Can this be modeled on top of async hooks

[jkrems]

Things like asynchronous operations at the very least could be expressed as (virtual) async resources. An interesting question would be if this is sufficient to express something like a "diagnostics channel" by filtering for specific async_hooks resources.

[MSLaguana]

I believe that async_hooks is not enough by itself, because of issues with userspace queueing. Essentially, if I have code like the following:

// Async context 1
function foo() {
  doAsyncThing("data", function () { console.log('Called back eventually')})
}

// Async context 2
function bar() {
  doAsyncThing("more data", function () { console.log('called back after starting in a different context');})
}

function doAsyncThing() {
  //...
  manager.jobs.push({data, callback});
  //...
}

// Elsewhere, asynchronously
manager.onFreeWorker = function () {
  worker.jobs = this.jobs;
  this.jobs = [];
  worker.work().then((completedJobs) => {
    for (job of completedJobs) {
      job.callback();
    }
  }
}

What context are the callbacks invoked in? There are at least 3 different asynchronous contexts here (suppose the first 2 are different HTTP requests coming in, and the worker is communicating the jobs with an out-of-process worker like a database).

My understanding of async hooks would let you associate a single stack of calls together, for example the promises in onFreeWorker might be associated with the context that invoked onFreeWorker, but that isn't either of the 2 http contexts. And each callback invoked at the end is invoked synchronously, within a single async context, so how would they be distinguished and assigned to the correct original context?

[jkrems]

I wasn't even talking about the async tracking, more about the data being emitted / the "diagnostics event bus". E.g. creating an AsyncResource that represents an individual mongodb interaction.

The userland callback queue can be implemented using the embedder API of async hooks so the context tracking shouldn't be an issue for properly instrumented libraries in node 8+.