HTTP Keep-Alive | Engineering Interview Kit

TL;DR

Keep-Alive is an HTTP feature that allows a single TCP connection to remain open for multiple HTTP requests/responses, drastically reducing the latency and CPU overhead caused by repeatedly performing the TCP/TLS handshake.

Mental Model

How It Works

By default, the built-in Node.js http.globalAgent (which http.request uses) does not have Keep-Alive enabled. This means every outgoing request from your Node.js backend to another API establishes a brand new TCP connection.

To enable it, you instantiate a new http.Agent with keepAlive: true and pass it to your requests (or configure libraries like Axios/Fetch to use it).

Example

const http = require('http');

// Create a custom agent with Keep-Alive enabled
const keepAliveAgent = new http.Agent({
    keepAlive: true,
    maxSockets: 100,      // Max concurrent sockets per host
    keepAliveMsecs: 1000  // How long to wait before closing idle sockets
});

// Use the agent in a request
const options = {
    hostname: 'api.example.com',
    port: 80,
    path: '/data',
    agent: keepAliveAgent // Attach the agent here!
};

http.request(options, (res) => {
    // ... handle response
}).end();

Common Interview Questions

Why is Keep-Alive critical for microservices?

Microservices make thousands of internal requests to each other. If Keep-Alive is disabled, the servers spend a massive amount of CPU and time just opening and closing TCP and TLS connections (which involves expensive cryptographic handshakes).

Does Node.js Express server enable Keep-Alive by default?

For incoming requests from clients, Node.js HTTP servers (and thus Express) automatically respect the Connection: keep-alive header sent by browsers and keep the connection open by default (usually with a 5-second timeout).