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！