Design Pattern: Pub/Sub

Ori · May 3, 2020, 8:20am

A useful design pattern for inter-canister comms:

dfinity/examples/blob/master/motoko/pubsub/README.md

# Design Pattern: Pub/Sub

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This sample project demonstrates how functions may be passed as arguments of inter-canister calls to be used as callbacks.

### Overview

A common problem in both distributed and decentralized systems is keeping separate services (or canisters) synchronized with one another. While there are many potential solutions to this problem, a popular one is the Publisher/Subscriber pattern or "PubSub". PubSub is an especially valuable pattern on the Internet Computer as its primary drawback, message delivery failures, does not apply.

### Implementation

The first canister (Publisher) exposes a `subscribe` method that other canisters can call to register a callback to be executed whenever its other method `publish` is called with an event matching the subscribed topic.

The second canister (Subscriber) updates its internal count when its `updateCount` method is called.

Note: There are many obvious improvements (keying subscribers by topic in Publisher, validating the topic in the callback) and callbacks can do much more complex things than update counters but hopefully this example illustrates the concepts in a simple way.

### Prerequisites

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Ori · June 26, 2020, 9:58am

Some good reference on passing actors and callbacks in Motoko for this: https://sdk.dfinity.org/docs/language-guide/sharing.html

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Design Pattern: Pub/Sub

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