pgmq

A message queue built on top of Postgres.

Purpose

Sometimes you have a Postgres database and need a queue. You could stand up more infrastructure (SQS, Redis, etc), or you could use your existing database. There are plenty of use cases for a persistent queue that do not require infinite scalability, snapshots or any of the other advanced features full fledged queues/event buses/message brokers have.

Features

• Best effort at most once delivery (messages are only delivered to one worker at a time)
• Automatic redelivery of failed messages
• Low latency delivery (near realtime, uses PostgreSQL's NOTIFY feature)
• Low latency completion tracking (using NOTIFY)
• Bulk sending and receiving
• Fully typed async Python client (using asyncpg)
• Persistent scheduled messages (scheduled in the database, not the client application)

Possible features:

• Exponential backoffs
• Dead letter queues
• Custom delivery ordering key
• Message attributes and attribute filtering
• FIFO delivery
• Backpressure / bound queues

Unplanned features:

• Sending back response data (currently it needs to be sent out of band)
• Supporting "subscriptions" (this is a simple queue, not a message broker)

Examples

from contextlib import AsyncExitStack

import anyio
import asyncpg  # type: ignore
from pgmq import create_queue, connect_to_queue, migrate_to_latest_version

async def main() -> None:

    async with AsyncExitStack() as stack:
        pool: asyncpg.Pool = await stack.enter_async_context(
            asyncpg.create_pool(  # type: ignore
                "postgres://postgres:postgres@localhost/postgres"
            )
        )
        await migrate_to_latest_version(pool)
        await create_queue("myq", pool)
        queue = await stack.enter_async_context(
            connect_to_queue("myq", pool)
        )
        async with anyio.create_task_group() as tg:

            async def worker() -> None:
                async with queue.receive() as msg_handle_rcv_stream:
                    # receive a single message
                    async with (await msg_handle_rcv_stream.receive()).acquire():
                        print("received")
                        # do some work
                        await anyio.sleep(1)
                        print("done processing")
                        print("acked")

            tg.start_soon(worker)
            tg.start_soon(worker)

            async with queue.send(b'{"foo":"bar"}') as completion_handle:
                print("sent")
                await completion_handle()
                print("completed")
                tg.cancel_scope.cancel()


if __name__ == "__main__":
    anyio.run(main)
    # prints:
    # "sent"
    # "received"
    # "done processing"
    # "acked"
    # "completed"

Development

1. Clone the repo
2. Start a disposable PostgreSQL instance (e.g docker run -it -e POSTGRES_PASSWORD=postgres -p 5432:5432 postgres)
3. Run make test

See this release on GitHub: v0.1.0