Supertester

A battle-hardened OTP testing toolkit with chaos engineering, performance testing, and zero-sleep synchronization for building robust Elixir applications.

Version 0.5.1 - Eliminates the private @doc warning in TestableGenServer shims.

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The Problem: Flaky OTP Tests

Are you tired of...

• Flaky tests that fail randomly due to race conditions?
• GenServer name clashes when running tests with async: true?
• Littering your test suite with Process.sleep/1 and hoping for the best?
• Struggling to test process crashes, restarts, and complex supervision trees?

Writing tests for concurrent systems is hard. Traditional testing methods often lead to fragile, non-deterministic, and slow test suites.

The Solution: Supertester

Supertester provides a comprehensive suite of tools to write clean, deterministic, and reliable tests for your OTP applications. It replaces fragile timing hacks with robust synchronization patterns and provides powerful helpers for simulating and asserting complex OTP behaviors.

With Supertester, you can build a test suite that is fast, parallel, and trustworthy.

Key Features

• Rock-Solid Test Isolation: Run all your tests with async: true without fear of process name collisions or state leakage.
• Zero Process.sleep: Complete elimination of timing-based synchronization. Use proper OTP patterns for deterministic testing.
• Powerful OTP Assertions: Go beyond assert. Use assert_process_restarted/2, assert_genserver_state/2, and assert_all_children_alive/1 for more expressive tests.
• Effortless Setup & Teardown: Start isolated GenServers and Supervisors with a single line and trust Supertester to handle all the cleanup.
• Chaos Engineering: Test system resilience with controlled fault injection, random process crashes, and resource exhaustion.
• Supervision Tree Testing: Verify restart strategies, trace supervision events, and validate tree structures.
• Performance Testing: Assert performance SLAs, detect memory leaks, and prevent regressions with built-in benchmarking.
• TestableGenServer: Automatic injection of sync handlers for deterministic async operation testing.
• Concurrent Harness: Describe multi-threaded scenarios once and let Supertester orchestrate calls, casts, chaos hooks, performance checks, and mailbox monitoring without extra glue.
• Telemetry Built-In: Structured :telemetry events for scenario start/stop, mailbox sampling, chaos injections, and performance metrics so observability does not become an afterthought.

Installation

Add supertester as a dependency in your mix.exs file. It's only needed for the :test environment.

def deps do
  [
    {:supertester, "~> 0.5.1", only: :test}
  ]
end

Then, run mix deps.get to install.

Quick Start: From Flaky to Robust

See how Supertester transforms a common, fragile test pattern into a robust, deterministic one.

Before: The Flaky Way

# test/my_app/counter_test.exs
defmodule MyApp.CounterTest do
  use ExUnit.Case, async: false # <-- Forced to run sequentially

  test "incrementing the counter" do
    # Manual setup, prone to name conflicts
    {:ok, _pid} = start_supervised({Counter, name: Counter})

    GenServer.cast(Counter, :increment)
    Process.sleep(50) # <-- Fragile, timing-dependent guess

    state = GenServer.call(Counter, :state)
    assert state.count == 1
  end
end

After: The Supertester Way

# test/my_app/counter_test.exs
defmodule MyApp.CounterTest do
  use Supertester.ExUnitFoundation, isolation: :full_isolation # <-- Fully parallel!

  # Import the tools you need
  import Supertester.OTPHelpers
  import Supertester.GenServerHelpers
  import Supertester.Assertions

  test "incrementing the counter" do
    # Isolated setup with automatic cleanup, no name clashes
    {:ok, counter_pid} = setup_isolated_genserver(Counter)

    # Deterministic sync: ensures the cast is processed before continuing
    :ok = cast_and_sync(counter_pid, :increment)

    # Expressive, OTP-aware assertion for checking state
    assert_genserver_state(counter_pid, fn state -> state.count == 1 end)
  end
end

Core Modules Overview

Supertester is organized into several powerful modules, each targeting a specific area of OTP testing.

• Supertester.ExUnitFoundation: Drop-in ExUnit.Case adapter that enables Supertester isolation with a single use, automatically configuring async support per isolation mode.

• Supertester.UnifiedTestFoundation: The isolation runtime powering Supertester. Call it directly from custom harnesses or advanced setups where you don’t want to use the ExUnit adapter.

• Supertester.TestableGenServer: A behavior that injects a :__supertester_sync__ handler into your GenServers. This enables cast_and_sync/2 to work deterministically: since GenServers process messages sequentially, a call that returns after a cast guarantees the cast was processed. No Process.sleep/1 needed.

• Supertester.OTPHelpers: Provides the essential tools for managing the lifecycle of isolated OTP processes. Functions like setup_isolated_genserver/3 and setup_isolated_supervisor/3 are your entry point for starting processes within the isolated test environment.

• Supertester.GenServerHelpers: Contains specialized functions for interacting with GenServers. The star of this module is cast_and_sync/2, which provides a robust way to test asynchronous cast operations deterministically. It also includes helpers for stress-testing and crash recovery scenarios.

• Supertester.SupervisorHelpers: A dedicated toolkit for testing the backbone of OTP applications: supervisors. You can verify restart strategies, validate supervision tree structures, and trace supervision events to ensure your application is truly fault-tolerant.

• Supertester.ConcurrentHarness: Compose complex concurrent scenarios declaratively. Provide thread scripts, invariants, optional chaos hooks, performance expectations, and mailbox monitoring, then receive a structured report plus telemetry events for each run.

• Supertester.PropertyHelpers: Build StreamData generators that output ready-to-run concurrency scenarios. Feed them into ConcurrentHarness for property-based testing of GenServers and supervisors.

• Supertester.MessageHarness: Trace messages delivered to any process while executing a function. Perfect for debugging mailbox growth or validating ordering without invasive instrumentation.

• Supertester.Telemetry: Single entry point for emitting :telemetry events with the [:supertester | ...] prefix, covering scenario lifecycle, chaos hooks, mailbox samples, and performance measurements so you can plug into dashboards with minimal wiring.

• Supertester.ChaosHelpers: Unleash controlled chaos to test your system's resilience. This module allows you to inject faults, kill random processes, and simulate resource exhaustion to ensure your system can withstand turbulent conditions.

• Supertester.PerformanceHelpers: Integrate performance testing directly into your test suite. Assert that your code meets performance SLAs, detect memory leaks, and prevent performance regressions before they hit production.

• Supertester.Assertions: A rich set of custom assertions that understand OTP primitives. Go beyond simple equality checks with assertions like assert_genserver_state/2, assert_all_children_alive/1, and assert_no_process_leaks/1 for more expressive and meaningful tests.

• Supertester.TelemetryHelpers: Async-safe telemetry testing with per-test event isolation. Attach handlers that only receive events tagged with the current test ID, assert telemetry events with pattern matching, and capture events during function execution.

• Supertester.LoggerIsolation: Per-process Logger level isolation and log capture. Set Logger levels that only affect the current test process, capture log messages during execution, and restore original levels automatically on test completion.

• Supertester.ETSIsolation: Isolated ETS table creation, mirroring, and table injection with automatic cleanup. Create test-scoped ETS tables, mirror production tables for safe testing, and inject isolated tables into processes under test.

Advanced Usage Examples

Chaos Engineering

Test your system's resilience to failures:

test "system survives random process crashes" do
  {:ok, supervisor} = setup_isolated_supervisor(MyApp.WorkerSupervisor)

  # Kill 50% of workers over 3 seconds
  report = chaos_kill_children(supervisor,
    kill_rate: 0.5,
    duration_ms: 3000,
    kill_interval_ms: 200
  )

  # Verify system recovered
  assert Process.alive?(supervisor)
  assert report.supervisor_crashed == false
  assert_all_children_alive(supervisor)
end

# Mix chaos scenarios with concurrent harness runs
suite = [
  %{type: :kill_children, kill_rate: 0.3, duration_ms: 500},
  %{
    type: :concurrent,
    build: fn sup ->
      Supertester.ConcurrentHarness.simple_genserver_scenario(
        MyWorker,
        [{:cast, :do_work}, {:call, :get_state}],
        4,
        setup: fn -> {:ok, sup, %{}} end,
        cleanup: fn _, _ -> :ok end
      )
    end
  }
]

report = run_chaos_suite(supervisor, suite)

Performance Testing

Ensure your code meets performance SLAs:

test "API response time SLA" do
  {:ok, api_server} = setup_isolated_genserver(APIServer)

  assert_performance(
    fn -> APIServer.handle_request(api_server, :get_user) end,
    max_time_ms: 50,
    max_memory_bytes: 500_000,
    max_reductions: 100_000
  )
end

test "no memory leak in message processing" do
  {:ok, worker} = setup_isolated_genserver(MessageWorker)

  assert_no_memory_leak(10_000, fn ->
    MessageWorker.process(worker, generate_message())
  end)
end

Supervision Tree Testing

Verify supervision strategies work correctly:

test "one_for_one restarts only failed child" do
  {:ok, supervisor} = setup_isolated_supervisor(MySupervisor)

  result = test_restart_strategy(supervisor, :one_for_one,
    {:kill_child, :worker_1}
  )

  assert result.restarted == [:worker_1]
  assert :worker_2 in result.not_restarted
  assert :worker_3 in result.not_restarted
end

test "supervision tree structure" do
  {:ok, root} = setup_isolated_supervisor(RootSupervisor)

  assert_supervision_tree_structure(root, %{
    supervisor: RootSupervisor,
    strategy: :one_for_one,
    children: [
      {:cache, CacheServer},
      {:worker_pool, WorkerPoolSupervisor}
    ]
  })
end

Concurrent Harness with Chaos and Telemetry

Orchestrate multi-threaded scenarios with chaos injection and observability:

test "counter stays non-negative under chaos" do
  scenario =
    Supertester.ConcurrentHarness.simple_genserver_scenario(
      CounterServer,
      [{:cast, :increment}, {:cast, :decrement}],
      5,
      chaos: Supertester.ConcurrentHarness.chaos_kill_children(kill_rate: 0.2, duration_ms: 500),
      performance_expectations: [max_time_ms: 100],
      mailbox: [sampling_interval: 2],
      metadata: %{test: "counter chaos"}
    )

  assert {:ok, report} = Supertester.ConcurrentHarness.run(scenario)
  assert report.metrics.total_operations == length(report.events)
end

Attach to telemetry events for scenario timing:

:telemetry.attach(
  "supertester",
  [:supertester, :concurrent, :scenario, :stop],
  fn _event, %{duration_ms: duration}, metadata, _ ->
    Logger.info("[supertester] scenario #{metadata.scenario_id} finished in #{duration}ms")
  end,
  nil
)

Documentation

For a comprehensive guide to all features, modules, and functions, please see the full User Manual.

The user manual includes:

• In-depth explanations of every module.
• Detailed function signatures, parameters, and return values.
• Practical code examples and recipes for common testing scenarios.
• Best practices for writing robust tests.

Additional documentation lives in guides/:

• Documentation Index
• Quick Start
• API Guide

Examples

If you want a full Mix app that demonstrates every Supertester module, start here:

• Examples Index
• EchoLab Example App

What's New in 0.5.1

• TestableGenServer warning fix: clears stale @doc attributes before defining the private sync helper.

See CHANGELOG.md for detailed changes.

Contributing

Contributions are welcome! If you'd like to help improve Supertester, please feel free to:

1. Fork the repository.
2. Create a new feature branch.
3. Add your feature or bug fix.
4. Ensure all new code is covered by tests.
5. Open a pull request.

License

This project is licensed under the MIT License.