GenServer

GenServer is a generic server behaviour and module in the Erlang/OTP framework, also exposed in the Elixir standard library. It provides a structured way to implement servers that maintain internal state and communicate with other processes through message passing. A GenServer runs in its own lightweight process and is typically supervised, making it suitable for building reliable, fault-tolerant applications.

The core idea is to implement a defined set of callbacks that describe how the server initializes, handles requests, and shuts down. The most important callbacks are init/1 to initialize the server state, handle_call/3 for synchronous requests that expect a reply, handle_cast/2 for asynchronous requests, and handle_info/2 for miscellaneous messages such as timer events. Additional callbacks, such as terminate/2 and code_change/3, support cleanup and hot code upgrades in OTP releases.

GenServer exposes a clean API for interacting with the server. Common operations include start_link/3 to start and link the server process, start/3 to start without linking, call/3 to perform a synchronous request, and cast/2 to send an asynchronous request. The server communicates results back to callers as part of the callback return values, and the server’s internal state is threaded through callback invocations.

In practice, GenServers are often placed under supervision in OTP supervision trees to provide fault tolerance and automatic restart behavior. They enable encapsulation of mutable state, sequential handling of requests, and scalable concurrency. Typical use cases include maintaining caches, protocol handlers, resource pools, and other long-running services that require controlled state management and robust error handling. GenServer remains a foundational abstraction for building robust, concurrent applications in both Erlang and Elixir.