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PubSub

View this sample's code on GitHub

Overview​

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

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.

Prerequisites​

This example requires an installation of:

  • Install the IC SDK.
  • Clone the example dapp project: git clone https://github.com/dfinity/examples

Begin by opening a terminal window.

Step 1: Navigate into the folder containing the project's files and start a local instance of the replica with the command:​

cd examples/motoko/pub-sub
dfx start --background

Step 2: Deploy the canister:​

dfx deploy

Step 3: Subscribe to the "Apples" topic:​

dfx canister call sub init '("Apples")'

Step 4: Publish to the "Apples" topic:​

dfx canister call pub publish '(record { "topic" = "Apples"; "value" = 2 })'

Step 5: Receive your subscription:​

dfx canister call sub getCount

The output should resemble the following:

(2 : nat)

Security considerations and best practices​

If you base your application on this example, we recommend you familiarize yourself with and adhere to the security best practices for developing on the Internet Computer. This example may not implement all the best practices.

For example, the following aspects are particularly relevant for this app, since it makes inter-canister calls: