异步上下文的示例

上下文跟踪用例包含在稳定的 API AsyncLocalStorage 中。 此示例仅说明异步钩子操作，但 AsyncLocalStorage 更适合此用例。

以下是一个示例，其中包含有关 before 和 after 调用之间对 init 的调用的附加信息，特别是对 listen() 的回调将是什么样子。 输出格式稍微复杂一点，使调用上下文更容易看到。

const async_hooks = require('node:async_hooks');
const fs = require('node:fs');
const net = require('node:net');
const { fd } = process.stdout;

let indent = 0;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      fd,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
    indent += 2;
  },
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
  },
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
  },
}).enable();

net.createServer(() => {}).listen(8080, () => {
  // 让我们在记录服务器启动之前等待 10 毫秒。
  setTimeout(() => {
    console.log('>>>', async_hooks.executionAsyncId());
  }, 10);
});

仅启动服务器的输出：

TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before:  6
  Timeout(7): trigger: 6 execution: 6
after:   6
destroy: 6
before:  7
>>> 7
  TickObject(8): trigger: 7 execution: 7
after:   7
before:  8
after:   8

如示例所示，executionAsyncId() 和 execution 各自指定当前执行上下文的值；通过调用 before 和 after 来描述。

仅使用 execution 绘制资源分配图结果如下：

  root(1)
     ^
     |
TickObject(6)
     ^
     |
 Timeout(7)

TCPSERVERWRAP 不是这个图表的一部分，尽管它是调用 console.log() 的原因。 这是因为绑定到一个没有主机名的端口是一个同步操作，但是为了维护一个完全异步的 API，用户的回调被放置在一个 process.nextTick() 中。 这就是为什么 TickObject 出现在输出中并且是 .listen() 回调的'父调用'。

该图只显示资源的创建时间，而不是创建原因，因此要跟踪使用 triggerAsyncId 的原因。 可以用下图表示：

 bootstrap(1)
     |
     ˅
TCPSERVERWRAP(5)
     |
     ˅
 TickObject(6)
     |
     ˅
  Timeout(7)

The context tracking use case is covered by the stable API AsyncLocalStorage. This example only illustrates async hooks operation but AsyncLocalStorage fits better to this use case.

The following is an example with additional information about the calls to init between the before and after calls, specifically what the callback to listen() will look like. The output formatting is slightly more elaborate to make calling context easier to see.

const async_hooks = require('node:async_hooks');
const fs = require('node:fs');
const net = require('node:net');
const { fd } = process.stdout;

let indent = 0;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      fd,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
    indent += 2;
  },
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
  },
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
  },
}).enable();

net.createServer(() => {}).listen(8080, () => {
  // Let's wait 10ms before logging the server started.
  setTimeout(() => {
    console.log('>>>', async_hooks.executionAsyncId());
  }, 10);
});

Output from only starting the server:

TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before:  6
  Timeout(7): trigger: 6 execution: 6
after:   6
destroy: 6
before:  7
>>> 7
  TickObject(8): trigger: 7 execution: 7
after:   7
before:  8
after:   8

As illustrated in the example, executionAsyncId() and execution each specify the value of the current execution context; which is delineated by calls to before and after.

Only using execution to graph resource allocation results in the following:

  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(). 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. Which can be represented with the following graph:

 bootstrap(1)
     |
     ˅
TCPSERVERWRAP(5)
     |
     ˅
 TickObject(6)
     |
     ˅
  Timeout(7)