A chaos engineering library for Elixir inspired by Netflix's Chaos Monkey. Mutineer allows you to inject controlled, random failures into your functions to test resilience and error handling.

Features

• Wrap functions to randomly trigger failures at configurable rates
• Multiple failure types: errors, exceptions, timeouts, nil returns, and process exits
• Minimal runtime overhead when disabled
• Two flexible APIs: attribute-based decorators or explicit macros
• Disabled by default for safety

Installation

Add mutineer to your list of dependencies in mix.exs:

def deps do
  [
    {:mutineer, "~> 0.2.0"}
  ]
end

Configuration

Configure Mutineer in your config/config.exs (or environment-specific config):

config :mutineer, Mutineer,
  enabled: true,
  default_failure_rate: 0.1,
  default_failure_types: :error

Usage

Attribute-based API

Add the use Mutineer statement to your module. You can then use the @chaos attribute to mark functions for chaos injection:

defmodule MyApp.Database do
  use Mutineer

  @chaos failure_rate: 0.2
  def query(sql) do
    # Your database query logic
  end

  @chaos failure_type: :timeout, delay: 3000
  def slow_query(sql) do
    # This will randomly timeout
  end
end

Macro-based API

Use defchaos or defchaosp (for private functions) for explicit chaos wrapping:

defmodule MyApp.ExternalService do
  use Mutineer

  defchaos call_api(endpoint), failure_rate: 0.3, failure_type: :raise do
    # Your API call logic
  end

  defchaosp internal_helper(data), failure_rate: 0.1 do
    # Private function with chaos
  end
end

Global configuration options

• enabled - Enables or disables chaos globally (default: false)
• default_failure_types - Sets the default failure types for all functions, can be a list of failure types or a single failure type (default: :error)
• default_failure_rate - Sets the default failure rate for all functions (default: 0.1)

Failure Types

• :error - Returns {:error, :mutineer_chaos} (default) or a random error from the errors option
• :raise - Raises either a Mutineer.ChaosError exception (default) or a custom error specified in the raised_errors option
• :delay - Introduces a random delay (1-5 seconds) before executing function
• :timeout - Same as :raise, but with a random delay before raising the exception
• :nil - Returns nil
• :exit - Calls exit(:mutineer_chaos); the atom can be specified in the exit_errors option

Failure Options

Options can be passed to @chaos, defchaos, or defchaosp:

• failure_rate is the probability of failure for a given function (0.0 - 1.0), where 1.0 or above will always fail
• failure_types (or failure_type) is either a list of failure types to trigger (e.g. [:error, :delay]) or a single failure type (e.g. :error)
• errors (or error) is either a list of objects to be randomly selected from or a single object to return when the :error type is triggered
• raised_errors (or raised_error) is a list of errors to be randomly selected from or a single error to be raised when the :raise type is triggered
• exit_errors (or exit_error) is a list of errors to be randomly selected from or a single error to be raised when the :exit type is triggered
• delay is the upperbound of the delay in milliseconds or a range of milliseconds for :timeout and :delay types

Example

defmodule MyApp.PaymentGateway do
  use Mutineer

  @chaos failure_rate: 0.1, failure_type: :error
  def process_payment(amount, card) do
    # Payment processing logic
    {:ok, %{transaction_id: "txn_123", amount: amount}}
  end

  @chaos failure_rate: 0.05, failure_type: :raise, message: "Gateway timeout"
  def verify_card(card) do
    # Card verification logic
    {:ok, :valid}
  end
end

# In your tests or staging environment:
case MyApp.PaymentGateway.process_payment(100, card) do
  {:ok, result} -> handle_success(result)
  {:error, reason} -> handle_error(reason)
end

# config/dev.exs
config :mutineer,
  Mutineer,
  enabled: true,
  default_failure_rate: 0.1

# config/prod.exs
config :mutineer,
  Mutineer,
  enabled: false