Distributed Systems Course. a.o. Univ.-Prof. Dr. Harald Kosch

Transcription

1 Distributed Systems Course a.o. Univ.-Prof. Dr. Harald Kosch

2 Topics Introduction Advantages, Disadvantages, Hard/Soft Concepts Network / Distributed Operating Systems Basics of Communication Models (Client/Server, Groups) Easter Java-RMI Middleware (CORBA, DCOM,.NET, Web Services, Globe) The Concept of Causality, Synchronization, Clock synchronization, mutual exclusion, transactions Consistency and Replication (Distributed File Systems and Distributed Shared Memory) Mobile Agents

3 2V : Useful and Necessary H. Kosch Wend HS C, start on March 9, AG: Ch. Timmerer, Ch. Spielvogel, M. Taschwer Wend und Precondition: Operating Systems + Computer Networks

4 Literature Andrew Tanenbaum: Distributed Systems, Prentice Hall, 2002 (auch in Deutsch: Pearson-Studium 2004) George Coulouris, Jean Dollimore, Tim Kindberg Verteilte Systeme Konzepte und Design, 3. überarbeitete Auflage, Addison-Wesley, 2002 (auch in Deutsch: Pearson-Studium 2004) Randy Chow and Theodore Johnson Distributed Operating Systems & Algorithms Addison- Wesley, 1997

5 Links Slides, Links, News to be viewed regularely Examen: June 24, 2005: HS A Theoratical with Practical Examples (Distributed Algorithms (not in AG!) and Programming)

6 Credits The Slides of Chapter 1 and 2 are based on the book: Andrew Tanenbaum and Maarten v. Steen Distributed Systems Prentice Hall, 2002 Slides are provided by Prof. Tanenbaum at: nenbaum/custom/dist_sys_1e/ you equally find there an example chapter, ToC and other links

7 Introduction Chapter 1

8 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.

9 Definition of a Distributed System (2) 1.1 A distributed system organized as middleware. Note that the middleware layer extends over multiple machines.

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

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

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

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

14 Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

15 Multiprocessors (1) coherent 1.7 Not coherent A bus-based multiprocessor.

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

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

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

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

20 Multiprocessor Operating Systems (1) monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count 1;} } A monitor to protect an integer against concurrent access.

21 Multiprocessor Operating Systems (2) 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; } } A monitor to protect an integer against concurrent access, but blocking a process.

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

23 Multicomputer Operating Systems (2): Communication: send/receive Blocking (synchronous) send The sender is blocked until the message is sent When is the message sent? Buffer is copied to the kernel Message leaves the own computer Message reaches the remote computer Message reaches the peer process (the receiver) Non-blocking (asynchronous) send The send procedure returns immediately and the sender process may proceed It must be prohibited that the sender process changes the buffer: Either by copying to the kernel (inefficient) or by interrupting the sender when it may reuse the buffer (uncomfortable to program)

24 Multicomputer Operating Systems (3) Blocking (synchronous) receive The receiving process is blocked until a message arrives (from a certain sender or from any sender) Non-blocking (asynchronous) receive How can the receiver get the message? Explicit wait statement Polling (maybe additionally to polling) Conditional receive (returns immediately if there is a message, waits otherwise for a given period of time) Blocking primitives are simpler, can be combined with threads.

25 The Message-Passing Interface (MPI) Primitive MPI_bsend MPI_send MPI_ssend MPI_sendrecv MPI_isend MPI_issend MPI_recv MPI_irecv Meaning Append outgoing message to a local send buffer Send a message and wait until copied to local or remote buffer Send a message and wait until receipt starts Send a message and wait for reply Pass reference to outgoing message, and continue Pass reference to outgoing message, and wait until receipt starts Receive a message; block if there are none Check if there is an incoming message, but do not block Some of the most intuitive message-passing primitives of MPI.

26 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 Inconsistency if replication used -> Strong/Weak Consistency Models

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

28 Network Operating System (1) 1-19 General structure of a network operating system -> Distributed File Server.

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

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

31 Positioning Middleware 1-22 General structure of a distributed system as middleware.

32 Middleware and Openness 1.23 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.

33 Comparison between Systems Item Distributed OS Multiproc. Multicomp. Network OS Middlewarebased OS 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 Files Model specific Resource management Global, central Global, distributed Per node Per node Scalability No Moderately Yes Varies Openness Closed Closed Open Open A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.

34 Clients and Servers 1.25 General interaction between a client and a server.

35 An Example Client and Server (1) The header.h file used by the client and server.

36 An Example Client and Server (2) A sample server.

37 An Example Client and Server (3) 1-27 b A client using the server to copy a file.

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

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

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

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