@hackage postgresql-simple-queue1.0.1

This module utilize PostgreSQL to implement a durable queue for efficently processing arbitrary payloads which can be represented as JSON.

Typically a producer would enqueue a new payload as part of larger database transaction

 createAccount userRecord = do
    'runDBTSerializable' $ do
       createUserDB userRecord
       'enqueueDB' "queue_schema" $ makeVerificationEmail userRecord

In another thread or process, the consumer would drain the queue.

  forever $ do
    -- Attempt get a payload or block until one is available
    payload <- lock "queue" conn

    -- Perform application specifc parsing of the payload value
    case fromJSON $ pValue payload of
      Success x -> sendEmail x -- Perform application specific processing
      Error err -> logErr err

    -- Remove the payload from future processing
    dequeue "queue" conn $ pId payload

To support multiple queues in the same database, the API expects a table name string
to determine which queue tables to use.