Overview

PyMom is a Python framework that enables the creation of event-driven systems leveraging messaging systems like Kafka and Google's Cloud Pub/Sub.

Installation

This repo can be installed with pip:

pip3 install pymom

Configuration

PyMom expects the environment variable PYMOM_CONFIG_FILE to be set to the name of the configuration file. This file specifies the Kafka bootstrap brokers and Zookeeper servers in this format:

[DEFAULT]
kafka-bootstrap-brokers = localhost:9092
zookeeper-servers = localhost:2181

Message Formats

The current implementation of PyMom uses the PyMomMessage format. All messages on Kafka are in JSON format and must include at least two elements:

• id is a key that correlates multiple related messages. It is used for partitioning Kafka topics.
• payload is the content of the message. It can be any valid JSON.

Producing Messages

PyMom provides access to a class for writing messages to a Kafka topic. PyMomTestProducer.py demonstrates how to use it:

pymom = PyMom()
producer = pymom.producer('test.pymom.consume')  # The topic name.
producer.write(sys.argv[1],sys.argv[2])  # The ID and payload.

Consuming Messages

To receive messages from Kafka, a class must derive from PyMomAbstractConsumer and implement the on_message method. The class must then be registered with PyMom. PyMomTestConsumer.py demonstrates how to do this:

class PyMomTestConsumer(PyMomAbstractConsumer):
    def __init__(self,pymom):
        self.pymom = PyMom()
        self.producer = self.pymom.producer('test.pymom.produce')

    def on_message(self,message):
        """ Process messages. """
        id = message['id']
        payload = message['payload']
        print("Received message:  ({}) {}".format(id,payload))
        try:
            self.producer.write(id,payload)
            print("Wrote message.")
        except Exception as error:
            print("Unable to send message:  {}".format(error))


if __name__ == "__main__":
    pymom = PyMom()
    consumer = PyMomTestConsumer(framework)
    pymom.register(consumer,'PyMomTestConsumerGroup','test.pymom.consume')
    pymom.run()

    print("PyMomTestConsumer terminated.")
    sys.exit(0)

on_message can either return normally or throw one of two exceptions. If a recoverable error (e.g an external API is temporarily unavailable) occurs, throw PyMomRecoverableError. If an unrecoverable error (e.g. the message format is invalid) occurs, throw PyMomUnrecoverableError. In both cases, the error is logged. In the case of a recoverable error, the message will be retried. In the case of an unrecoverable error, processing will continue with the next message.