ConsumerExample.hs

{-# LANGUAGE OverloadedStrings   #-}
{-# LANGUAGE ScopedTypeVariables #-}
module ConsumerExample

where

import Control.Arrow     ((&&&))
import Control.Exception (bracket)
import Kafka.Consumer
import Data.Text         (Text)

-- Global consumer properties
consumerProps :: ConsumerProperties
consumerProps = brokersList ["localhost:9092"]
             <> groupId "consumer_example_group"
             <> noAutoCommit
             <> setCallback (rebalanceCallback printingRebalanceCallback)
             <> setCallback (offsetCommitCallback printingOffsetCallback)
             <> logLevel KafkaLogInfo

-- Subscription to topics
consumerSub :: Subscription
consumerSub = topics ["kafka-client-example-topic"]
           <> offsetReset Earliest

-- Running an example
runConsumerExample :: IO ()
runConsumerExample = do
    print $ cpLogLevel consumerProps
    res <- bracket mkConsumer clConsumer runHandler
    print res
    where
      mkConsumer = newConsumer consumerProps consumerSub
      clConsumer (Left err) = return (Left err)
      clConsumer (Right kc) = maybe (Right ()) Left <$> closeConsumer kc
      runHandler (Left err) = return (Left err)
      runHandler (Right kc) = processMessages kc

-------------------------------------------------------------------
processMessages :: KafkaConsumer -> IO (Either KafkaError ())
processMessages kafka = do
    mapM_ (\_ -> do
                   msg1 <- pollMessage kafka (Timeout 1000)
                   putStrLn $ "Message: " <> show msg1
                   err <- commitAllOffsets OffsetCommit kafka
                   putStrLn $ "Offsets: " <> maybe "Committed." show err
          ) [0 :: Integer .. 10]
    return $ Right ()

printingRebalanceCallback :: KafkaConsumer -> RebalanceEvent -> IO ()
printingRebalanceCallback _ e = case e of
    RebalanceBeforeAssign ps ->
        putStrLn $ "[Rebalance] About to assign partitions: " <> show ps
    RebalanceAssign ps ->
        putStrLn $ "[Rebalance] Assign partitions: " <> show ps
    RebalanceBeforeRevoke ps ->
        putStrLn $ "[Rebalance] About to revoke partitions: " <> show ps
    RebalanceRevoke ps ->
        putStrLn $ "[Rebalance] Revoke partitions: " <> show ps

printingOffsetCallback :: KafkaConsumer -> KafkaError -> [TopicPartition] -> IO ()
printingOffsetCallback _ e ps = do
    print ("Offsets callback:" ++ show e)
    mapM_ (print . (tpTopicName &&& tpPartition &&& tpOffset)) ps