在 ZeroMQ 的 Response/Request 模式中，服务端回调函数逻辑内调用 fs.readFile 时，为何会阻塞执行？（内有代码）

Node 版本 10.16.0 ZeroMQ 版本 5.1.0 使用 ZeroMQ 的代码如下：

服务端 /zmq-rep.js

"use strict";
const fs = require("fs");
const zmq = require("zeromq");
const moment = require("moment");
const responder = zmq.socket("rep");

responder.on("message", data => {
  let reqData = JSON.parse(data);
  let filename = reqData.path;
  console.log(`Request received:${filename}`);
  fs.readFile(filename, (err, data) => {
    if (err) {
      throw Error(err);
    }
    console.log(`Sending response for ${filename} at ${moment().format()}`);
    responder.send(
      JSON.stringify({
        content: data.toString(),
        timestamp: moment().format("x"),
        pid: process.pid
      })
    );
  });
});

responder.bind("tcp://127.0.0.1:50221", err => {
  if (err) {
    throw Error(err);
  }
  console.log("Now listening for requests...");
});

process.on("SIGINT", () => {
  console.log("Now closing server...");
  responder.close();
});

客户端 /zmq-req.js

"use strict";
const zmq = require("zeromq");
const moment = require("moment");
const filename = process.argv[2];
const request = zmq.socket("req");
request.on("message", data => {
  let response = JSON.parse(data);
  console.log(
    `Fetch file content:${response.content} at ${
      response.timestamp
    } processed by ${response.pid}`
  );
});
request.connect("tcp://127.0.0.1:50221");

for (let i = 1; i <= 5; i++) {
  console.log(
    `Sending request to get ${filename} for ${i} time(s) at ${moment().format()}`
  );
  request.send(
    JSON.stringify({
      path: filename
    })
  );
}

使用如下方式调用：

# Terminal 1
$ node zmq-rep.js
# Terminal 2
$ node zmq-req.js target.txt

服务端输出如下：

Now listening for requests...
Request received:target.txt
Sending response for target.txt at 2019-07-27T07:36:47+08:00
Request received:target.txt
Sending response for target.txt at 2019-07-27T07:36:47+08:00
Request received:target.txt
Sending response for target.txt at 2019-07-27T07:36:47+08:00
Request received:target.txt
Sending response for target.txt at 2019-07-27T07:36:47+08:00
Request received:target.txt
Sending response for target.txt at 2019-07-27T07:36:47+08:00

客户端输出如下

Sending request to get target.txt for 1 time(s) at 2019-07-27T07:36:47+08:00
Sending request to get target.txt for 2 time(s) at 2019-07-27T07:36:47+08:00
Sending request to get target.txt for 3 time(s) at 2019-07-27T07:36:47+08:00  
Sending request to get target.txt for 4 time(s) at 2019-07-27T07:36:47+08:00
Sending request to get target.txt for 5 time(s) at 2019-07-27T07:36:47+08:00  
Fetch file content:50221 at 1564184207708 processed by 12984
Fetch file content:50221 at 1564184207734 processed by 12984
Fetch file content:50221 at 1564184207747 processed by 12984
Fetch file content:50221 at 1564184207766 processed by 12984
Fetch file content:50221 at 1564184207794 processed by 12984

可以看到，客户端代码发送请求是非阻塞的，但服务端代码是阻塞的。《 Node.js 8 the right way 》对这个现象的解释是："Node.js event loop was left spinning while the fs.readFile for each request was being processed."但查阅文档，fs.readFile 本身是非阻塞的，如果这一解释成立的话，fs.readFile 不就是阻塞函数了吗？

我另外写了一份只使用 net 模块进行通讯的代码，现象与使用 ZeroMQ 进行通信不同，在这种情况下，服务端的回调并没有阻塞。

服务端 /tcp-server.js

"use strict";
const fs = require("fs");
const net = require("net");
const moment = require("moment");

const server = net
  .createServer(connection => {
    console.log("Request received");
    connection.on("data", data => {
      let reqData = JSON.parse(data);
      console.log(`Reading ${reqData.path} at ${moment().format()}`);
      fs.readFile(reqData.path, (err, data) => {
        connection.write(
          JSON.stringify({
            content: data.toString(),
            timestamp: moment().format()
          })
        );
      });
    });
  })
  .listen({ port: 50221, hostname: "127.0.0.1" })
  .on("listening", () => {
    console.log("Now listening...");
  });

客户端 /tcp-client.js

"use strict";
const net = require("net");
const filename = process.argv[2];
const connectionPool = [];

for (let i = 1; i <= 5; i++) {
  connectionPool.push(
    net
      .createConnection(50221, "127.0.0.1", () => {
        console.log(
          `Connetcion ${i} established and start fetching file ${filename} on server...`
        );
      })
      .on("data", data => {
        console.log(`Receive response:${data}`);
      })
  );
}
connectionPool.forEach(connection => {
  connection.write(
    JSON.stringify({
      path: filename
    })
  );
});

process.on("SIGINT", () => {
  connectionPool.forEach(connection => {
    connection.end();
  });
});

调用如下：

# Terminal 1
$ node tcp-server.js
# Terminal 2
$ node tcp-client.js target.txt

客户端输出如下：

Connetcion 1 established and start fetching file target.txt on server...
Connetcion 2 established and start fetching file target.txt on server...
Connetcion 3 established and start fetching file target.txt on server...      
Connetcion 4 established and start fetching file target.txt on server...      
Connetcion 5 established and start fetching file target.txt on server...      
Receive response:{"content":"50221","timestamp":"2019-07-27T07:44:53+08:00"}
Receive response:{"content":"50221","timestamp":"2019-07-27T07:44:53+08:00"}
Receive response:{"content":"50221","timestamp":"2019-07-27T07:44:53+08:00"}  
Receive response:{"content":"50221","timestamp":"2019-07-27T07:44:53+08:00"}  
Receive response:{"content":"50221","timestamp":"2019-07-27T07:44:53+08:00"}

服务端输出如下：

Now listening...
Request received
Request received
Request received
Request received
Request received
Reading target.txt at 2019-07-27T07:44:53+08:00
Reading target.txt at 2019-07-27T07:44:53+08:00
Reading target.txt at 2019-07-27T07:44:53+08:00
Reading target.txt at 2019-07-27T07:44:53+08:00
Reading target.txt at 2019-07-27T07:44:53+08:00

于是可以看到，不论在服务端还是客户端的 io 都是非阻塞的。

这种不同是否与 ZeroMQ 的实现有关呢？感谢大佬们的回复！