Redis

Reviewed last on 2020-10-14

This library provides functions for working with Redis and JSON-serialized haskell records.

Because JSON encoding is quite lenient for containers (e.g. lists, maps), using Hedis alone makes it easy to accidentally write a record to redis, and only later discover that, when reading, you actually get back a list of that record.

This library helps avoid those problem by levering specific Key-Value mapping that we call an API.

At present this library implements a subset of the available redis commands. If you miss a command please feel free to add it!

How to use me

1. make a handler

(optional) Set up some environmental variables.

these are the defaults:

REDIS_CONNECTION_STRING=redis://localhost:6379
# we also support unix sockets via a scheme we provide:
# REDIS_CONNECTION_STRING=redis+unix:///path/to/redis.sock[?db=DB_NUM]
REDIS_CLUSTER=0 # 1 is on
REDIS_DEFAULT_EXPIRY_SECONDS=0 # 0 is no expiration
REDIS_QUERY_TIMEOUT_MILLISECONDS=1000 # 0 is no timeout

main : IO ()
main =
  -- use nri-env-parser to parse settings from the environment
  settings <- Environment.decode Redis.decoder
  -- get a handler
  Conduit.withAcquire (Redis.handler settings) <| \redisHandler ->
    callSomeRedisFuncs redisHandler

2. Define a codec type

data Key = Key
  { userId :: Int,
    quizId :: Int
  }
data User =
  User {
    name :: Text,
    favoriteColor :: Text
  }
  deriving (Generic)
-- payload needs this!
instance Aeson.ToJSON User
instance Aeson.FromJSON User
-- using this enforces this key-value mapping
redisApi :: Redis.Api Key User
redisApi =
  Redis.Hash.jsonApi toKey
  where
    toKey :: Key -> Text
    toKey Key {userId, quizId} =
      Text.join
        "-"
        [ "quiz",
          Text.fromInt userId,
          Text.fromInt quizId,
          "v1" -- good idea to version your keys!
        ]

3. use the codec to read and writee!

_ <- Redis.query handler (Redis.set redisApi (Key 1 2) (User "alice" "orange"))
Redis.query (Redis.get redisApi (Key 1 2))  -- Just (User "alice" "orange")
-- or, perhaps better
[ (Redis.set redisApi (Key 1 2) (User "alice" "orange"))
, (Redis.get redisApi (Key 1 2))
]
  |> Redis.sequence
  |> Redis.transaction -- Transaction ensures these commands are run together

FAQ

Your default env variables cause a collision. What do I do?

add a prefix, and use decoderWithEnvVarPrefix instead of decoder