Eagerly start subscription heartbeats

[tninesling]

Overview

Eagerly start subscription heartbeats in case the subscription has long-running setup steps before returning its async generator. When a subscription is configured, subscribe can return either an AsyncGenerator or a Promise<AsyncGenerator>. If an AsyncGenerator is returned, the previous code works fine. We get the generator right away and set up the heartbeat to keep things alive, even if the generator has long-running setup code before returning values.

On the other hand, if the Promise<AsyncGenerator> takes longer than the hearbeat interval to resolve, the router will terminate the subscription. When the subscription actually starts generating data, the callback returns 404, as it has already been cleaned up. We want the system to gracefully wait for the setup code to complete, since we will do so if the setup code is placed inside the iterator itself.

Example

In this example, subscribe returns a Promise<AsyncGenerator>, but the Promise resolves quickly. This results in the heartbeat being started prior to the long-running setup. The heartbeat keeps the subscription alive until the setup is completed, then it can start sending data to the client.

return {
    subscribe: async () => {
        return {
            async *[Symbol.asyncIterator]() {
                // The heartbeat is already started, so this can wait as long as it wants
                await setupWithLongDelay();
                while (running) {
                    yield data;
                }
            },
        };
    },
}

On the other hand, waiting for the setup before resolving the Promise leads to complications.

return {
    subscribe: async () => {
        // The router kills this request because it doesn't start the heartbeat in time
        await setupWithLongDelay(); 
        return {
            async *[Symbol.asyncIterator]() {
                while (running) {
                    yield data;
                }
            },
        };
    },
}

Solution

This change eagerly starts the heartbeat interval prior to awaiting the returned value from subscribe. This allows long-running setup code to run concurrently with the heartbeats. Previously the second example would result in a SUBSCRIPTION_HEARTBEAT_ERROR and subscription termination. Now, the router receives heartbeats so it correctly waits for the setup and data in both cases.

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[trevor-scheer]

Looks great!