems (2) False sharing of a page between two independent processes. 1.18

Network Operating System (1) General structure of a network operating system. Services: rlogin, rcp, file servers, 1-19

Network Operating System (2) Two clients and a server in a network operating system. 1-20

Network Operating System (3) Different clients may mount the servers in different places. 1.21

Middleware Models –Distributed file systems –Remote Procedure Calls –Distributed objects –Distributed Documents Services –Communication –Naming –Persistence –Distributed Transactions –Security

Positioning Middleware General structure of a distributed system as middleware:communication, naming, data persistence, distributed tx, security. 1-22

Middleware and Openness: for interoperability 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

Comparison between Systems A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems. Item Distributed OS Network OS Middleware- based OS Multiproc.Multicomp. Degree of transparencyVery HighHighLowHigh Same OS on all nodesYes No Number of copies of OS1NNN Basis for communication Shared memory MessagesFilesModel specific Resource management Global, central Global, distributed Per node ScalabilityNoModeratelyYesVaries OpennessClosed Open

Cluster Computing Systems Cluster Computing System is a kind of DS Cluster computing is based on a collection of similar workstations and the same operating systems connected by a high speed LAN It is used for parallel computing A true cluster is expected to gibe a transparent process migration

Grid Computing Systems Grid computing has heterogenity, regarding computers, operating systems, networks, administrative domains, security policies The heterogeneous resources are provided to a virtual organization that is formed by different users. The resources are individual computers with possibly different OS, super computers, cluster computers, storage facilities, databases, etc.

Clients and Server Models Client Server model has been agreed on as one of the universal DS model, by many researchers

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 b

Processing Level The general organization of an Internet search engine into three different layers 1-28

Client-Server Architectures Multi-tiered Architectures: Vertical: put different components on differen machines Horizontal: clients and servers may be physically split into different parts and distributed to run on own data. Peer-to-peer: No client server role is taken, the applications cooperate to conduct certain task…

Multitiered Architectures (1) Two-tiered architecture: Example1 for vertical distribution: Alternative client-server organizations (a) – (e). 1-29

Multitiered Architectures (2) Three-tiered Architecture: vertical distribution example2 An example of a server acting as a client. 1-30

Modern Architectures An example of horizontal distribution of a Web service. 1-31