Use ExUnit’s test supervisor to start a fresh process for each test, exercise a GenServer through its public API, and verify supervision behavior by triggering a controlled exit and checking the configured restart outcome. Choose the startup helper, exit reason, and assertions to match the process’s contract; avoid arbitrary sleeps and account for shared resources when enabling asynchronous tests.
How do I start a process in ExUnit and clean it up?
Prefer start_supervised!/2 over starting a linked process directly in each test. ExUnit’s test supervisor owns the child and stops it before the next test begins, preventing a process from one test from lingering into another. The official ExUnit.Callbacks documentation describes these helpers and their lifecycle.
use ExUnit.Case, async: true
setup do
server = start_supervised!({MyApp.Counter, 0})
%{server: server}
end
test "increments the counter", %{server: server} do
assert MyApp.Counter.value(server) == 0
assert MyApp.Counter.increment(server) == 1
endThe module and argument tuple must fit the process’s child specification and start_link contract. Use the raising helper when startup failure should fail setup immediately and return the child PID.
Choose the helper based on the failure you need to observe
| Test need | Mechanism | What it establishes |
|---|---|---|
| Start a process and clean it up at test completion | start_supervised!/2 |
Returns the PID on success; raises if startup fails. The child is managed by ExUnit’s test supervisor, not linked to the test process. |
| Inspect a startup success or error result | start_supervised/2 |
Lets the test examine the returned {:ok, pid} or {:error, reason}. |
| Make a child crash propagate to the test | start_link_supervised!/2 |
Starts a linked child so an unexpected failure can fail the test. |
| Stop a test-supervised child before the test ends | stop_supervised/1 |
Stops the child by its child ID. Simply terminating a restartable child may prompt its supervisor to restart it. |
These helpers do not make every process failure equivalent: a monitor lets you assert termination as an expected event, while a link makes a linked process’s unexpected failure propagate. Choose according to whether the test should observe a crash or fail because of it.
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How do I test a GenServer in Elixir?
Test ordinary behavior through the GenServer’s public client API. Call its synchronous or asynchronous functions and assert replies or state transitions that callers depend on. The GenServer guide demonstrates client-server testing and test-supervised startup.
Assert observable outcomes, not incidental implementation
- For a synchronous operation, assert its returned value and, if relevant, the next result visible through the API.
- For an asynchronous contract, use
assert_receivewith a bounded timeout to check the expected message. - If termination is part of the contract, monitor the PID and assert the
:DOWNmessage and reason. - Avoid reading internal state or testing callback details unless that internal detail is itself part of the required contract.
Use replies, messages, and monitor signals as synchronization. An arbitrary sleep only waits for a guessed interval; it does not prove the process reached the condition the test needs.
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How do I test that a supervisor restarts a process?
Start the supervisor or relevant subtree under ExUnit’s test supervisor, identify the child by its child ID, and induce a controlled failure. Then assert the configured result: whether the child returns, whether its PID changes, and whether affected siblings restart. A child specification defines the child’s start, shutdown, and restart behavior; the supervisor strategy defines how a failure affects the rest of the tree. See the versioned Supervisor documentation for the applicable configuration details.
Account for restart mode and exit reason
| Child restart mode | Expected response to exit |
|---|---|
:permanent |
The child is restarted regardless of exit reason. |
:transient |
The child is restarted after an abnormal exit, not a normal termination. |
:temporary |
The child is not restarted. |
Assert the configured supervision strategy
| Strategy | Expected effect of a child failure | Useful assertion |
|---|---|---|
:one_for_one |
The failed child is the restart focus. | Check the failed child’s new PID and initialized behavior; check that unrelated siblings retain their PIDs if that matters to the contract. |
:one_for_all |
All children in the group are restarted. | Compare captured PIDs before and after for the failed child and each sibling in the group. |
:rest_for_one |
The failed child and children started after it are restarted. | Check that the failed child and later siblings change PID, while earlier siblings retain theirs where relevant. |
Use a test-only message, deliberately failing input, or another controlled trigger suitable for the child. Synchronize on an explicit message, a supervisor API result, or monitoring rather than waiting an arbitrary duration. A new PID alone proves that a process restarted, not that it returned to the required state; call its public API to check that state as well.
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test "restarts a permanent worker after an abnormal exit" do
supervisor = start_supervised!({MyApp.WorkerSupervisor, []})
old_pid = MyApp.WorkerSupervisor.worker_pid(supervisor)
send(old_pid, :crash_for_test)
assert_receive {:worker_restarted, new_pid}
refute old_pid == new_pid
assert MyApp.Worker.get_state(new_pid) == :initial_state
endThis is a pattern, not a drop-in implementation: adapt the child ID, crash trigger, restart mode, and synchronization signal to the application. The child must be configured to restart for the induced exit; for example, a transient child requires an abnormal exit. Avoid identifying a child by module alone if the tree can contain multiple children of that module. Supervisor details can vary by Elixir version, so consult documentation matching the project’s pinned version.
How do I test a DynamicSupervisor?
Start a fresh DynamicSupervisor for the test, then add and remove children through its API. Assert that a child is present after a successful start and absent after a stop or termination consistent with its restart mode. ExUnit’s outer test supervisor provides lifecycle cleanup for the test’s processes. The DynamicSupervisor guide covers the dynamic-child model.
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Should I use async tests for OTP processes?
Use async: true only if concurrently running tests cannot interfere through shared mutable state or external resources. Per-test process ownership isolates those supervised processes, but it does not isolate registered names, files, ports, external services, or other global resources.
- Give registered processes unique names when tests run concurrently.
- Use resources scoped to the individual test where possible.
- Disable async execution for tests that share state or cannot safely isolate an external resource.
ExUnit documentation also covers test grouping and parameterized runs in newer releases; verify the features available in the Elixir version pinned by the project. The current Elixir documentation index reported Elixir v1.20.4 as stable on October 4, 2026, with Erlang/OTP 27, 28, and 29 listed as supported. The linked ExUnit and Supervisor API pages here are versioned v1.18.0 and v1.15.8 references, respectively; match implementation details to the versions your application uses.
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