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https://github.com/PaulGilmartin/django-pgpubsub

A distributed task processing framework for Django built on top of the Postgres NOTIFY/LISTEN protocol.
https://github.com/PaulGilmartin/django-pgpubsub

django message-queue postgres

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A distributed task processing framework for Django built on top of the Postgres NOTIFY/LISTEN protocol.

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django-pgpubsub

===============



``django-pgpubsub`` provides a framework for building an asynchronous

and distributed message processing network on top of a Django application

using a PostgreSQL database. This is achieved by leveraging Postgres'

[LISTEN/NOTIFY](https://www.postgresql.org/docs/current/sql-notify.html)

protocol to build a message queue at the database layer.

The simple user-friendly interface,

minimal infrastructural requirements and the ability to leverage Postgres'

transactional behaviour to achieve exactly-once messaging, makes

``django-pgpubsub`` a solid choice as a lightweight alternative to AMPQ

messaging services, such as

[Celery](https://docs.celeryq.dev/en/stable/search.html?q=ampq)



Documentation

-------------



https://django-pgpubsub.readthedocs.io/en/latest/



Primary Authors

---------------

* [Paul Gilmartin](https://github.com/PaulGilmartin)

* [Wesley Kendall](https://github.com/wesleykendall)



Highlights

==========



- **Minimal Operational Infrastructure**: If you're already running a Django application

  on top of a Postgres database, the installation of this library is the sum total

  of the operational work required to implement a framework for a distributed

  message processing framework. No additional frameworks or technologies

  are required.



- **Integration with Postgres Triggers (via django-pgtrigger)**:

  To quote the [official](https://www.postgresql.org/docs/current/sql-notify.html)

  Postgres docs:



  *"When NOTIFY is used to signal the occurrence of changes to a particular table,

  a useful programming technique is to put the NOTIFY in a statement trigger that is triggered

  by table updates.

  In this way, notification happens automatically when the table is changed,

  and the application programmer cannot accidentally forget to do it."*



  By making use of the ``django-pgtrigger``

  [library](https://pypi.org/project/django-pgtrigger/), ``django-pgpubsub``

  offers a Django application layer abstraction of the trigger-notify Postgres

  pattern. This allows developers to easily write python-callbacks which will

  be invoked (asynchronously) whenever a custom ``django-pgtrigger`` is invoked.

  Utilising a Postgres-trigger as the ground-zero for emitting a

  message based on a database table event is far more robust than relying

  on something at the application layer (for example, a ``post_save`` signal,

  which could easily be missed if the ``bulk_create`` method was used).



- **Lightweight Polling**: we make use of the Postgres ``LISTEN/NOTIFY``

  protocol to have achieve notification polling which uses

  [no CPU and no database transactions unless there is a message to read.](https://www.psycopg.org/docs/advanced.html#asynchronous-notifications)

  (Note: This is true when using the `psycopg2` library, which is the default. `psycopg3` is supported,

   but polling incurs a small CPU cost to fetch the notifications.

   We hope to fix this in the future once `psycopg3` makes it easier to poll for notifications client side).



- **Exactly-once notification processing**: ``django-pgpubsub`` can be configured so

  that notifications are processed exactly once. This is achieved by storing

  a copy of each new notification in the database and mandating that a notification

  processor must obtain a postgres lock on that message before processing it.

  This allows us to have concurrent processes listening to the same message channel

  with the guarantee that no two channels will act on the same notification. Moreover,

  the use of Django's ``.select_for_update(skip_locked=True)`` method allows

  concurrent listeners to continue processing incoming messages without waiting

  for lock-release events from other listening processes.



- **Durability and Recovery**: ``django-pgpubsub`` can be configured so that

  notifications are stored in the database before they're sent to be processed.

  This allows us to replay any notification which may have been missed by listening

  processes, for example in the event a notification was sent whilst the listening

  processes were down.



- **Atomicity**: The Postgres ``NOTIFY`` protocol respects the atomicity

  of the transaction in which it is invoked. The result of this is that

  any notifications sent using ``django-pgpubsub`` will be sent if and only if

  the transaction in which it sent is successfully committed to the database.



Quick start

===========



Prerequisites

-------------



Before using this library, you must be running Django 2.2 (or later) on top

of a (single) PostgreSQL 11 (or later) database.



Installing

----------



    pip install django-pgpubsub



``django-pgpubsub`` ships with a ``Notification`` model. This table must

be added to the app's database via the usual django ``migrate`` command.

We should also add `pgpubsub` and `pgtrigger` into `INSTALLED_APPS`.

Additionally, if we wish to run the `pgpubsub` tests, we need to add

`pgpubsub.tests` into `INSTALLED_APPS` too.



Minimal Example

---------------



Let's get a brief overview of how to use ``pgpubsub`` to asynchronously

create a ``Post`` row whenever an ``Author`` row is inserted into the

database. For this example, our notifying event will come from a

postgres trigger, but this is not a requirement for all notifying events.

A more detailed version of this example, and an example which

does not use a postgres trigger, can be found in the

**Documentation (by Example)** section below.



**Define a Channel**



Channels are the medium through which we send notifications.

We define our channel in our app's ``channels.py`` file as a dataclass

as follows:



```python

from dataclasses import dataclass



from pgpubsub.channel import TriggerChannel

from pgpubsub.tests.models import Author



@dataclass

class AuthorTriggerChannel(TriggerChannel):

    model = Author

```



**Define a Listener**



A *listener* is the function which processes notifications sent through a channel.

We define our listener in our app's ``listeners.py`` file as follows:



```python

import datetime



import pgpubsub

from pgpubsub.tests.channels import AuthorTriggerChannel

from pgpubsub.tests.models import Author, Post



@pgpubsub.post_insert_listener(AuthorTriggerChannel)

def create_first_post_for_author(old: Author, new: Author):

    print(f'Creating first post for {new.name}')

    Post.objects.create(

        author_id=new.pk,

        content='Welcome! This is your first post',

        date=datetime.date.today(),

    )

```



**Note that since ``AuthorTriggerChannel`` is a trigger-based channel, we need

to perform a ``migrate`` command after first defining the above listener

so as to install the underlying trigger in the database.**



Finally, we must also ensure  that this listeners.py module is imported into the app's config

class. In this example, our app is calls "tests":



```python

# tests/apps.py

from django.apps import AppConfig



class TestsConfig(AppConfig):

    name = 'tests'



    def ready(self):

        import pgpubsub.tests.listeners

```



**Start Listening**



To have our listener function listen for notifications on the ``AuthorTriggerChannel``,

we use the ``listen`` management command:



    ./manage.py listen



Now whenever an ``Author`` is inserted into our database, our listener process creates

a ``Post`` object referencing that ``Author``:



https://user-images.githubusercontent.com/18212082/165683416-b5cbeca1-ea94-4cd4-a5a1-81751e1b0feb.mov



Live Demos

==========



`bulk_create` over several processes

------------------------------------



In the below example we show how `pgpubsub` handles a bulk creation

of ``Author`` objects when several processes are listening to the

``AuthorTriggerChannel`` channel. For the sake of the below demonstration,

we added a `time.sleep(3)` statement into the `create_first_post_for_author`

listener function. Note how only one processes is able to process any given

notification:



https://user-images.githubusercontent.com/18212082/165823588-df91e84a-47f2-4220-8999-8556665e3de3.mov