Explain the concept of Domain in Node.js

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an error event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code.

Domain error handlers are not a substitute for closing down a process when an error occurs. The safest way to respond to a thrown error is to shut down the process. In a normal web server, the better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

var domain = require('domain');
var d = domain.create();
// Domain emits 'error' when it's given an unhandled error
d.on('error', function(err) {
    console.log(err.stack);
    // Our handler should deal with the error in an appropriate way
});

// Enter this domain
d.run(function() {
    // If an un-handled error originates from here, process.domain will handle it
    console.log(process.domain === d); // true
});

// domain has now exited. Any errors in code past this point will not be caught.