--- title: "Distributed objects and communication" subject: "Distributed System" chapter: 2 key: "semester_7/distributed_system:distributed-objects-communication" source: https://www.bctengineer.com/q/semester_7/distributed_system/distributed-objects-communication site: BCT Engineering --- # Distributed objects and communication Distributed System · Chapter 2: Distributed Objects and File System - Asked in: 2076 Asoj; Model question (asked 2 times) - Video: [3.17 Communication Distributed Object Based Systems](https://www.youtube.com/watch?v=g3daKYO5GSQ) — OU Education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/distributed-objects-communication - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-2 ## Question Define distributed objects and explain communication between distributed objects. ## Short answer Distributed objects encapsulate state and offer methods via a remote interface with unique references. Communication occurs through RMI using client-side proxy and server-side skeleton that marshal/unmarshal parameters, with at-most-once semantics; other forms include RPC, messaging, and publish-subscribe events. ## Answer #### Distributed objects A **distributed object** is an object whose methods can be invoked by processes on other computers. Its state is encapsulated and is accessible only through methods defined in a **remote interface**, and each remote object has a unique **remote object reference** (network address, object id, interface) that can be passed as a parameter or result. Examples are Java RMI remote objects and CORBA objects. **Parts of the model:** *remote interface* (set of remotely callable methods), *remote object reference*, *proxy* at the client, *skeleton* and *dispatcher* at the server, and *remote reference module*. Local calls are made as usual; invocations of remote objects use RMI. #### Communication between distributed objects Distributed objects communicate in these ways. 1. **Remote method invocation (RMI):** a client holding a remote reference calls a method through a *proxy*; the *skeleton/dispatcher* at the server calls the object. Request-reply protocol with call semantics (at-most-once), as in Java RMI and CORBA. 2. **Remote procedure call (RPC):** procedural communication through stubs, used with non-object servers. 3. **Message passing:** direct send/receive messages (sockets, message queues). 4. **Event notification (publish-subscribe):** an object of interest publishes events and subscribers get notification asynchronously (Jini distributed events, CORBA event service). 5. **Group (multicast) communication** for replicated objects. ```text Client obj -> Proxy -> Request ----> Skeleton -> Remote obj <- <- Reply <---- ``` Marshalling converts arguments and results to a common external form (CDR in CORBA, serialisation in Java). Key issues: invocation semantics (maybe, at-least-once, at-most-once), failures (exceptions such as `RemoteException`), parameter passing (by value for serialisable objects, by reference for remote objects) and binding through a naming service/registry. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the state of a distributed object accessed? A. Only through methods in its remote interface, using a remote object reference B. Directly by any process through shared memory C. Through the proxy's private fields D. By reading its disk file Answer: A. State is encapsulated behind the remote interface. (Key point: Distributed object) 2. What does a remote object reference contain? A. Only the object's name B. Network address, object id and interface C. Only the IP address D. The object's source code Answer: B. It uniquely identifies the object and can be passed as parameter or result. (Key point: Remote reference) 3. How does event notification (publish-subscribe) differ from RMI? A. The client blocks waiting for a reply B. It uses stubs and skeletons only C. Subscribers are notified asynchronously when the object of interest publishes an event D. It cannot involve more than one receiver Answer: C. Events are pushed asynchronously rather than invoked by request-reply. (Key point: Event notification) 4. Which call semantics does RMI request-reply typically use, as in Java RMI and CORBA? A. Maybe B. At-least-once C. Exactly-once D. At-most-once Answer: D. A request id and reply history filter duplicates. (Key point: Invocation semantics) ## Related in Distributed System - [Event notification among distributed objects](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aevent-notification-architecture) — Distributed System, chapter 2 - [DFS; RMI communication between objects](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Armi-dfs-comm) — Distributed System, chapter 2 - [Object communication; RPC vs RMI](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aobjects-communicate-rpc-rmi) — Distributed System, chapter 2 - [RPC and its communication semantics](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Arpc-semantics) — Distributed System, chapter 2, asked 3× - [Short note: Reliable group communication](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areliable-group-comm-note) — Distributed System, chapter 9 - [Reliable client-server communication](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areliable-client-server) — Distributed System, chapter 9 ## Related in other subjects - [Lumped vs distributed analysis, applications, protocols](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Alumped-distributed-protocols) — RF and Microwave Engineering, chapter 1