Chap 01

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. Hide whether a (software) resource is in memory or on disk Persistence Hide the failure and recovery of a resource Failure Hide that a resource may be shared by several competitive users Concurrency Hide that a resource may be shared by several competitive users Replication Hide that a resource may be moved to another location while in use Relocation Hide that a resource may move to another location Migration Hide where a resource is located Location Hide differences in data representation and how a resource is accessed Access Description Transparency

Scalability Problems Examples of scalability limitations. Doing routing based on complete information Centralized algorithms A single on-line telephone book Centralized data A single server for all users Centralized services Example Concept

Scaling Techniques (1) 1.4 The difference between letting: a server or 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 crossbar switch An omega switching network 1.8

Homogeneous Multicomputer Systems Grid Hypercube 1-9

Software Concepts An overview of DOS (Distributed Operating Systems) NOS (Network Operating Systems) Middleware Provide distribution transparency Additional layer atop of NOS implementing general-purpose services Middleware Offer local services to remote clients Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) NOS Hide and manage hardware resources Tightly-coupled operating system for multi-processors and homogeneous multicomputers DOS Main Goal Description System

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. Necessary No Block sender until message delivered Necessary No Block sender until message received Not necessary No Block sender until message sent Not necessary Yes Block sender until buffer not full Reliable comm. guaranteed? Send buffer Synchronization point

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

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

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

Chap 01

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Chap 01