In node.js, how to declare a shared variable that can be initialized by master process and accessed by worker processes?

Question

I want the following

• During startup, the master process loads a large table from file and saves it into a shared variable. The table has 9 columns and 12 million rows, 432MB in size.
• The worker processes run HTTP server, accepting real-time queries against the large table.

Here is my code, which obviously does not achieve my goal.

var my_shared_var;
var cluster = require('cluster');
var numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  // Load a large table from file and save it into my_shared_var,
  // hoping the worker processes can access to this shared variable,
  // so that the worker processes do not need to reload the table from file.
  // The loading typically takes 15 seconds.
  my_shared_var = load('path_to_my_large_table');

  // Fork worker processes
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  // The following line of code actually outputs "undefined".
  // It seems each process has its own copy of my_shared_var.
  console.log(my_shared_var);

  // Then perform query against my_shared_var.
  // The query should be performed by worker processes,
  // otherwise the master process will become bottleneck
  var result = query(my_shared_var);
}

I have tried saving the large table into MongoDB so that each process can easily access to the data. But the table size is so huge that it takes MongoDB about 10 seconds to complete my query even with an index. This is too slow and not acceptable for my real-time application. I have also tried Redis, which holds data in memory. But Redis is a key-value store and my data is a table. I also wrote a C++ program to load the data into memory, and the query took less than 1 second, so I want to emulate this in node.js.

Answer 1

If I translate your question in a few words, you need to share data of MASTER entity with WORKER entity. It can be done very easily using events:

From Master to worker:

worker.send({json data});    // In Master part

process.on('message', yourCallbackFunc(jsonData));    // In Worker part

From Worker to Master:

process.send({json data});   // In Worker part

worker.on('message', yourCallbackFunc(jsonData));    // In Master part

I hope this way you can send and receive data bidirectionally. Please mark it as answer if you find it useful so that other users can also find the answer. Thanks

Answer 2

You are looking for shared memory, which node.js just does not support. You should look for alternatives, such as querying a database or using memcached.

Answer 3

In node.js fork works not like in C++. It's not copy current state of process, it's run new process. So, in this case variables isn't shared. Every line of code works for every process but master process have cluster.isMaster flag set to true. You need to load your data for every worker processes. Be careful if your data is really huge because every process will have its own copy. I think you need to query parts of data as soon as you need them or wait if you realy need it all in memory.

Answer 4

If read-only access is fine for your application, try out my own shared memory module. It uses mmap under the covers, so data is loaded as it's accessed and not all at once. The memory is shared among all processes on the machine. Using it is super easy:

const Shared = require('mmap-object')

const shared_object = new Shared.Open('table_file')

console.log(shared_object.property)

It gives you a regular object interface to a key-value store of strings or numbers. It's super fast in my applications.

There is also an experimental read-write version of the module available for testing.

Answer 5

You can use Redis.

Redis is an open source, BSD licensed, advanced key-value cache and store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets, sorted sets, bitmaps and hyperloglogs.

redis.io