Effectful effects for hw-kafka-client.

kafka-effectful

Effectful effects and interpreters for hw-kafka-client, a Haskell binding to Apache Kafka via librdkafka.

Provides typed, composable KafkaProducer and KafkaConsumer effects for the effectful ecosystem.

Status: experimental. This package is on its first release. The API may change in breaking ways in subsequent 0.x versions. Pin to an exact version in production until 1.0 is tagged.

Features

• KafkaProducer -- send messages and flush the producer queue
• KafkaConsumer -- poll messages (single or batch), manage offsets, assign/pause/resume/seek partitions, and query committed offsets, positions, assignments, and subscriptions
• Resource-safe interpreters that acquire and release Kafka handles via bracket
• Errors surfaced through Effectful.Error.Static (Error KafkaError)

Usage

Producer

import Kafka.Effectful

example :: (IOE :> es, Error KafkaError :> es) => Eff es ()
example =
  runKafkaProducer producerProps $ do
    produceMessage record
    flushProducer

Consumer

import Kafka.Effectful

example :: (IOE :> es, Error KafkaError :> es) => Eff es ()
example =
  runKafkaConsumer consumerProps subscription loop
  where
    loop = do
      mbMsg <- pollMessage (Timeout 1000)
      case mbMsg of
        Nothing  -> loop
        Just msg -> do
          commitOffsetMessage OffsetCommit msg
          loop

pollMessage returns Nothing when the timeout elapses without a message arriving; non-timeout failures are thrown via the Error KafkaError effect.

Running it

The effect handlers runKafkaProducer and runKafkaConsumer require IOE and Error KafkaError in the effect stack. A complete program wires them with runEff and runError:

{-# LANGUAGE TypeApplications #-}

import Effectful
import Effectful.Error.Static (runError)
import Kafka.Effectful

main :: IO ()
main = do
  result <- runEff . runError @KafkaError $ runProgram
  case result of
    Left (_, err) -> putStrLn ("Kafka error: " <> show err)
    Right ()      -> pure ()
  where
    runProgram =
      runKafkaProducer producerProps $ do
        produceMessage record
        flushProducer

Replace producerProps and record with your own ProducerProperties and ProducerRecord values (see the Kafka.Effectful.Producer module for the available builders).

Module Structure

Requirements

• GHC >= 9.12
• librdkafka (system dependency)

License

MIT.