Topic Subscription Filtering

[tjholm]

Add the ability for topic subscriptions to be filtered by some attributes or data related to messages published to that topic, so subscribers can opt in to a subset of messages being published to a topic.

Current Workarounds:

At the moment multiple topics can be created for specialised subscribers.

Challenges

Supporting all existing implementations of the nitric eventing service:

• AWS SNS: Supports attributes and message based filtering (byte based payloads cannot be filtered).
• GCP Pubsub: Only supports simple string attribute based filtering.
• Azure Eventgrid: Supports only message based filtering.

Any and all proposals/suggestions/ideas welcome.

[tjholm]

Proposal - Simple Map based attribute filtering

Add attributes to nitric publish requests that allows additional metadata to be attached to event:

import { topic } from "@nitric/sdk";

const notify = topic('notify');
const notifier = notify.for('publishing');

async function publishEmailNotification(message: string) {
   await notifier.publish(
     { message }, 
     // Publish with attributes
     { type: "email" }
   );
}

notify.subscribe(async () => {
   // handle email notification
}, {
   // subscribe to message with a key 'type' and value 'email'
   type: "email"
});

Existing azure eventgrid implementation can be supported by shimming attributes into the payload and removing them in the nitric azure gateway