解决未决的并行承诺

Question

I have recently upgraded a HAPI installation to v17, which uses promises, and am facing a situation I do not know how to resolve. This is not HAPI-specific...

When a certain route is called, a check is made to see if the results of a particular service call have been cached. If so, we skip the call and use the cached results - otherwise, a service call is made, which involves a promise.

The problem is, if the server is started up and the results have not been cached, and we run a load test, suddenly there's a flood of calls to the service to get the results, because they haven't been cached, yet, and a bunch of requests just came in at the same time.

What I want to do is only let the first request fire the service call, and just let all subsequent requests wait (via a promise?) for that call to return - then all pending promises can be resolved to let the other requests continue.

How do I structure this? In the past, using callbacks, I would just add the callbacks to a list, then call all the callbacks in that list once the results were in. Can I somehow do something similar, but stacking up promises until the call comes back, then resolve them all? I'm still new to promises, so having trouble visualizing it.

The basic simplified logic in place, currently, is:

function getData() {
    return new Promise(...);
}

...

if (!data) {
    data = await getData();
}
return data;

Rather than just calling getData() if data is not cached, I want to set a flag when getData sends a request to get it, and have subsequent calls wait for that request to complete to pick up data and move on, rather than sending yet another request for it.

Thanks!

Answer 1

You may cache promise itself instead of caching data + flag-to-see-if-data-retrieved.

function loadData() {
    someOuterVariable = fetch(...).then(/*transforming data structure*/)
}

async function dataConsumer() {
    const data = await someOuterVariable; // does not matter if it has been retrieved or not
    .... 
}

async function anotherConsumer() {
    ....
    const item = (await someOuterVariable).filter(/* just an example of inline processing*/)
}

And it does not matter if data has already been retrieved or not - Promise is still Promise.

The only limitation is - you can update that someOuterVariable with new Promise but if any consumer function has already started to await - it will get old data from previous Promise. But according to your description it's not your case to substitute promise until it has been fulfilled, is it?