Elixir runs on the BEAM, the Erlang virtual machine, which allows Elixir to seamlessly interoperate with Erlang code and packages. This enables developers to leverage the extensive ecosystem available in Erlang while writing in Elixir’s modern and expressive syntax.

Early in Erlang’s development, the Open Telecom Platform (OTP) was created, codifying Erlang’s principles into a framework. Most Elixir and Erlang applications and libraries treat OTP as a primitive, inheriting and implementing its behaviours. One of the most common behaviours is the gen_server, a standardized way to implement a server process.

Key Features of GenServer

A gen_server is a process, which is a lightweight, isolated thread that communicates with other processes solely through message passing.

  • State Management: gen_server provides a structured way to manage a process’s state.
  • Synchronous Messages: gen_server uses call/2 for synchronous message passing, making it suitable for logic that needs to wait for a result.
  • Asynchronous Messages: gen_server uses cast/2 for asynchronous message passing, where calls do not wait for a response, making them fire-and-forget.
  • Handling Info Messages: The handle_info/2 callback is used to handle arbitrary messages sent to the gen_server process that are not call/2 or cast/2 requests. This includes messages from other processes, timeout events, or any custom messages.
  • Fault Tolerance: gen_server processes can be monitored and restarted by supervision trees.

Building a GenServer

First, it’s important to understand how Erlang/Elixir builds complexity.

Functional Programming builds complexity by composing simple functions into more complex ones. These languages often feature strong type systems to ensure that functions interact correctly and safely, facilitating secure and effective functional composition.

Object-Oriented Programming manages complexity through class inheritance and interfaces, creating a hierarchy that extends and reuses behaviors. This paradigm uses an interface type system and encapsulates both state and functionalities within objects, promoting modular and scalable code design.

Erlang, although a functional language, uniquely handles complexity by extending behaviors from simpler, foundational modules. It simplifies complex interactions by abstracting them into manageable, extendable modules.

When extending a GenServer, you are effectively saying, “I work like a GenServer,” which means you need to define the behaviors outlined in a GenServer.

  • Required:
    • init/1: Initializes the state.
    • handle_call/3: Manages synchronous calls (if needed).
    • handle_cast/2: Manages asynchronous messages (if needed).
  • Optional:
    • handle_info/2: Handles other messages.
    • terminate/2: Manages cleanup on termination.

You need to implement the essential behaviors, init/1 and at least one of handle_call/3 or handle_cast/2.