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doc: improve async_hooks asynchronous context example

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* use writeFile(1) everywhere to log
* prettify execution id graph
* add clearer explanation for TickObject presence
* add causation graph via triggerAsyncId

PR-URL: https://github.com/nodejs/node/pull/33730
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
This commit is contained in:
Denys Otrishko
2020-06-04 20:58:52 +03:00
committed by James M Snell
parent ee3243fe3f
commit b4fe76d656
1 changed files with 28 additions and 9 deletions
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37
doc/api/async_hooks.md
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@@ -331,20 +331,17 @@ async_hooks.createHook({
},
before(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeFileSync('log.out',
`${indentStr}before: ${asyncId}\n`, { flag: 'a' });
fs.writeSync(process.stdout.fd, `${indentStr}before: ${asyncId}\n`);
indent += 2;
},
after(asyncId) {
indent -= 2;
const indentStr = ' '.repeat(indent);
fs.writeFileSync('log.out',
`${indentStr}after: ${asyncId}\n`, { flag: 'a' });
fs.writeSync(process.stdout.fd, `${indentStr}after: ${asyncId}\n`);
},
destroy(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeFileSync('log.out',
`${indentStr}destroy: ${asyncId}\n`, { flag: 'a' });
fs.writeSync(process.stdout.fd, `${indentStr}destroy: ${asyncId}\n`);
},
}).enable();
@@ -380,16 +377,38 @@ the value of the current execution context; which is delineated by calls to
Only using `execution` to graph resource allocation results in the following:
```console
Timeout(7) -> TickObject(6) -> root(1)
root(1)
^
|
TickObject(6)
^
|
Timeout(7)
```
The `TCPSERVERWRAP` is not part of this graph, even though it was the reason for
`console.log()` being called. This is because binding to a port without a host
name is a *synchronous* operation, but to maintain a completely asynchronous
API the user's callback is placed in a `process.nextTick()`.
API the user's callback is placed in a `process.nextTick()`. Which is why
`TickObject` is present in the output and is a 'parent' for `.listen()`
callback.
The graph only shows *when* a resource was created, not *why*, so to track
the *why* use `triggerAsyncId`.
the *why* use `triggerAsyncId`. Which can be represented with the following
graph:
```console
bootstrap(1)
|
˅
TCPSERVERWRAP(5)
|
˅
TickObject(6)
|
˅
Timeout(7)
```
##### `before(asyncId)`