When you move from a single BEAM node to a fleet of nodes, the way processes talk to each other has to change. This article shows, step by step, how to turn a simple local application into a fault‑tolerant cluster without sacrificing the familiar GenServer abstraction.

Why Coordination Matters Across Nodes

In a single node you can safely hand a large value to a central process that serialises access to a shared resource. The call travels inside the node, so the cost is just the copy made in the VM’s memory. Across a network, however, that same copy

When you move from a handful of modules to a full‑blown system, you soon discover the need for a framework that can:

• Group related modules together.
• Describe what other components your code needs.
• Start everything with a single call.
• Provide a clean way to ship a self‑contained release.

That framework is the OTP application. In the Erlang/Elixir world an OTP application is a small, self‑describing package that the runtime can start, stop, and supervise. This article explains the anatomy of an OTP application, shows how mix helps you generate it, and demonstrates how to bring in