6: Inter-canister calls

Beginner

Rust

Tutorial

Overview

Inter-canister calls can be used to update information between two or more canisters.

To demonstrate these inter-canister calls, you'll use an example project called "PubSub".

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

Before getting started, assure you have set up your developer environment according to the instructions in the developer environment guide.

Then, download the sample project's files with the commands:

git clone https://github.com/dfinity/examples/
cd examples/rust/pub-sub/

Viewing the canister code

This project is comprised of two canisters: publisher and subscriber.

The subscriber canister contains a record of topics. The publisher canister uses inter-canister calls to add topics to the record within the subscriber canister.

Let's take a look at the src/lib.rs file for each of these canisters.

src/publisher/src/lib.rs:

use candid::{CandidType, Principal};
use ic_cdk::update;
use serde::Deserialize;
use std::cell::RefCell;
use std::collections::BTreeMap;

type SubscriberStore = BTreeMap<Principal, Subscriber>;

thread_local! {
    static SUBSCRIBERS: RefCell<SubscriberStore> = RefCell::default();
}

#[derive(Clone, Debug, CandidType, Deserialize)]
struct Counter {
    topic: String,
    value: u64,
}

#[derive(Clone, Debug, CandidType, Deserialize)]
struct Subscriber {
    topic: String,
}

#[update]
fn subscribe(subscriber: Subscriber) {
    let subscriber_principal_id = ic_cdk::caller();
    SUBSCRIBERS.with(|subscribers| {
        subscribers
            .borrow_mut()
            .insert(subscriber_principal_id, subscriber)
    });
}

#[update]
async fn publish(counter: Counter) {
    SUBSCRIBERS.with(|subscribers| {
        // This example is explicitly ignoring the error.
        for (k, v) in subscribers.borrow().iter() {
            if v.topic == counter.topic {
                let _call_result: Result<(), _> =
                    ic_cdk::notify(*k, "update_count", (&counter,));
            }
        }
    });
}

In this code, you can see two inter-canister update calls: fn subscribe(subscriber: Subscriber) and async fn publish(counter: Counter). The first method allows for the publisher canister to make a call to the subscriber canister and subscribe to topics. The second method allows the publisher canister to publish information into a topic in the subscribers canister.

src/subscriber/src/lib.rs:

use candid::{CandidType, Principal};
use ic_cdk::{update, query};
use serde::Deserialize;
use std::cell::Cell;

thread_local! {
    static COUNTER: Cell<u64> = Cell::new(0);
}

#[derive(Clone, Debug, CandidType, Deserialize)]
struct Counter {
    topic: String,
    value: u64,
}

#[derive(Clone, Debug, CandidType, Deserialize)]
struct Subscriber {
    topic: String,
}

#[update]
async fn setup_subscribe(publisher_id: Principal, topic: String) {
    let subscriber = Subscriber { topic };
    let _call_result: Result<(), _> =
        ic_cdk::call(publisher_id, "subscribe", (subscriber,)).await;
}

#[update]
fn update_count(counter: Counter) {
    COUNTER.with(|c| {
        c.set(c.get() + counter.value);
    });
}

#[query]
fn get_count() -> u64 {
    COUNTER.with(|c| {
        c.get()
    })
}

In this code, there are three main methods: two inter-canister update methods and a query method.

The first method, async fn setup_subscribe(publisher_id: Principal, topic: String) provides functionality for the publisher canister to subscribe to topics within the subscriber canister. This function is called by the publisher canister.

The second method, fn update_count(counter: Counter) updates the counter record for each published value in a topic within the subscriber canister.

The third method, fn get_count() -> u64 allows the Counter value to be queried and returned in a call.

Deploying the canisters

Now that you've taken a look at your canisters, let's deploy them.

Open a terminal window on your local computer, if you don’t already have one open.

Then run the commands:

dfx start --clean --background
dfx deploy

Making inter-canister calls

First, let's subscribe to a topic. For example, to subscribe to the "Apples" topic, use the command:

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

Then, to publish a record to the "Apples" topic, use the command:

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

Then, you can query and receive the subscription record value with the command:

dfx canister call subscriber getCount

The output should resemble the following:

(2 : nat)

Next steps

Next, let's cover how to upgrade canisters.