Elixir Test Runner

This skill helps run ExUnit tests efficiently with proper filtering, debugging, and error analysis.

When to Use

• Running full test suite
• Testing specific files or tests
• Debugging test failures
• Running tests with coverage
• Testing in different environments

Basic Test Execution

Run All Tests

mix test

Run Specific Test File

mix test test/my_app/accounts_test.exs

Run Specific Test by Line Number

mix test test/my_app/accounts_test.exs:42

Run Tests Matching Pattern

# Run all tests with "user" in the name
mix test --only user

Test Filtering

By Tag

# In test file
@tag :integration
test "complex integration test" do
  # ...
end

# Run only integration tests
mix test --only integration

# Exclude slow tests
mix test --exclude slow

# Run everything except integration
mix test --exclude integration

By Module Pattern

# Run all controller tests
mix test test/**/controllers/*_test.exs

# Run all LiveView tests
mix test test/**/*_live_test.exs

Umbrella App Filtering

# Run tests for specific app
mix test apps/my_app/test

# Run all umbrella tests
mix test --only apps

Debugging Options

With Trace (Detailed Output)

mix test --trace

Shows each test as it runs - useful for hanging tests.

With Verbose Failures

mix test --max-failures 1

Stops after first failure for faster debugging.

With Test Seed

# Tests run in random order by default
# To reproduce specific order:
mix test --seed 123456

# See seed in output:
# "Randomized with seed 123456"

With IEx for Debugging

# Add IEx.pry() in test code
mix test --trace

Test Output Control

Show Only Failures

mix test --failed

Reruns only previously failed tests.

Stale Tests

mix test --stale

Only runs tests for changed files.

With Coverage

mix test --cover

Generates coverage report in

cover/

directory.

Quiet Mode

mix test --quiet

Less verbose output.

Test Environments

Standard Test Run

mix test

Uses MIX_ENV=test automatically.

With Database Reset

mix ecto.reset && mix test

Fresh database for each run.

CI Mode

# Ensure warnings fail, run with coverage
mix test --warnings-as-errors --cover

Common Test Patterns

Test Suite Organization

Unit Tests (fast, isolated):

mix test test/my_app/accounts/user_test.exs

Integration Tests (slower, with database):

mix test --only integration

Controller Tests:

mix test test/my_app_web/controllers/

LiveView Tests:

mix test test/my_app_web/live/

Parallel Testing

# In test_helper.exs - already default
ExUnit.start()

# In DataCase
use MyApp.DataCase, async: true  # Parallel execution

# Run with more cores
mix test --max-cases 8

Analyzing Test Failures

Read Failure Output Carefully

Example failure:

1) test creates user with valid attrs (MyApp.AccountsTest)
   test/my_app/accounts_test.exs:42
   ** (RuntimeError) Database not started

What to check:

1. Test name: "creates user with valid attrs"
2. Module: MyApp.AccountsTest
3. File and line: test/my_app/accounts_test.exs:42
4. Error: Database not started

Common Failure Patterns

Database Not Started:

# Start database
mix ecto.create
MIX_ENV=test mix ecto.migrate

Async Test Conflicts:

# Change to synchronous if tests conflict
use MyApp.DataCase, async: false

Missing Setup:

# Check for missing setup block
setup do
  :ok = Ecto.Adapters.SQL.Sandbox.checkout(Repo)
end

Factory/Fixture Issues:

# Verify factory data is valid
mix test test/support/fixtures.exs --trace

Performance Optimization

Identify Slow Tests

# Run with timing
mix test --trace | grep -E "^\s+test.*\([0-9]+\.[0-9]+s\)"

Profile Test Suite

# With profiling
mix test --profile

Parallel Execution

# Enable async for fast unit tests
use MyApp.DataCase, async: true

# Disable for integration tests
use MyApp.DataCase, async: false

Test Coverage

Generate Coverage Report

mix test --cover

View report:

cover/excoveralls.html

Coverage Configuration

# In mix.exs
def project do
  [
    test_coverage: [tool: ExCoveralls],
    preferred_cli_env: [
      coveralls: :test,
      "coveralls.detail": :test,
      "coveralls.post": :test,
      "coveralls.html": :test
    ]
  ]
end

With ExCoveralls

# HTML report
mix coveralls.html

# Detailed console report
mix coveralls.detail

# Check coverage threshold
mix coveralls --min-coverage 80

CI/CD Integration

GitHub Actions

- name: Run tests
  run: mix test --warnings-as-errors --cover

GitLab CI

test:
  script:
    - mix ecto.create
    - mix ecto.migrate
    - mix test --cover

Pre-commit Hook

#!/bin/bash
# .git/hooks/pre-commit
mix test --failed || exit 1

Troubleshooting

Tests Hang

# Use --trace to see which test hangs
mix test --trace

# Look for:
# - Database connection issues
# - Infinite loops
# - Missing async: false for conflicting tests

Tests Pass Locally, Fail in CI

Common causes:

1. Missing MIX_ENV=test
2. Database not created
3. Dependencies not fetched
4. Different Elixir/OTP versions
5. Async test conflicts

Debug:

# Reproduce CI environment locally
MIX_ENV=test mix do deps.get, ecto.create, ecto.migrate, test

Flaky Tests

# Run same test multiple times
mix test test/my_app/accounts_test.exs:42 --trace
mix test test/my_app/accounts_test.exs:42 --trace
mix test test/my_app/accounts_test.exs:42 --trace

# Check for:
# - Time-dependent logic
# - Random data without seeds
# - Async conflicts
# - External dependencies

Memory Issues

# Run with more memory
elixir --erl "+hms 4294967296" -S mix test

Best Practices

1. Run tests frequently during development
2. Use --stale for fast feedback loop
3. Tag slow tests and exclude during dev
4. Fix failures immediately - don't accumulate
5. Use --trace when debugging
6. Run full suite before committing
7. Check coverage for critical code paths
8. Keep tests fast - mock external services
9. Use factories for consistent test data
10. Run in CI to catch environment issues

Quick Reference

# Development workflow
mix test --stale                    # Fast feedback
mix test test/my_app/file_test.exs # Specific file
mix test --failed                   # Rerun failures

# Debugging
mix test --trace                    # See each test
mix test --max-failures 1          # Stop at first failure
mix test --seed 123456             # Reproduce order

# Coverage
mix test --cover                    # Basic coverage
mix coveralls.html                  # Detailed HTML

# Filtering
mix test --only integration        # Tagged tests
mix test --exclude slow            # Exclude tagged

# CI/CD
mix test --warnings-as-errors --cover