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

Transparency in a Distributed System Different forms of transparency in a distributed system. TransparencyDescription Access Hide differences in data representation and how a resource is accessed LocationHide where a resource is located MigrationHide 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

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

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.

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

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

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;} }

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; }

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) 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. 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

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. 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

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

Multitiered Architectures (1) Alternative client-server organizations (a) – (e). 1-29

Multitiered Architectures (2) An example of a server acting as a client. 1-30

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