Chapter 16

Chapter 16 Distributed IPC

Objectives

To study how to achieve a distributed implementation of IPC.

Points to emphasise

The subject matter so far in Part 2 is relevant to both centralised and distributed systems. We are now focussing on what is needed, in addition, to achieve a distributed IPC implementation.
Refer to Section 5.5. The absence of global time is not a big issue in this chapter. Nor is inconsistent or incomplete state. The independent failure modes of components of a system is the main concern.
The client-server model can be seen as a simple case of a more general object model. Servers manage objects on behalf of clients and invoke them on request.
Any IPC mechanism can be distributed. We show how to distribute message passing and procedure call.
An RPC system provides session and presentation services and can be integrated with a language in which applications are written. That is, starting from language-level IPC we are, in addition, supporting data representation and transfer and cross-machine synchronisation. The RPC protocol allows for congestion and failure.
ISO have to consider communication among heterogeneous systems. Many systems are homogeneous and can be implemented more efficiently than ISO presentation, for example.

Possible difficulties

The students may lack experience and understanding of communications and the communications services which distributed IPC must use. For the first time we are devising a protocol (RPC) which is assumed to sit above a transport service, such as UDP, and below an application.

When we move to a distributed rather than centralised system the issue of system-wide naming (also location and binding) must be considered. I believe a full treatment of naming etc. belongs in an advanced course which covers large-scale systems.

Teaching hints

It is good to look at independent failure modes first in the context of a simple RPC system (one client, one server). The protocols of Chapter 21 (e.g. two and three phase commit) and the Chapter 22 (distributed mutual exclusion) rely on this basic knowledge.
If the students have already studied communications in another course make use of this.
Some curricula may cover the material of Chapters 15 and 22 in a later course. In this case it would be useful to end Part II with an outline of how IPC can be distributed in order to form a link to that course.