hw-kafka-streamly

Streamly streaming integration for hw-kafka-client — the Haskell binding to Apache Kafka via librdkafka.

hw-kafka-streamly exposes Kafka consumers as composable Streamly Streams and Kafka producers as Streamly Folds, so message processing can be expressed in the same vocabulary used elsewhere in a Streamly pipeline. Resource handling (consumer/producer creation, close, flush) is layered so the caller picks the amount of bracketing they want.

Installation

Add to your .cabal file:

build-depends:
  , hw-kafka-streamly  >=0.2 && <0.3
  , hw-kafka-client    >=5.3 && <6
  , streamly-core      >=0.3 && <0.5

hw-kafka-client needs the librdkafka C library at build and run time (Homebrew: brew install librdkafka; Debian/Ubuntu: apt install librdkafka-dev).

Modules

• Kafka.Streamly.Stream — consumer streams, error predicates and filters, value-mapping helpers built on Bifunctor/Bitraversable.
• Kafka.Streamly.Fold — producer folds and a withKafkaProducer bracket helper.
• Kafka.Streamly.Combinators — batching combinators and helpers that throw Left values as exceptions.

Consuming

The stream module ships three variants that differ only in how they manage the underlying KafkaConsumer:

• kafkaStream — creates the consumer from ConsumerProperties and Subscription, closes it when the stream ends. Use this when the stream fully owns the consumer's lifecycle.
• kafkaStreamAutoClose — wraps a caller-supplied KafkaConsumer and closes it on stream end. Use this when the consumer is created elsewhere but its lifetime matches the stream.
• kafkaStreamNoClose — wraps a caller-supplied KafkaConsumer and leaves it open. Use this when the consumer outlives the stream.

import Kafka.Consumer
import Kafka.Streamly.Stream (kafkaStream, skipNonFatal)
import Streamly.Data.Stream qualified as Stream

main :: IO ()
main = do
  let props = brokersList ["localhost:9092"]
           <> groupId "my-group"
           <> noAutoCommit
      sub  = topics ["events"] <> offsetReset Earliest
  Stream.fold (Fold.drainBy print)
    . skipNonFatal
    $ kafkaStream props sub (Timeout 1000)

Producing

Producer folds are Streamly Folds:

import Kafka.Producer
import Kafka.Streamly.Fold (kafkaFold, withKafkaProducer)
import Streamly.Data.Fold qualified as Fold
import Streamly.Data.Stream qualified as Stream

main :: IO ()
main = do
  let props = brokersList ["localhost:9092"]
  result <- withKafkaProducer props $ \producer ->
    Stream.fold (kafkaFold producer)
      . fmap mkRecord
      $ Stream.fromList ["a", "b", "c"]
  print result
  where
    mkRecord v =
      ProducerRecord
        { prTopic     = TopicName "events"
        , prPartition = UnassignedPartition
        , prKey       = Nothing
        , prValue     = Just v
        , prHeaders   = mempty
        }

Cookbook

For end-to-end runnable examples — concurrent consumers, batching producers, error handling, transform pipelines — see the companion hw-kafka-streamly-jitsurei package in this repository. It is not published to Hackage; clone the repo and run the executables locally.

Design notes

The original design plan for the bindings lives at docs/plans/1-streamly-bindings-for-hw-kafka-client.md.

License

MIT — see LICENSE.