Pierre Rochard

Realtime Flask with Socket.io and PostgreSQL

Previous post: Realtime Admin Architecture Experiment

My working demo: https://github.com/PierreRochard/realtime-flask-experiment

While doing research I came across a project that did what I was looking for: send realtime updates from Flask to the browser. Check out Luke Yeager's demo here. The only wrinkle was that he is sending the update to a message queue at the application level while I'm looking to do it at the database level.

There are trade-offs between these two approaches. For performance or scalability I would have to run empirical tests. Architecturally, I think it shouldn't matter what client modifies the record: a notification should always be emitted. To make that happen without repeating notification code in every client app we have to use PostgreSQL's LISTEN/NOTIFY feature. Having said that, it may be the case that those notifications should go to an intermediate message queue like Redis or ZeroMQ. TBD.

I took Luke's demo and extended it to use Brent Tubbs pgpubsub module instead of Redis or ZeroMQ. The relevant code is here. Pgpubsub is a light but incredibly useful wrapper around raw SQL queries, select, and psycopg2.

The minimalism of Luke's demo led me to an interesting insight. He uses DOM manipulation with Javascript to append to an HTML list. I extended this to update or delete existing list items based on incoming socket.io messages here. I realized that I could do this same kind of DOM manipulation with a Flask-Admin Jinja2 template, even if it's rendered server-side. This deviates from my initial plan to create a REST API and use client-side rendering with React, but it would allow me to leverage what the Flask-Admin team has already built.

pierre@rochard.org