The word “clustering” can mean different things to different people. Some people would say clustering is simply having a replicated system on standby available to be turned on when the primary system fails. To others, clustering is having several systems working in concert with one another, with replicated data, fully redundant, and infinitely expandable. For most people, it’s probably somewhere between those two extremes.

In this chapter, we’re going to explore the possibilities for clustering that exist with Asterisk at a high level, giving you the knowledge and direction to start planning your system into the future. As examples, we’ll discuss some of the tools that we’ve used in our own large deployments; while there is no single way to go about building an Asterisk cluster, the topologies we’ll cover have proven reliable over the years.

Our examples will delve into building a distributed call center, one of the more popular reasons for building a distributed system. In some cases this is necessary simply because a company has satellite offices it wants to tie into the primary system. For others, the goal is to integrate remote employees, or to be able to handle a large number of seats. We’ll start by looking at a simple, traditional PBX system, and see how that system can eventually grow into something much larger.