Elixir: function Foo.bar/1 is undefined (module Foo is not available)

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Ask the runtime whether the module is there at all
iex -S mix
iex> Code.ensure_loaded?(MyApp.Worker)
iex> MyApp.Worker.__info__(:functions)

# A stale build directory outlives a renamed module
mix clean && mix compile --force

# Dependency present but not started: its modules load, its supervisor does not
mix deps.get
Application.ensure_all_started(:telemetry)

# In a release, a module only used through apply/3 can be stripped out
# Add it to the release config rather than calling it dynamically
# mix.exs: releases: [app: [applications: [runtime_tools: :permanent]]]

# Erlang modules use a different form and a different failure
:crypto.strong_rand_bytes(16)

How to diagnose Elixir errors

Elixir errors are shaped by the actor model: a GenServer.call timeout does not mean the server crashed, it means the server was busy for longer than the caller was willing to wait. The right question is usually "what is that process doing?" rather than "why did the call fail?". Because supervisors restart failed processes automatically, transient errors can also hide in the logs while the system appears healthy.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Attach to a running node with iex --remsh and inspect the process: Process.info(pid, :message_queue_len). A growing mailbox means the process is the bottleneck.
2. Use :observer.start() to see the supervision tree, process memory and message queues visually.
3. Move long-running work out of handle_call. Use handle_cast, a Task, or a dedicated pool so the GenServer stays responsive.
4. Check restart intensity. A supervisor that exceeds max_restarts takes down its own supervisor, producing a cascade that looks like an unrelated failure at the top.
5. For compile errors in dependencies, run mix deps.compile --force and check that any required native toolchain (make, gcc, erlang headers) is installed.

Tools worth reaching for

• :observer.start()
• iex --remsh
• Process.info/2
• mix deps.tree
• :recon

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 hexdocs.pm 📖 hexdocs.pm

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.