From Coulouris, Dollimore, Kindberg and Blair Distributed Systems: Concepts and Design Edition 5, © Addison-Wesley 2012 System Models by Dr. Sarmad Sadik

System models  Introduction  Physical models  Architectural models  Fundamental models  Software layers 2

3 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Generations of distributed systems

Architectural elements  Objects  Components  Web services 4

Communication paradigms  Inter-process communication  Remote invocation  Indirect communication 5

6 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Communicating entities and communication paradigms

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Clients invoke individual servers

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Peer-to-peer architecture

Variations  Services provided by multiple servers  Proxy servers and caches  Mobile code  Mobile Agents  Network computers  Thin clients  Mobile devices and spontaneous interoperation 9

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 A service provided by multiple servers

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Web proxy server

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Web applets

Architectural Patterns  Layering  Tiered architecture 13

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Software and hardware service layers in distributed systems

Tiered architecture  Two tier and three tier  Presentation logic  Application logic  Data logic 15

16 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Two-tier and three-tier architectures

17 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 AJAX example: soccer score updates new Ajax.Request('scores.php? game=Arsenal:Liverpool’, {onSuccess: updateScore}); function updateScore(request) {..... ( request contains the state of the Ajax request including the returned result. The result is parsed to obtain some text giving the score, which is used to update the relevant portion of the current page.)..... } 17

Architectural Pattern  Thin clients  Virtual Network Computing (VNC)  Proxy pattern  Brokerage  Reflection  Middleware 18

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Thin clients and compute servers Thin Client Application Process Network computer or PC Compute server network

20 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 The web service architectural pattern

21 Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Categories of middleware

Fundamental models  Interaction  Failure  Security 22

Interaction model  Distributed algorithm  Communication performance is often a limiting characteristic  It is impossible to maintain a single global notion of time  Performance of communication channels 23

Synchronous distributed systems  The time to execute each step of a process has known lower and upper bounds.  Each message transmitted over a channel is received within a known bounded time.  Each process has a local clock whose drift rate from real time has a known bound.  Asynchronous distributed systems  Event ordering 24

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Real-time ordering of events

Failure model  Omission failures  Process omission failure  Communication omission failure  Arbitrary failure  Timing failure  Masking failure  Reliable communication  Validity  Integrity 26

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Processes and channels

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Omission and arbitrary failures Class of failure AffectsDescription Fail-stop ProcessProcess halts and remains halted. Other processes may detect this state. CrashProcessProcess halts and remains halted. Other processes may not be able to detect this state. OmissionChannelA message inserted in an outgoing message buffer Never arrives at the other end’s incoming message buffer. Send-omissionProcessA process completes asend, but the message is not put in its outgoing message buffer. Receive-omissionProcessA message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process or channel Process/channel exhibits arbitrary behaviour: it may send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step.

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Timing failures Class of FailureAffectsDescription ClockProcessProcess’s local clock exceeds the bounds on its rate of drift from real time. PerformanceProcessProcess exceeds the bounds on the interval between two steps. PerformanceChannelA message’s transmission takes longer than the stated bound.

Security model  Protecting objects  Securing process and interactions  Threats to processes  Threats to communication channels  Defeating security threats  Cryptography and shared secrets  Authentication  Secure channels 30

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Objects and principals

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 The enemy Communication channel Copy of m Process p q m The enemy m’

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Secure channels Principal A Secure channel Process p q Principal B

Thank you! 34