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Building Distributed Systems in Elixir: Part 1 - Building a Stateful Process in Elixir Without GenServer

The article explains a minimal stateful server in Elixir by turning a counter into a process that owns its private state. Clients do not read that state directly; they send requests with send/2, wait for replies with receive, and rely on the process to update and return values.

Reading notes
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  • A stateful server is described as a process with private state that receives messages, updates state, sends replies, and waits for the next message.
  • The example uses only spawn/1, send/2, receive, and recursion, without GenServer, Agent, or other OTP behaviours.
  • The counter is modeled as a process because BEAM processes do not share memory.
  • The protocol uses {:get, caller_pid}, {:increment, caller_pid}, and :stop, and replies with {:counter_value, value}.
  • start/1 calls spawn/1 and begins the recursive loop/1 with the initial value as private state.
  • get/1 sends a request and then waits for a new reply message; it does not read the message it sent.
  • increment/1 follows the same request-reply pattern, but the counter process performs the state update.
  • The receive loop matches incoming messages, handles each operation, and calls loop/1 again with the next state.
  • Because the process handles one mailbox message at a time, requests are serialized and clients do not modify the state concurrently.
  • GenServer is presented as a higher-level abstraction that adds request handling, correlation, timeouts, monitoring, supervision, debugging support, system messages, and consistent APIs.

Series
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Building Distributed Systems in Elixir, by krishnadaspc. Research: Distributed Systems Patterns.