Introduction to the cluster module in Node.js

This article mainly introduces the introduction of the cluster module in Node.js. It has a certain reference value. Now I share it with you. Friends in need can refer to it.

Node’s single-threaded design is no longer available In order to fully "squeeze" machine performance, Node has added a built-in module cluster, which can implement cluster functions by managing a bunch of child processes through a parent process. This article mainly introduces an in-depth analysis of Node.js cluster Module, interested friends can refer to

cluster module overview

node instances are single-threaded operations. In server-side programming, multiple node instances are usually created to handle client requests to improve system throughput. For such multiple node instances, we call it a cluster.

With the help of node's cluster module, developers can obtain the benefits of cluster services without modifying the original project code with almost no modifications.

The cluster has the following two common implementation solutions, and the cluster module that comes with node adopts the second solution.

Option 1: Multiple node instances and multiple ports

The node instances in the cluster each listen to different ports, and then the reverse proxy implements the request to multiple Distribution of ports.

1. Advantages: Simple implementation, each instance is relatively independent, which is good for service stability.

2. Disadvantages: increased port occupancy, communication between processes is more troublesome.

Option 2: The main process forwards the request to the child process

In the cluster, create a main process (master) and several child processes ( worker). The master monitors client connection requests and forwards them to workers according to specific policies.

1. Advantages: Usually only one port is occupied, communication is relatively simple, and the forwarding strategy is more flexible.

2. Disadvantages: The implementation is relatively complex and requires high stability of the main process.

Getting Started Example

In the cluster module, the main process is called master and the child process is called worker.

The example is as follows, create server instances with the same number of CPUs to handle client requests. Note that they are all listening on the same port.

// server.js
var cluster = require('cluster');
var cpuNums = require('os').cpus().length;
var http = require('http');

if(cluster.isMaster){
 for(var i = 0; i < cpuNums; i++){
  cluster.fork();
 }
}else{
 http.createServer(function(req, res){
  res.end(`response from worker ${process.pid}`);
 }).listen(3000);

 console.log(`Worker ${process.pid} started`);
}

Create batch script: ./req.sh.

#!/bin/bash

# req.sh
for((i=1;i<=4;i++)); do  
 curl http://127.0.0.1:3000
 echo ""
done

The output is as follows. As you can see, the responses come from different processes.

response from worker 23735
response from worker 23731
response from worker 23729
response from worker 23730

cluster Module implementation principle

To understand the cluster module, we mainly need to understand three questions:

1. How do master and workers communicate?

2. How to achieve port sharing for multiple server instances?

3. Multiple server instances, how to distribute requests from clients to multiple workers?

The following will be introduced based on the schematic diagram. For source code level introduction, you can refer to the author's github.

Question 1: How to communicate between master and worker

This question is relatively simple. The master process creates worker processes through cluster.fork(). Cluster.fork() internally creates child processes through child_process.fork().

In other words:

1. The master process and the worker process are the relationship between parent and child processes.

2. The master process and the worker process can communicate through the IPC channel. (Important)

Question 2: How to implement port sharing

In the previous example, servers created in multiple wokers listened to the same port. port 3000. Generally speaking, if multiple processes listen to the same port, the system will report an error.

Why is our example okay?

The secret is that in the net module, the listen() method is specially processed. Depending on whether the current process is a master process or a worker process:

1. master process: listen to requests normally on this port. (No special processing)

2. #Worker process: Create a server instance. Then send a message to the master process through the IPC channel, so that the master process also creates a server instance and listens for requests on this port. When a request comes in, the master process forwards the request to the server instance of the worker process.

To sum up, it is: the master process listens to a specific port and forwards customer requests to the worker process.

As shown below:

Question 3: How to distribute requests to multiple workers

Every When the worker process creates a server instance to listen for requests, it will be registered on the master through the IPC channel. When a client request arrives, the master will be responsible for forwarding the request to the corresponding worker.

Which worker will it be forwarded to specifically? This is determined by the forwarding strategy. It can be set through the environment variable NODE_CLUSTER_SCHED_POLICY, or passed in when cluster.setupMaster(options).

默认的转发策略是轮询（SCHED_RR）。

当有客户请求到达，master会轮询一遍worker列表，找到第一个空闲的worker，然后将该请求转发给该worker。

master、worker内部通信小技巧

在开发过程中，我们会通过 process.on('message', fn) 来实现进程间通信。

前面提到，master进程、worker进程在server实例的创建过程中，也是通过IPC通道进行通信的。那会不会对我们的开发造成干扰呢？比如，收到一堆其实并不需要关心的消息？

答案肯定是不会？那么是怎么做到的呢？

当发送的消息包含cmd字段，且改字段以NODE_作为前缀，则该消息会被视为内部保留的消息，不会通过message事件抛出，但可以通过监听'internalMessage'捕获。

以worker进程通知master进程创建server实例为例子。worker伪代码如下：

// woker进程
const message = {
 cmd: 'NODE_CLUSTER',
 act: 'queryServer'
};
process.send(message);

master伪代码如下：

worker.process.on('internalMessage', fn);

相关链接

官方文档：https://nodejs.org/api/cluster.html

Node学习笔记：https://github.com/chyingp/nodejs-learning-guide

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