Building Distributed Systems in Elixir: Part 6 — Named Processes

In the previous part of this series, we built a tiny supervisor from scratch. When a worker crashed, the supervisor started a replacement. That replacement had a new PID: old worker -> #PID<0.102.0> new worker -> #PID<0.105.0> This reveals an important limitation of sharing PIDs as a public interface. A PID identifies one running incarnation of a process. It is excellent for sending a reply, setting up a monitor, or creating a link. It is not a stable address for a service that may stop and later be replaced. In this part, we'll use named processes to give a worker a discoverable address: :worker We'll build three small examples using: Process . register / 2 Process . whereis / 1 :global . register_name / 2 :global . whereis_name / 1 send / 2 No GenServer . No OTP Registry . The goal is to understand the lookup problem that registries solve before reaching for those abstractions. The PID-Sharing Problem Suppose one process starts a worker and gives its PID to a client: worker = spawn ( fn -> worker_loop () end ) send ( client , { :worker_started , worker }) The client can now send work directly: send ( worker , { :work , self (), "hello" }) This works while that particular worker process is alive. But process IDs are temporary. If the worker exits, the PID is no longer a route to the service: Client Worker holds #PID<0.102.0> #PID<0.102.0> | | | X exits | | send(#PID<0.102.0>, work) |------------------------------> no worker receives it Sending to a dead local PID does not raise an error and does not restart a process. The message is simply not delivered to a living worker. One answer is to tell every client about every new PID after a restart. That spreads lifecycle knowledge throughout the system. Another answer is to make clients depend on a name and resolve that name when sending. Registering a Local Name Our first worker waits for a stop message: defmodule Worker do def start do spawn ( fn -> receive do :stop -> :ok end end ) end end Starting it gives us a PID: