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To debug a crashing Elixir GenServer, start with its termination reason and stack trace, identify the message and callback being processed, then check that callback’s return value and any linked-process or supervisor events. A GenServer.call/3 timeout is not proof that the server crashed: it means the caller did not receive a reply before its wait limit.
1. Capture the termination evidence
Before changing code, collect the error log and the details that let you connect the failure to a particular event:
- The exception or exit reason and full stack trace.
- The GenServer’s PID or registered name and the time of the failure.
- The request, message, or state involved immediately before termination.
- Any supervisor log showing whether the child stopped or restarted.
Distinguish a server exit from a caller exit. With GenServer.call/3, the timeout is the caller’s maximum wait for a reply. If the reply does not arrive in time, the caller exits; a late reply can still arrive in the caller’s mailbox. A timeout alone does not establish that the server terminated. See the GenServer API reference.
2. Map the triggering event to its callback
Use the actual message shape and the API that sent it to find the callback that should handle it. The Elixir client-server guide describes the distinction:
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| Event | Callback to inspect | What to check |
|---|---|---|
Synchronous request via GenServer.call/3 |
handle_call/3 |
Request pattern, reply, next state, and any stop condition. |
Asynchronous request via GenServer.cast/2 |
handle_cast/2 |
Request pattern, next state, and whether the work can safely run without a reply. |
Other message, including ordinary send/2 messages or monitor :DOWN notifications |
handle_info/2 |
Whether the message has an intentional matching clause and how it affects state. |
A missing clause, a pattern that assumes a narrower input shape than callers actually send, or an unhandled message can lead to failure. Inspect the relevant caller and the exact incoming term rather than reasoning from the intended request format.
3. Check callback return values and exceptions
For each branch in the callback implicated by the trace, verify the return tuple’s form and arity against that specific callback’s contract. An exception, explicit exit, {:stop, ...} return, or invalid return value can terminate the process. init/1 has a separate startup contract, so an invalid initialization result or startup failure is different from a server that crashes later while handling a message. The GenServer API reference documents termination and callback return forms.
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For an expected, recoverable bad request, consider validating it and returning a useful error response while keeping the server alive. But if the failure means an internal invariant is broken, stopping may be safer than continuing with corrupted state. Avoid broad rescue clauses that hide defects or leave the server’s state inconsistent; rescue only errors the application can meaningfully recover from.
4. Inspect state and events when the process is available
If the server is still alive or the problem recurs intermittently, Elixir’s :sys facilities can help narrow down what happened:
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:sys.get_state(pid)retrieves the callback state.:sys.get_status(pid)retrieves process status details.:sys.trace(pid, true)enables tracing of system events such as received messages, replies, and state changes; use:sys.trace(pid, false)to turn it off.
Tracing is useful for a focused reproduction, but it can generate substantial output. State and messages may contain secrets or large values, so treat diagnostic output accordingly. The debugging section of the GenServer API reference describes these tools.
5. Check linked exits and supervisor behavior
A GenServer started with start_link/3 is linked to its parent. The observed termination may therefore follow an exit from a linked process or parent, rather than an exception in the server’s message handler. Also check whether a supervisor initiated shutdown. The GenServer API reference notes that terminate/2 is not guaranteed to run for every exit, so it is not a reliable place for cleanup that must always happen.
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Then inspect the child specification and supervisor strategy. The Supervisor API reference explains restart behavior and strategies such as :one_for_one and :one_for_all:
- Choose a strategy based on whether sibling processes depend on one another. Restarting only a failed child is different from restarting a group of related children.
- Check whether the child is configured to restart permanently, only after abnormal exits, or never. The exit reason matters; do not assume every stopped child should restart.
- Review shutdown settings. A shutdown timeout or
:brutal_killcan affect whetherterminate/2has an opportunity to run. - If restarts repeat, correlate the original exit reason with supervisor logs and restart intensity. A supervisor can itself stop restarting children after reaching its configured restart limit.
6. Choose the remedy that matches the failure
Expected input that the server can reject safely
Validate the incoming term and return a clear error reply from handle_call/3 when a synchronous caller needs to know the result. Add a deliberate fallback clause where appropriate. For asynchronous work, decide how failure will be surfaced, since a cast does not provide a reply.
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Operation that needs a reply or back-pressure
The client-server guide says synchronous calls are generally the default: waiting for a reply provides back-pressure. A cast is asynchronous and does not guarantee the server received the message. Choose between call and cast based on whether the sender needs a response and whether the system should regulate work by waiting.
Broken invariant or unrecoverable state
Stop the process if continuing could make the state unsafe, and let supervision handle recovery according to the application’s restart design. Do not change the restart policy merely to silence a crash report.
Restarted process loses necessary state
A restarted GenServer begins again from its initialization logic. The Supervisor documentation demonstrates this with a counter that returns to its initial value after a crash. If important state exists only in process memory, decide whether it must instead be persisted or reconstructed from another source before relying on restart for recovery.
7. Reproduce the failure and verify the fix
- Replay the request or message that caused the failure, using the observed term rather than only the intended input.
- Confirm that the relevant callback now returns a valid result, or stops intentionally with a reason the supervisor is configured to handle.
- Check the caller’s behavior separately: a successful server response, a deliberate error response, and a call timeout are different outcomes.
- Review supervisor logs and restart history to confirm whether the child remained alive, restarted as expected, or triggered broader tree recovery.
- Check the post-restart state against the application’s requirements, especially when it must be rebuilt or recovered outside process memory.
Elixir describes a GenServer as a process that can keep state and execute code asynchronously, not as a process immune to ordinary process failure. That is why the callback defect, message path, linked exit, and restart policy all matter to a complete diagnosis; see the GenServer API reference.
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