Solve the problem of how to share data among multiple processes in the cluster module in Nodejs

As mentioned above

nodejs added a module cluster after v0.6.x to implement multi-process, using the child_process module to create and manage processes, and increase the performance of the program on multi-core CPU machines. This article will introduce how multi-threads created using the cluster module share data.

Inter-process data sharing

First, let’s take a simple example. The code is as follows:

var cluster = require('cluster');
var data = 0;//这里定义数据不会被所有进程共享，各个进程有各自的内存区域
if (cluster.isMaster) { //主进程
 var numCPUs = require('os').cpus().length;
 for (var i = 0; i < numCPUs; i++) {
  var worker = cluster.fork();
 }
 data++;
 console.log(&#39;DATA VALUE in MainProcess: %d &#39; , data);
} else { //子进程,会被调用numCPUs次
 data++;
 console.log(&#39;DATA VALUE in ChildProcess %d: %d &#39; cluster.worker.id, data);
}

The running results are as follows:

Why are we outside the main process code block and sub-process code block? Shouldn't the declared variables be global variables? the answer is negative. Because each process has its own area in memory, the data++ operation is performed in its own area, which means that the variable data is not shared. So how to share data between processes? Let’s look at the following code:

var cluster = require('cluster');
var http = require('http');
  
if (cluster.isMaster) {
 var numCPUs = require('os').cpus().length;
 var data = 0;
 // 启动多个进程.
 for (var i = 0; i < numCPUs; i++) {
 //增加一个进程
 var worker_process = cluster.fork();
 //侦听子进程的message事件
 worker_process.on(&#39;message&#39;, function(msg) {
  if (msg.cmd && msg.cmd == &#39;notifyRequest&#39;) {
  data++;
  console.log(&#39;DATA VALUE : %d &#39;, data);
  }
 });
 }
} else {
 process.send({ cmd: &#39;notifyRequest&#39; });
}

The running results are as follows:

So if you need to share data, you need to use message notifications between processes to achieve this purpose.