Designing for Scalability with Erlang/OTP

Chapter 4. Generic Servers

Having broken up the radio frequency allocator into generic and specific modules and investigated some of the corner cases that can occur when dealing with concurrency, you will have figured out there is no need to go through this process every time you have to implement a client-server behavior. In this chapter, we introduce the gen_server OTP behavior, a library module that contains all of the generic client-server functionality while handling a large number of corner cases. Generic servers are the most commonly used behavior pattern, setting the foundations for other behaviors, all of which can be (and in the early days of OTP were) implemented using this module.

Generic Servers

The gen_server module implements the client-server behavior we extracted in the previous chapter. It is part of the standard library application and available as part of the Erlang/OTP distribution. It contains the generic code that interfaces with the callback module through a set of callback functions. The callback module, in our example containing the code specific to the frequency server, is implemented by the programmer. The callback module has to export a series of functions that follow naming and typing conventions, so that their inputs and return values conform to the protocol required by the behavior.

As seen in Figure 4-1, the functions of both the behavior and callback module execute within the scope the same server process. In other words, a process loops in the generic ...