ExLoggerMock

A mock logging backend for Elixir unit tests. A clean way to get rid of log output in the test run.

Installation

The package can be installed by adding ex_logger_mock to the list of dependencies in mix.exs:

def deps do
  [
    {:ex_logger_mock, "~> 1.0", only: :test, runtime: false}
  ]
end

Documentation can be found at https://hexdocs.pm/ex_logger_mock.

Configuration

In the test environment configuration, replace the default :console logging backend with this package in your config/test.exs file:

config :logger,
  backends: [{ExLoggerMock.Backend, :ex_logger_mock}]

Some packages, e.g. Rollbax, don't like receiving unexpected messages. You can limit the applications that will be sent a message with extra configuration:

config :logger,
  backends: [{ExLoggerMock.Backend, :ex_logger_mock}]

config :ex_logger_mock, application_reject: [:rollbax]

or:

config :logger,
  backends: [{ExLoggerMock.Backend, :ex_logger_mock}]

config :ex_logger_mock, application_filter: [:my_app, :my_app_web]

You can optionally reject messages using a callback.

config :logger,
  backends: [{ExLoggerMock.Backend, :ex_logger_mock}]

config :ex_logger_mock, message_reject: fn message -> String.match?(message, ~r/MyXQL.Connection/) end

Use

The log calls themselves do not change. Without the :console backend, you won't see log output during the mix test run.

In unit tests, you can now assert for specific messages being logged:

assert_receive {:ex_logger_mock, {:info, "test log message", _timestamp, _metadata}}

NOTE: assert_receive here, not assert_received. Logging happens asynchronously across processes, so we need to give the message a little time to make it's way back to the test process.

Why

I don't like seeing log output during the mix test run. Traditionally, we solved that problem by setting the log level in the test configuration to :warn. But that solution makes testing "normal" log output with log_capture fail. Logs at :info level won't ever be produced, so cannot be checked.

Inspired by José's Mocks and explicit contracts post, and with many thanks to Brandon Richey for his The Ultimate Guide To Logging In Elixir post, this package solves both those problems.

Contributing

The typical fork, branch and PR dance.

To check all is well:

mix compile --force --warnings-as-errors && \
mix coveralls && \
mix credo --strict

License

MIT - do what you want with it.