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CS551 UML for Distributed Objects (Chapter 2 of Engineering Distributed Objects)

CS551 UML for Distributed Objects (Chapter 2 of Engineering Distributed Objects). Yugi Lee STB #555 (816) 235-5932 yugi@cstp.umkc.edu www.cstp.umkc.edu/~yugi. Contents. Local versus Distributed Objects Method

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CS551 UML for Distributed Objects (Chapter 2 of Engineering Distributed Objects)

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  1. CS551 UML for Distributed Objects(Chapter 2 of Engineering Distributed Objects) Yugi Lee STB #555 (816) 235-5932 yugi@cstp.umkc.edu www.cstp.umkc.edu/~yugi CS551 - Lecture 5

  2. Contents Local versus Distributed Objects Method • Many will have experience with designing local objects that reside in the run-time environment of an OO programming lang. • Designing distributed objects is different! • Explain the differences. • Avoid some serious pitfalls CS551 - Lecture 5

  3. Local vs. Distributed Objects • References • Activation/Deactivation • Migration • Persistence • Latency of Requests • Concurrency • Communication • Security • Several Pitfalls are lurking here CS551 - Lecture 5

  4. Object Lifecycle • OOPL objects reside in one virtual machine. • Distributed objects might be created on a different machine. • Distributed objects might be copied or moved (migrated) from one machine to another. • Deletion by garbage collection does not work in a distributed setting. • Lifecycle needs attention during the design of distributed objects. CS551 - Lecture 5

  5. Object References • References to objects in OOPL are usually pointers to memory addresses • sometimes pointers can be turned into references (C++) • sometimes they cannot (Smalltalk,Java) • References to distributed objects are more complex • Location information • Security information • References to object types • References to distributed objects are bigger (e.g 40 bytes with Orbix). CS551 - Lecture 5

  6. Latency of Requests • Performing a local method call requires a couple of hundred nanoseconds. • An object request requires between 0.1 and 10 milliseconds. • Interfaces of distributed objects need to be designed in a way that • operations perform coarse-grained tasks • do not have to be requested frequently CS551 - Lecture 5

  7. Vector List +size():int +elementAt(i:int):Object ... +long list (in how_many:long, out l:sequence<object>, out bi:Iterator i) Iterator +next_one(out o:Object): boolean +next_n(in how_many:long, out l:sequence<object>):boolean Example: Iteration over a Sequence • Java • Distributed Objects CS551 - Lecture 5

  8. Activation/Deactivation • Objects in OOPL are in virtual memory between creation and destruction. • This might be inappropriate for distributed objects • sheer number of objects • objects might not be used for a long time • some hosts might have to be shut down without stopping all applications • Distributed object implementations are • brought into main memory (activation) • discarded from main memory (deactivation) CS551 - Lecture 5

  9. Activation/Deactivation (cont’d) Tony:Trainer BvB:Team bookGoalies object activated object deactivation CS551 - Lecture 5

  10. Activation/Deactivation (cont’d) • Several questions arise • Repository for implementations • Association between objects and processes • Explicit vs. implicit activation • When to deactivate objects • How to treat concurrent requests • Who decides answers to these questions? • Designer • Programmer • Administrator • How to document decisions? CS551 - Lecture 5

  11. Persistence • Stateless vs. statefull objects • Statefull objects have to save their state between • object deactivation and • object activation onto persistent storage • Can be achieved by • externalization into file system • mapping to relational database • object database • To be considered during object design CS551 - Lecture 5

  12. Parallelism • Execution of OOPL objects is often • sequential • concurrent (with multi-threading) • Distributed objects execute in parallel • Can be used to accelerate computations CS551 - Lecture 5

  13. Communication • Method invocations of OOPL objects are synchronous • Alternatives for distributed objects: • synchronous requests • oneway requests • deferred synchronous requests • asynchronous requests • Who decides on request • Designer of server? • Designer of client? • How documented? CS551 - Lecture 5

  14. Failures • Distributed object requests are more likely to fail than local method calls • Different request reliabilities are available for distributed objects • Clients have an obligation to validate that servers have executed request CS551 - Lecture 5

  15. Security • Security in OO applications can be dealt with at session level. • OOPL Objects do not have to be written in a particular way. • For distributed objects: • Who is requesting an operation execution? • How can we know that subject is who it claims to be? • How do we decide whether or not to grant that subject the right to execute the service? • How can we prove that we have delivered a service so as to make the requester pay CS551 - Lecture 5

  16. Key Points • Distributed objects evolved from research and development in object-oriented programming languages and distribution middleware • The Unified Modeling Language can be used to design distributed objects • Meta object models determine the characteristics of distributed objects • Designers need to be aware of differences between local and distributed objects CS551 - Lecture 5

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