implement domain-like hooks (asynclistener) for userland

[trevnorris]

Here's the current API planned for implementation:

// Class that will store information about a specific request.
function Storage() { }

// This will be called every time asynchronous work is queued.
// The returned data will propagate to the callbackObject's callbacks.
function onCreation() {
  return new Storage();
}

// Set of methods that will run at specific points in time according to
// their cooresponding callbacks. These methods are always run FIFO if
// multiple are queued.
// "context" is the "this" of the request object.
var callbackObject = {
  before: function asyncBefore(context, storageValue) {
  },
  // "returnValue" is the value returned from the callback.
  after: function asyncAfter(context, storageValue) {
  },
  // If this callback returns "true" then the error handler will assume
  // the error was properly handled, and process will continue normally.
  // If multiple error handlers are queued, and any one of those returns
  // true, then Node will assume the error was properly handled.
  // This will not currently be passed the context (or "this") of the
  // callback that threw. A simple way of achieving this is currently
  // being investigated, and the feature will be added when one is found.
  error: function asyncError(storageValue, err) {
  }
};

/**
 * process.addAsyncListener(callback[, object[, value]]);
 *
 * Arguments:
 *
 * callback - Function that will be run when an asynchronous job is
 *    queued.
 *
 * object - Object with the optional callbacks set on:
 *    before - Callback called just before the asynchronous callback
 *        will be called.
 *    after - Callback called directly after the asynchronous callback.
 *    error - Callback called if there was an error.
 *
 * The returned key is an Object that serves as the unique key for the
 * call (much like how Timers work).
 */
var key = process.addAsyncListener(onAsync, callbackObject);


/**
 * process.createAsyncListener(callback[, object[, value]]);
 *
 * Adding an async listener will immediately add it to the queue and
 * being listening for events. If you wish to create the listener in
 * advance, to say attach to the returned value object, it's possible
 * to get the key and pass it to process.addAsyncListener() later.
 */
var key = process.createAsyncListener(onAsync, callbackObject, value);

// Then activate like so:
process.addAsyncListener(key);


/**
 * process.removeAsyncListener(key);
 *
 * Remove any async listeners and associated callbackObjects. All
 * listeners will live independent of each other, and there will be no
 * method given to clear all async listeners.
 */
process.removeAsyncListener(key);

[othiym23]

This would meet my requirements, as long as the async listener also fires for the timer functions and is invoked from Make(Domain)Callback. Also, I'm pretty sure that if you're going to expose a generic hook like this, it's going to need the ability to accept multiple listeners. If those are pieces you plan to add to this, 👍 from me.

[trevnorris]

@othiym23 thanks for the feedback. that was along the lines of what I was attempting. exposing multiple listeners is easy. already have a patch for it, but removed it when I realized that users could easily wrap callbacks in callbacks in callbacks. and there's no guaranteed order of firing. also, do I only allow the same listener to be added once, or can they add multiple? it was getting complicated fast so it was dropped so I could get something solid posted.

i'll happily add it back if there's a clear road of where it should go.

[othiym23]

I think the most sensible, intuitive way for this to work would be to run the listeners in the order that they were registered via addAsyncListener, with each listener receiving the callback returned by the previous listener. It's up for the writers of the listeners to ensure that they're safe in how they create the closures. Regardless, the order in which they're run must be deterministic and stable (i.e. using an array for storage, not an object).

It's hard to avoid some difficulty with ordering dependencies, though. It might be enough to offer two calls to add a listener -- insertAsyncListener and addAsyncListener. If that's insufficient, giving modules that register listeners the power to dump and reorder the list of listeners might be necessary. I can't imagine a situation in which you'd want to have more than a couple of these listeners at a time (oh, the performance costs!), but you're right that there's an important semantic distinction to draw between outer and inner wrappers.

I don't really have it clear in my head, but it seems like it could be possible to use that object you were talking about yesterday as a means for these wrappers to share their state, and maybe externalize whatever data might otherwise subject to dependencies. But down that road lies a whole bunch of other annoying complications and complexity.

[creationix]

I just sent pull requests to CLS to use this API and to CLS-Glue to prollyfill this API.

• Integrate with process.addAsyncListener othiym23/node-continuation-local-storage#2
• Implement process.addAsyncListener polyfill othiym23/node-continuation-local-storage-glue#2

@trevnorris did I implement the proposed API correctly in the polyfill?

[creationix]

@othiym23 Also, once/if this lands in core and CLS becomes peer to Domains instead of domains being implemented on top of CLS, I recommend simplifying the CLS API and implementation to be just data storage. https://gist.github.com/creationix/d531157ad587a4af9f4e#file-multi-cls-js

[othiym23]

@creationix: I went ahead and split your proposed changes out into an entirely separate module, async-listener. I then went and included that polyfill directly in continuation-local-storage, putting the relevant tests in relevant places. I pulled Trevor's tests from this PR, and a few of his test cases are currently failing, but I'll bang on that some more tomorrow.

@trevnorris, what this means is that we now have a polyfill for versions of node < (whatever version of Node this lands in). I'll keep it up to date with your changes, and you can grab a copy of continuation-local-storage and run its tests against your fork if you want another source of test coverage.

[trevnorris]

@othiym23 Thanks. Trying to have basics working by NodeConf.eu, but working around everything already in place has been a little painful. To be honest one problem is that I'm catching more asynchronous callbacks then expected. Which make the tests unpredictable. So I need to trace down where every one is occurring so they can be accounted for.

[bnoordhuis]

I thought domains were going to be implemented in terms of async listeners?

[trevnorris]

Still in the middle of figuring that out. Domains are so tightly coupled
with the EventEmitter that it's been a little difficult.

[isaacs]

Everything relevant is landed in master now, right? Reopen if there's still work to do here.

Thanks for all the work on this, @othiym23, @jacobgroundwater, and especially @trevnorris.

[refack]

@trevnorris why arenet incomming events created out-of band?
for the following code the second assert fails

                namespace.set('test', TEST_VALUE);
                server = net.createServer();
                server.on('connection', function OnServerConnection(socket) {
                    expect(namespace.get('test')).equal(TEST_VALUE, "state should be preserved");  // Still have state
                    socket.on("data", function OnServerSocketData(data) {
                        expect(namespace.get('test')).equal(TEST_VALUE, "state should still be preserved :(");  // cls is lost

[bnoordhuis]

@refack I'm not sure why you direct that question to me.

[refack]

Sorry ment to ping @trevnorris

[trevnorris]

@refack If I understand the question, it's because internally we don't know whether you're creating a Pipe or a TCP connection until you call listen(). It's a total PITA. I have another PR open that does some work on this, but still under debate how it should be merged.

[RobinQu]

Is it landed on any released version?
I still cannot find it in the latest(v0.12.2)

[trevnorris]

@RobinQu Never landed.

[gobwas]

So, will node have this feature some day?

[trevnorris]

@gobwas It partially exists as process.binding('async_wrap'). Though the ability to capture exceptions was removed. Became a massive hair ball. Would still like to do this, but time constraints.

EDIT: It's in io.js, not node.js.

[gobwas]

Thank you, @trevnorris!

Does this feature present in iojs docs?

Any way, I am asking about async-listener functionality at the moment..?

[trevnorris]

Oh, that died a horrible death. After 3 months of work I still couldn't figure out a few key edge cases. So instead I've been working on the set of hooks necessary to implement a feature like async-listener. This way others can experiment and publish their own modules.

[gergelyke]

Hi @trevnorris ,

could you give us an update on the current status of this? What would be your suggestion to use these days if we want to active the same functionality?

Thanks!

cc @Peteyy

[trevnorris]

@gergelyke Unfortunately there isn't an alternative. Will have to continue using domains for the time being. Though discussion about reimplementing this has started back up. Hopefully it will lead to something.

[gergelyke]

thanks!

[naorye]

What is wrong with the Domain solution?

[trevnorris]

@naorye It's a complete hack that injects itself all over core code. By design it can't properly catch everything. There are edge cases it's missing. This is because it's injected at the JS API level, not at the native level where the asynchronous requests are actually made.

[Olegas]

What alternatives are now being discussed?

[jiaqifeng]

@trevnorris Is there any other solutions based on native layer of this problem currently available? I am a Java coder, I think if this should be done on VM layer which make things easier and more efficient.

[trevnorris]

@jiaqifeng In master there's a native API that can be used (see all functions that use async_context in node.h). This will be backported to v8.x, but no further.