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Eagerly start subscription heartbeats (#7871)
# 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.- Loading branch information
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,5 @@ | ||
| --- | ||
| "@apollo/server": patch | ||
| --- | ||
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| Subscription heartbeats are initialized prior to awaiting subscribe(). This allows long-running setup to happen in the returned Promise without the subscription being terminated prior to resolution. |
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