Code Reading: The Open Source Perspective by Diomidis Spinellis

Author:Diomidis Spinellis
Language: eng
Format: epub, pdf
Publisher: Addison-Wesley
Published: 2010-07-14T16:00:00+00:00

9.1.1 Centralized Repository and Distributed Approaches

A centralized repository architectural model relies on a central process or data structure acting as a control or information hub for the system. In larger-scale systems the coordination role can be distributed across several hub processes. You will find centralized repository architectures employed when several semiautonomous processes need to cooperate to access shared information or resources. Examples in this category include window managers,1,2 file 3 and print4 servers, and network resource management applications.5 Centralized repository architectures are also the basis for many collaborative applications such as the World Wide Web, on-line messaging, player games, and revision control systems like CVS and RCS.

As a typical small-scale example, consider the description of the method used for connecting two independent Unix talk processes in a one-to-one on-line chat session.6

The talk server acts [as] a repository of invitations, responding to requests by clients wishing to rendezvous for the purpose of holding a conversation. In normal operation, a client, the caller, initiates a rendezvous by sending a CTL_MSG to the server of type LOOK_UP. This causes the server to search its invitation tables to check if an invitation currently exists for the caller (to speak to the callee specified in the message). If the lookup fails, the caller then sends an ANNOUNCE message causing the server to broadcast an announcement on the callee's login ports requesting contact. When the callee responds, the local server uses the recorded invitation to respond with the appropriate rendezvous address and the caller and callee client programs establish a stream connection through which the conversation takes place.

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