async_hooks: AsyncLocalStorage to diagnostics_channel integration #45277

Qard · 2022-11-02T02:55:40Z

This is an alternate and simplified way to express the integration between AsyncLocalStorage and diagnostics_channel attempted in #44894. I think this might be a better approach. What do other @nodejs/diagnostics folks think? I'll write up some docs for it soon, if this seems like a clearer, more coherent approach.

Depends on #44943

nodejs-github-bot · 2022-11-02T02:55:46Z

Review requested:

@nodejs/http
@nodejs/net

mcollina

I find this significantly easier to understand! I have one clarifying question still: why do we need TracingChanel at all if most of this could be achieved by having a convention of .start and .end in diagnostic_channels? The various trace* methods could be implemented as static utilities.

lib/diagnostics_channel.js

mcollina · 2022-11-02T07:58:00Z

lib/diagnostics_channel.js

+      ctx.result = result;
+      asyncEnd.publish(ctx);
+      return result;
+    };


These closures can be costly if there are no subscribers.

Flarna · 2022-11-02T07:47:33Z

lib/async_hooks.js

@@ -351,6 +353,49 @@ class AsyncLocalStorage {
      return resource[this.kResourceStore];
    }
  }
+
+  bindToTracingChannel(channel, transform = (v) => v) {
+    if (typeof channel === 'string') {


Should we throw if it is not a string? If passing a channel object is allowed should be somehow validate it?
Symbols as channel names are also allowed.

Yes, will be adding some validation that if it's not a string it's a tracing channel, just didn't get that far yesterday. Also, while symbols are valid for regular channels, they currently are not for tracing channels due to string concatenation use.

Flarna · 2022-11-02T08:04:24Z

lib/_http_server.js

@@ -1063,6 +1069,10 @@ function parserOnIncoming(server, socket, state, req, keepAlive) {
    server.emit('request', req, res);
  }

+  if (onRequestEndChannel.hasSubscribers) {
+    onRequestEndChannel.publish(dcMessage);


I guess a try block is needed to ensure end is called even if an exception is thrown.

Yep. And also should have the error channel to fully match tracing channel.

Flarna · 2022-11-02T08:04:58Z

lib/async_hooks.js

+      if (store.enabled) {
+        ctx[store.kResourceStore] = store.getStore();
+      }
+      store.enterWith(transform(ctx.data));


I guess transform requires exception handling.

Probably, yes.

Flarna · 2022-11-02T08:09:16Z

lib/diagnostics_channel.js

  unsubscribe,
-  Channel
+  Channel,
+  TracingChannel


Why is it needed to export this?

Oh, I was going to use that for an instanceof check on the AsyncLocalStorage side, just hadn't got to that before I stopped yesterday.

Flarna · 2022-11-02T08:22:39Z

Could you please provide some more background why this is needed in core? I'm sure you have a valid real world usecase in mind so it would be nice if you could share it here.

I'm also a bit concerned about hiding the use of AsyncLocalStorage#enterWith. To my understanding enterWith is still experimental because it's seen as unsafe. Now it's potentially called automagically via diagnostics channel. If the corresponding end event is not emitted (or emitted async or emitted in wrong order) the local store is in bad state.

This is quite problematic in my opinion because the DC publisher doesn't know/control if it's channels get bound to a local store. So they might be not aware at all about this implicit binding and a simple await added in their code can have significant side effects.

If consensus is that enterWith is unsafe and has to stay experimental because of this I would assume this is inherited by TracingChannel. We should have a path out of experimental here to avoid discussions like parts of #45084

Qard · 2022-11-04T20:14:01Z

The intent of this API is to avoid people using enterWith directly. Using it here it's in a controlled environment as the TracingChannel interface handles the event ordering so we can be sure of correctness. As for the use case, there's several reasons for this.

First off, we want AsyncLocalStorage and diagnostics_channel in other environments which necessitates more strictly defining a way to express scoping an AsyncLocalStorage run around trace events from diagnostics_channel. For example, in a Cloudflare worker we would like to have a storage scoped to a request. To do this we can use this interface and Cloudflare providing tracing channel events around the sync start and end of their request handler call. In Node.js core, we can wrap these events around http to scope to an http request.

Additionally we can use it to scope around any tracing channel scope, so an APM could do something like this to manage their span stack:

storage.bindToTracingChannel('mysql.query', data => {
  const parentSpan = storage.getStore()
  return new Span('mysql', data, parentSpan)
})

dc.subscribe('mysql.query.asyncEnd', () => {
  const span = storage.getStore()
  span.end()
})

This will set this span object as the current value of the store between the start and end events of the tracing channel and any descending async activity, and will call the end when the asyncEnd event is reached. Note that this will safely exit the storage scope when the end event happens rather than the current state where we leave the context entered until the end of the tick, which is the concern we have with enterWith currently.

As TracingChannel is designed currently, it is possible to manually emit on the individual named channels and produce out-of-order data, but that's part of why we're specifically trying to provide an API for this, to ensure that people don't do it wrong. Also, the binding is to the individual store and needs to be done explicitly, so I feel it's somewhat on the store owner to ensure they trust the correctness of the tracing channel they want to bind.

Flarna · 2022-11-08T09:09:31Z

lib/async_hooks.js

+      channel = tracingChannel(channel);
+    }
+
+    channel.traceSync(runner, { data });


I took me a while to find the transformation to move the published start data into the .data property which is used in start() closure to call transform.

So it seems like a publisher should use this API from AsyncLocalStore to publish to ensure its TracingChannel fits to AsyncLocalStore. I know it's not strictly required as creating a context object with a data property can be also done in producer code.

But isn't the target to decouple the producing side using TracingChannel and consuming side subscribing and maybe bind to AsyncLocalStore?

Flarna · 2022-11-08T10:53:47Z

As TracingChannel is designed currently, it is possible to manually emit on the individual named channels and produce out-of-order data, but that's part of why we're specifically trying to provide an API for this, to ensure that people don't do it wrong

Do you have already a path in mind to get rid of this possibility to produce out-of-order data?

I fully agree and support the new additions here and a lot use case have no need for enterWith anymore:

in your own code one can use run already now
via TracingChannel a producer signals start/end consistent and by binding a consumer has no longer the need to use enterWith

So the tools are there so only the (small) task left to transform the world to use it :o).

Qard · 2022-11-08T19:22:17Z

Not a specific plan yet, but working on some ideas. We'll see what I can come up with. 🤔

Qard · 2022-11-08T20:51:54Z

I added a new change which now omits enterWith entirely. This eliminates the storage breakage risk. Let me know what you think.

There's some additional things I want to try to make the tracing channel level safer too, but shouldn't matter here anymore. Also, while the binding here now does only depend on one channel, I would still like to keep this on the TracingChannel interface as the purpose is both to bind the store and to provide a convenient tracing wrapper so we don't need to do both things separately everywhere.

…ames

lib/async_hooks.js

lib/diagnostics_channel.js

mcollina

Good work so far.

I'm still unconvinced that we need a new TracingChannel to achieve this vs a bunch of static utilities. We could save a few allocations of TracingChannel as it does not hold any state.

This would also be confusing for users because of the DiagnosticChannel singleton nature.

Flarna · 2022-11-14T18:57:01Z

lib/diagnostics_channel.js

+    for (const key in handlers) {
+      const channel = this.#channels[key];
+      if (!channel || !channel.unsubscribe(handlers[key])) {
+        return false;


Why end the loop just because there is maybe an extra key in the object?

Flarna · 2022-11-14T19:51:18Z

lib/diagnostics_channel.js

+  }
+
+  channel = new TracingChannel(name);
+  channel._weak = new WeakReference(channel);


Suggested change

channel._weak = new WeakReference(channel);

channel._weak = tracingChannels[name] = new WeakReference(channel);

TracingChannel#_weak is a setter, there is no getter otherwise this is always undefined

Flarna · 2022-11-14T20:20:00Z

lib/async_hooks.js

+    };
+  }
+
+  static runInTracingChannel(channel, data, runner) {


I assume this needs thisArg, ...args to be forwarded to the runner via traceSync.

just noticed that AsyncLocalStore#run has no support to provide thisArg...

Flarna · 2022-11-14T20:43:57Z

AsyncLocalStorage.runInTracingChannel() calls now traceSync.
Do you plan to add similar functions for in AsyncLocalStorage which wrap traceCallback and tracePromise?

Which criteria do we define for producers to decide if they should call traceSync vs runInTracingChannel?

Flarna · 2022-11-14T20:44:24Z

lib/async_hooks.js

@@ -9,6 +9,7 @@ const {
  FunctionPrototypeBind,
  NumberIsSafeInteger,
  ObjectDefineProperties,
+  ObjectPrototypeHasOwnProperty,


Flarna · 2022-11-14T20:45:27Z

lib/_http_server.js

-    });
+    } : undefined;
+
+  if (onRequestStartChannel.hasSubscribers) {


I assume this needs to be changed now to call AsyncLocalStore.runInTracingChannel

Qard · 2022-12-15T02:55:25Z

Merging the concepts of this into #44943.

lib: add tracing channel to diagnostics_channel

0438150

Qard added http Issues or PRs related to the http subsystem. async_hooks Issues and PRs related to the async hooks subsystem. diagnostics_channel Issues and PRs related to diagnostics channel labels Nov 2, 2022

nodejs-github-bot added the needs-ci PRs that need a full CI run. label Nov 2, 2022

Qard mentioned this pull request Nov 2, 2022

diagnostics_channel: add storage channel #44894

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Flarna mentioned this pull request Nov 8, 2022

async_hooks: rename AsyncLocalStore#enterWith to set #45373

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diagnostics_channel: make tracing channels safer and support symbol n…

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diagnostics_channel: Avoid wrapping with no subscribers

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Qard force-pushed the als-diagnostics-channel-integration branch from fc22266 to 2ba33f3 Compare November 10, 2022 01:11

async_hooks: AsyncLocalStorage to diagnostics_channel integration

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Qard force-pushed the als-diagnostics-channel-integration branch from 2ba33f3 to e72265b Compare November 10, 2022 01:12

Flarna mentioned this pull request Nov 10, 2022

Stability of built in diagnostics channels #45400

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diagnostics_channel: add tracing channel #44943

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async_hooks: AsyncLocalStorage to diagnostics_channel integration #45277

async_hooks: AsyncLocalStorage to diagnostics_channel integration #45277

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Qard commented Dec 15, 2022

	channel._weak = new WeakReference(channel);
	channel._weak = tracingChannels[name] = new WeakReference(channel);

async_hooks: AsyncLocalStorage to diagnostics_channel integration #45277

async_hooks: AsyncLocalStorage to diagnostics_channel integration #45277

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