[Waterluvian]

I have never found a good example of a Python web server that provides some mechanism for statefulness. Is it just fundamentally not possible to have shared state among requests handled by the threads of a process? Sanic's examples seem to be the same as Flask's: self-contained function calls attached to endpoints.

I keep hitting a wall with Python when I want to do something like:

1. subscribe to a websocket connection and keep the last received message in state 2. expose an http endpoint to let a client GET that last message.

[bobbyi_settv]

You normally use something like redis to store the state.

If you were going to share state in memory between threads, how would you handle the case where the second request goes to a different server or that the process has restarted? You'd need redis anyway, so you might as well just use it in all cases.

[Waterluvian]

I get that everyone's responses are thinking some big public thing. I'm thinking small toy implementation for my home network.

The toy experiment is how to do what's trivial in Node with Python. Mainly because I like working with python. I think the answer might be: Python is the wrong tool for the job.

[rcarmo]

Erm, no. You can do shared thread storage, in Python, it's just that it doesn't really scale. I've done it for small daemons without significant hassle, and even wrote my own Go-like CSP helper: https://github.com/rcarmo/python-utils/blob/master/taskkit.p...

[zeptomu]

The problem is that accessing shared state concurrently in a multi-process context is a non-trivial problem, so specific software emerged that handles these problems for you.

The simplest solution is to use a small DB system like sqlite. It is built into Python (import sqlite3) performs reasonably well and you do not have to run an additional service.

Now if a small DB like sqlite already feels overblown to you (and it really is simple and small) you might not need concurrent access either, so the simplest solution is to just use a file where you store your state.

[icebraining]

I'd say the simplest solution is a global (or just shared) variable using a thread-safe container like queue.Queue. There's also multiprocessing.Queue, which supports sharing the queue across multiple workers.

[bpicolo]

Having multiple nodejs processes is the same thing as having multiple python processes w/ regards to sharing state.

What you're referring to works equally well in the single-process case for both.

[rbutcher]

This maybe seems complicated because Node has 1 obvious way to run (single threaded with asynchronous functions) but Python has a few ways (single threaded, multithreaded, ioloops kind of like Node, greenlets).

Python is excellent for toy implementations, and real ones too in many cases.

[netheril96]

Is multithreading really necessary for a toy?

[mkj]

It probably is for long-polling or websockets?

https://github.com/mkj/wort-templog/blob/master/web/templog.... is my not-quite-toy example - a single process runs from uwsgi with Bottle (like Flask) and gevent. The long polling waits on a global Event variable that's updated by another request, nice and simple.

[raziel2p]

Is that really something you want to do in-memory? Once you have to start multiple worker processes or application servers behind a load balancer, you'll have to re-implement it with some sort of shared persistant store like Redis.

[icebraining]

Not really, you can use a multiprocessing.Manager to share a plain old dict or list across multiple worker processes: https://docs.python.org/3/library/multiprocessing.html#shari...

[cwilkes]

I think the OP was talking about different VMs behind a load balancer where there is no shared memory at all.

[icebraining]

Wasn't my impression (using worker processes in the same machine is common), but fair enough. On the other hand, message passing across machines is overrated. We run a SaaS service on 25 VMs with no communication between them for regular operation.

[weberc2]

Flask lets you share state between requests; just have the route methods reference some global variable. You could also apply the route decorator to an instance method (although probably not using the decorator syntax).

[quinnftw]

You can use caching to mimic this behaviour in Flask, I.e.

http://flask.pocoo.org/docs/0.12/patterns/caching/

I'm not sure how this works with multiple threads though, I imagine you would have to synchronize it yourself.

[tyingq]

Redis's lpush, and rpop plus some naming scheme might suffice. Everything is atomic, so the thread bit is covered.

[pdmccormick]

Your request has stayed on my mind over the past few days, so I put this together for you: https://github.com/pdmccormick/sample-socketio-chat-app

Enjoy!

[chronial]

I am a little confused here. What's keeping you from storing your state in a global variable?