Études for Erlang

Chapter 11. Getting Started with OTP

In order to help me understand how the gen_server behavior works, I drew the diagram shown in Figure 11-1.

Figure 11-1. Processing a call in gen_server

The client does a gen_server:call(Server, Request). The server will then call the handle_call/3 function that you have provided in the Module that you told gen_server to use. gen_server will send your module the client’s Request, an identifier telling who the request is From, and the server’s current State.

Your handle_call/3 function will fulfill the client’s Request and send a {reply, Reply, NewState} tuple back to the server. It, in turn, will send the Reply back to the client, and use the NewState to update its state.

In Introducing Erlang and in the next two études, the client is you, using the shell. The module that handles the client’s call is contained within the same module as the gen_server framework, but, as the preceding diagram shows, it does not have to be.

Note

You can learn more about working with OTP basics in Chapters 11 and 12 of Erlang Programming, Chapters 16 and 18 of Programming Erlang, Chapter 4 of Erlang and OTP in Action, and Chapters 14 through 20 of Learn You Some Erlang For Great Good!.