pypubsub

PyPubSub - An asynchronous pubsub protocol written in Python 3

Introduction

PyPubSub is a simple publisher/subscriber service, where clients can connect and either deliver a payload (in JSON format) or listen for specific payloads as a stream of events. It is written as an asynchronous Python service, and can handle thousands of connections at any given time on a single core. It utilizes the HTTP protocol and JSON for a simplistic delivery scheme.

A working copy of this program is in service by the Apache Software Foundation, listing all development events going on at the organization (see this page for an introduction to their service).

Installing

• Download or clone this repository: git clone https://github.com/Humbedooh/pypubsub.git
• Install dependencies: pip3 install -r requirements.txt
• Edit pypubsub.yaml and (for ACL) pypubsub_acl.yaml to fit your needs
• Launch the program in the foreground or as a systemd service: python3 pypubsub.py
• Check that your pubsub service is working: curl -I http://localhost:2069

Topics and publishing/subscribing

PyPubSub is designed around topics for both publishing and subscribing. I client can use topics to describe what an event is for when publishing, as well as what a client expects to subscribe to. Subscriptions are made on a "highest common denominator" basis, meaning the more topics you subscribe to, the fewer events you will receive, as the topics of an event must, at least, match all the topics a subscriber has subscribed to. Topics are set using the path segment of a URI, and are order agnostic, meaning fruits and apples is the same as apples and fruits internally.

As an example, let's imagine we wish to subscribe to all events for the topics surrounding apples, which is a sub-topic of fruits. We would then subscribe to http://localhost:2069/fruits/apples and listen for events.
If a payload with fruits/apples comes in, we would receive it. If a payload with just fruits come in, we would not receive it, because we are specifically asking for apples to be present as a topic. Neither would fruit/oranges match our subscription, while fruits/apples/macintosh would, as it contains our topics (and a bit more).

The below matrix shows how subscription paths match topics:

Topics	/fruits	/fruits/apples	/fruits/apples/red	/fruits/oranges
fruits	✓	✗	✗	✗
fruits + apples	✓	✓	✗	✗
fruits + apples + red	✓	✓	✓	✗
fruits + oranges	✓	✗	✗	✓

Pushing an event to PyPubSub

Event payloads requires that the IP or IP range (Ipv4 or IPv6) is listed in pypubsub.yaml under payloaders first. Once whitelisted, clients can do a POST or PUT to the pubsub service on port 2069, passing a JSON object as the request body, for instance:

curl -XPUT -d '{"text": "Apples are delicious"}' http://localhost:2069/fruits/apples

Event payloads MUST be in dictionary (hash) format, or they will be rejected.

On the subscription side, any client listening to http://localhost:2069/fruits or http://localhost:2069/fruits/apples will receive the following event in their stream:

{
  "text": "Apples are delicious",
  "pubsub_topics": ["fruits", "apples"],
  "pubsub_path": "/fruits/apples"
}

Pushing an event via Python

To push an event to PyPubSub via Python, you can make use of the requests library in Python:

import requests
requests.put('http://localhost:2069/fruits/apples', json = {"cultivar": "macintosh"})

Listening for events in PyPubSub via cURL

You can subscribe to topics via cURL like so: curl http://localhost:2069/topics/here where topics/here are the topics you are subscribing to, with / as a delimiter between topics. To subscribe to all events, you can omit the topics.

Listening for events via Python

For Python, you can import the asfpy package via pip and utilize its pubsub plugin:

import asfpy.pubsub

def process_event(payload):
    print("we got an event from pubsub")
    ...

def main():
    pubsub = asfpy.pubsub.Listener('http://localhost:2069')
    pubsub.attach(process_event) # poll forever

Access-Control-List and private events

PyPubSub supports private events that only authenticated clients can receive.

Pushing a private event

To mark an event as private, simply prepend private as the first topic when you push the event:

curl -XPUT -d '{"private_text": "Squeamish Ossifrage"}' http://localhost/private/topics/here

Retreiving private events

Clients ACL is defined in pypubsub_acl.yaml (and is entirely optional, you can omit the file). See the example ACL configuration for an example. Access is, as with public events, defined with "highest common denominator" in mind, meaning access to topics is granted to the specific topic group specified in the yaml and its sub-groups. Thus, if you grant access to internal and foo in one ACL segment, events pushed to private/internal/foo would be seen by that client, whereas pushes to private/internal/bar would not.

To authenticate and receive private events, use Basic authentication, such as:

curl -u 'user:pass' http://localhost:2069/internal/topics/here

LDAP-based ACL

PyPubSub supports ACL via asynchronous LDAP, either through group memberships or single users via their dn.

See pypubsub.yaml for an LDAP example.

License

PyPubSub is licensed under the Apache License v/2.