Service Downtime Simulator

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🚨 If you work at Deliveroo and you're contributing to this project, please bear in mind that this repository is public.

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This is a piece of Rack middleware that simulates failures you would want to tolerate in upstream services.

Installation

Rails

Add the following in application.rb:

config.middleware.use(
  ServiceDowntimeSimulator::Middleware,
  config # See below for info about how to configure this
)

Configuration

The middleware takes a config argument in the form of a hash. Said hash should have the following shape:

{
  enabled: Boolean,
  mode: Symbol,
  excluded_paths: Array<String>,
  logger: Logger?
}

Here's what you can supply for each of those options:

• enabled (Boolean)
  • true will enable simulation of failures (assuming you supply a valid mode, see below)
  • false will disable simulation and your application will function as normal
• mode (Symbol)
  • :hard_down will cause all requests to return a 500 error
  • :intermittently_down will cause 50% of requests to return a 500 error
  • :successful_but_gibberish will return a 200, but with a response body that is not machine readable
  • :timing_out will wait for 15 seconds on each request, and then return a 503
• excluded_paths (Array<String>)
  • You can supply a list of paths that you don't want to be affected by the simulation here (e.g. ['/foobar'])
  • The most common thing you're going to want to include here is your service's health check endpoint, as if it is returning a 5xx thanks to this middleware your application will not deploy
• logger (Logger?)
  • If supplied, useful debug information will be sent here

In order for the middleware to kick in, enabled must be explicitly set to true and mode must be a valid option. Unless both are explicitly supplied, the underlying application will continue to function as normal.

Examples

Here's a couple of example configurations:

Hard-coded Hard Down

This example will always return a 500 for all requests.

config.middleware.use(
  ServiceDowntimeSimulator::Middleware,
  {
    enabled: true,
    mode: :hard_down,
    excluded_paths: ['/health'],
    logger: Rails.logger
  }
)

Environment-variable Controlled Simulation

This is a more practical example, allowing failure simulation to happen based on environment variables. It requires an environment variable with a specific value to enable the failure simulation, and also requires a mode to be provided. If either are missing, the app continues as normal. You can also use this pattern for feature flagging. Probably.

config.middleware.use(
  ServiceDowntimeSimulator::Middleware,
  {
    enabled: ENV['FAILURE_SIMULATION_ENABLED'] == 'I_UNDERSTAND_THE_CONSEQUENCES_OF_THIS',
    mode: ENV.fetch('FAILURE_SIMULATION_MODE', '').to_sym,
    excluded_paths: ['/health'],
    logger: Rails.logger
  }
)

Development

• Clone this repository
• Ensure you have Ruby 2.5.1 installed
• make install to get the dependencies
• make test to run the tests
• make lint to lint your code
• ???
• Profit

Gem Publishing

TBC, but very manual and involved flow is:

• Update version in lib/service_downtime_simulator.rb and commit
• Tag version via git tag XXX
• Push (git push origin head --tags)
• Release to Rubygems (make publish)