Elixir Tutorial: How Unbounded Mailboxes Cause OOM Crashes and How to Fix Them

Part 9 of a DEV Community series on building distributed systems in Elixir tackles backpressure, a critical flow-control concept in concurrent programming. The article explains how Elixir's non-blocking send/2 primitive allows fast producers to overwhelm slow consumers, causing unbounded mailbox growth and eventual out-of-memory crashes. Using only raw BEAM process primitives — spawn/1, send/2, receive/1, and Process.info/2 — the tutorial demonstrates the failure mode in real time without relying on higher-level libraries like GenStage or Broadway. Two fundamental solutions are then implemented from scratch: a Stop-and-Wait protocol and a credit-based Demand-Driven Windowing mechanism, the latter being the same core design that powers GenStage. The guide targets Elixir developers seeking a ground-up understanding of how backpressure prevents cascading failures in distributed systems.