Node.js v25.0.0 文档

警告：不要忽略错误！#

域错误处理程序不能替代在发生错误时关闭进程。

根据 JavaScript 中 throw 的工作原理，几乎没有任何方法可以安全地“从中断的地方继续”，而不会泄漏引用，或造成其他某种未定义的脆弱状态。

响应抛出错误的最安全方式是关闭进程。当然，在一个正常的 Web 服务器中，可能有很多打开的连接，因为别人触发的一个错误而突然关闭这些连接是不合理的。

更好的方法是向触发错误的请求发送一个错误响应，同时让其他请求在正常时间内完成，并停止在该工作进程中监听新的请求。

通过这种方式，domain 的使用与 cluster 模块紧密相连，因为主进程可以在一个工作进程遇到错误时 fork 一个新的工作进程。对于扩展到多台机器的 Node.js 程序，终止代理或服务注册中心可以注意到这个故障，并作出相应的反应。

例如，这不是一个好主意

// XXX WARNING! BAD IDEA!

const d = require('node:domain').create();
d.on('error', (er) => {
  // The error won't crash the process, but what it does is worse!
  // Though we've prevented abrupt process restarting, we are leaking
  // a lot of resources if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
  require('node:http').createServer((req, res) => {
    handleRequest(req, res);
  }).listen(PORT);
}); copy

通过使用域的上下文，以及将我们的程序分离到多个工作进程中的弹性，我们可以更恰当地作出反应，并以更高的安全性处理错误。

// Much better!

const cluster = require('node:cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isPrimary) {
  // A more realistic scenario would have more than 2 workers,
  // and perhaps not put the primary and worker in the same file.
  //
  // It is also possible to get a bit fancier about logging, and
  // implement whatever custom logic is needed to prevent DoS
  // attacks and other bad behavior.
  //
  // See the options in the cluster documentation.
  //
  // The important thing is that the primary does very little,
  // increasing our resilience to unexpected errors.

  cluster.fork();
  cluster.fork();

  cluster.on('disconnect', (worker) => {
    console.error('disconnect!');
    cluster.fork();
  });

} else {
  // the worker
  //
  // This is where we put our bugs!

  const domain = require('node:domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests. How it works, caveats, etc.

  const server = require('node:http').createServer((req, res) => {
    const d = domain.create();
    d.on('error', (er) => {
      console.error(`error ${er.stack}`);

      // We're in dangerous territory!
      // By definition, something unexpected occurred,
      // which we probably didn't want.
      // Anything can happen now! Be very careful!

      try {
        // Make sure we close down within 30 seconds
        const killtimer = setTimeout(() => {
          process.exit(1);
        }, 30000);
        // But don't keep the process open just for that!
        killtimer.unref();

        // Stop taking new requests.
        server.close();

        // Let the primary know we're dead. This will trigger a
        // 'disconnect' in the cluster primary, and then it will fork
        // a new worker.
        cluster.worker.disconnect();

        // Try to send an error to the request that triggered the problem
        res.statusCode = 500;
        res.setHeader('content-type', 'text/plain');
        res.end('Oops, there was a problem!\n');
      } catch (er2) {
        // Oh well, not much we can do at this point.
        console.error(`Error sending 500! ${er2.stack}`);
      }
    });

    // Because req and res were created before this domain existed,
    // we need to explicitly add them.
    // See the explanation of implicit vs explicit binding below.
    d.add(req);
    d.add(res);

    // Now run the handler function in the domain.
    d.run(() => {
      handleRequest(req, res);
    });
  });
  server.listen(PORT);
}

// This part is not important. Just an example routing thing.
// Put fancy application logic here.
function handleRequest(req, res) {
  switch (req.url) {
    case '/error':
      // We do some async stuff, and then...
      setTimeout(() => {
        // Whoops!
        flerb.bark();
      }, timeout);
      break;
    default:
      res.end('ok');
  }
} copy

对 Error 对象的补充#

任何时候一个 Error 对象通过一个域进行路由时，都会给它添加一些额外的字段。

• error.domain 第一个处理该错误的域。
• error.domainEmitter 触发了带有该错误对象的 'error' 事件的事件发射器。
• error.domainBound 绑定到该域的回调函数，并且错误作为其第一个参数被传入。
• error.domainThrown 一个布尔值，指示错误是被抛出、触发，还是传递给一个绑定的回调函数。

隐式绑定#

如果正在使用域，那么所有 新 的 EventEmitter 对象（包括 Stream 对象、请求、响应等）将在其创建时隐式地绑定到活动域。

此外，传递给低级事件循环请求（例如 fs.open() 或其他接受回调的方法）的回调函数将自动绑定到活动域。如果它们抛出错误，那么域将捕获该错误。

为了防止过度的内存使用，Domain 对象本身不会作为活动域的子对象被隐式添加。如果这样做，将太容易导致请求和响应对象无法被正确地垃圾回收。

要将 Domain 对象作为父 Domain 的子对象进行嵌套，必须显式地添加它们。

隐式绑定会将抛出的错误和 'error' 事件路由到 Domain 的 'error' 事件，但不会在 Domain 上注册 EventEmitter。隐式绑定只处理抛出的错误和 'error' 事件。

显式绑定#

有时，正在使用的域可能不是应该用于特定事件发射器的域。或者，事件发射器可能在一个域的上下文中创建，但应该被绑定到某个其他的域。

例如，可能有一个用于 HTTP 服务器的域，但我们可能希望为每个请求使用一个单独的域。

这可以通过显式绑定来实现。

// Create a top-level domain for the server
const domain = require('node:domain');
const http = require('node:http');
const serverDomain = domain.create();

serverDomain.run(() => {
  // Server is created in the scope of serverDomain
  http.createServer((req, res) => {
    // Req and res are also created in the scope of serverDomain
    // however, we'd prefer to have a separate domain for each request.
    // create it first thing, and add req and res to it.
    const reqd = domain.create();
    reqd.add(req);
    reqd.add(res);
    reqd.on('error', (er) => {
      console.error('Error', er, req.url);
      try {
        res.writeHead(500);
        res.end('Error occurred, sorry.');
      } catch (er2) {
        console.error('Error sending 500', er2, req.url);
      }
    });
  }).listen(1337);
}); copy

domain.create()#

• 返回: <Domain>

类: Domain#

• 继承自：<EventEmitter>

Domain 类封装了将错误和未捕获的异常路由到活动 Domain 对象的功能。

要处理它捕获的错误，请监听其 'error' 事件。

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.bind((er, data) => {
    // If this throws, it will also be passed to the domain.
    return cb(er, data ? JSON.parse(data) : null);
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
}); copy

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.intercept((data) => {
    // Note, the first argument is never passed to the
    // callback since it is assumed to be the 'Error' argument
    // and thus intercepted by the domain.

    // If this throws, it will also be passed to the domain
    // so the error-handling logic can be moved to the 'error'
    // event on the domain instead of being repeated throughout
    // the program.
    return cb(null, JSON.parse(data));
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
}); copy

const domain = require('node:domain');
const fs = require('node:fs');
const d = domain.create();
d.on('error', (er) => {
  console.error('Caught error!', er);
});
d.run(() => {
  process.nextTick(() => {
    setTimeout(() => { // Simulating some various async stuff
      fs.open('non-existent file', 'r', (er, fd) => {
        if (er) throw er;
        // proceed...
      });
    }, 100);
  });
}); copy

域和 Promise#

自 Node.js 8.0.0 起，Promise 的处理程序在调用 .then() 或 .catch() 本身所在的域内运行。

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then((v) => {
    // running in d2
  });
}); copy

回调函数可以使用 domain.bind(callback) 绑定到特定的域。

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then(p.domain.bind((v) => {
    // running in d1
  }));
}); copy

域不会干扰 Promise 的错误处理机制。换句话说，对于未处理的 Promise 拒绝，不会触发 'error' 事件。