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# hasql-queue

This module utilizes PostgreSQL to implement a durable queue for efficently processing payloads.

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

```haskell
createAccount userRecord = transaction Serializable Write $ do
  createUser userRecord
  enqueueNotification "queue_channel" emailEncoder [makeVerificationEmail userRecord]
```

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

```haskell
  -- Wait for a single new record and try to send the email 5 times for giving
  -- up and marking the payload as failed.
  forever $ withDequeue "queue_channel" conn emailDecoder 5 1 $
    mapM_ sendEmail
```

In the example above we used the `Session` API for enqueuing and the `IO` for
dequeuing.

The `Session` API is useful for composing larger transactions and the `IO` utilizes PostgreSQL notifications to avoid polling.

## Installation

```bash
stack install hasql-queue
```