Network.FeatureFlags

Use feature flags in WAI applications.

A feature flag is a way to toggle functionality on or off without having to redeploy the application. Feature flags have many possible uses, one is making it safer to release new functionality by providing a way to turn it off immediately if it misbehaves.

An application using this library first needs to define which feature flags is supports. This is done by creating a record type containing only boolean fields and adding a Flags instance to it.

data Features
  = Features
      { openWindow :: Bool,
        feedPigeons :: Bool
      }
  deriving (Generic)

instance Flags Features

Then we need a place to persist flag data. This library provides a memoryStore but it doesn't remember flag states across restarts. For production applications it's probably best to implement a Store that reads and writes flag data to the database or key,value store backing your project.

mkApplication provides a frontend from which each feature flag can be fully enabled, fully disabled, or enabled for a specific percentage of traffic. It is compatible with Wai-based web frameworks like spock, scotty, and servant. Setup instructions will be different for each. If you're having trouble integrating this in your choice of web framework please feel free to open an issue.

Now you're all set up. You can use fetch to read your feature flags from your store and can use their values in conditionals. For a full example check out this sample application.

Flags

class Flags flags

The feature flags you define are described by a type you create yourself. It needs to be a record though, with every field a boolean. Then we add a Flags instance to it so this library is able to work with the type.

data Features
  = Features
      { openWindow :: Bool,
        feedPigeons :: Bool
      }
  deriving (Generic)

instance Flags Features

fetch :: forall flags. Flags flags => Store flags -> IO flags

Read feature flag states out of the store. The states of flags enabled for part of the traffic will be determined by die-roll.

The default state for new flags and flags we cannot find values for in the store is off. This library offers no way to set other defaults to keep it as simple as possibe. You are encouraged to phrase your flag names in such a way that off corresponds to what you'd like the default value to be, i.e. enableExperimentalDoodad is likely safer than disableExperimentalDoodad.

Store

data Store flags

A type describing a store in which feature flag data can be saved. You are recommended to define your own stores using a persistence mechanism of your choice.

Constructors

Store

Fields

• readKeys :: IO [(ByteString, ByteString)]

  Read all key,value pairs from the store.

• writeKey :: ByteString -> ByteString -> IO ()

  Save a key,value pair to the store. Create the key,value pair if it does not exist yet and overwrite it otherwise.

memoryStore :: IO (Store flags)

An in-memory store that does not persist feature flag data across application restarts. Suitable for experimentation but not recommended for production use.

Feature flag frontend

mkApplication :: Flags flags => Store flags -> IO Application

Create a WAI application that serves a frontend for modifying feature flag states. How you embed this into your real application depends on the web framework you're using.