How to Configure Keep-Alive in Axios for Node.js

Configuring HTTP keep-alive connections in Axios for Node.js allows your application to reuse underlying TCP connections across multiple HTTP/HTTPS requests. By default, Node.js terminates connections after each request, introducing latency and unnecessary resource overhead. This guide explains how to enable and configure persistent keep-alive connections in Axios using Node.js native http and https agents.

Why Enable Keep-Alive?

When making repeated requests to the same server, establishing a new TCP connection (and performing the TLS handshake for HTTPS) for every request wastes CPU and adds network latency. Enabling HTTP Keep-Alive keeps the socket open, allowing multiple requests to share the same connection.

Implementing Keep-Alive in Axios

Axios relies on Node.js's built-in http and https modules to dispatch network requests on the server side. To enable keep-alive, create custom instances of http.Agent and https.Agent with the keepAlive option set to true, then pass them to an Axios instance.

const axios = require('axios');
const http = require('http');
const https = require('https');

// 1. Create HTTP and HTTPS agents with keep-alive enabled
const httpAgent = new http.Agent({
  keepAlive: true,
  keepAliveMsecs: 1000, // Duration (ms) to keep idle sockets alive
  maxSockets: 50,       // Max simultaneous sockets allowed per host
  maxFreeSockets: 10,   // Max free sockets to keep open in idle state
  timeout: 60000        // Socket timeout in milliseconds
});

const httpsAgent = new https.Agent({
  keepAlive: true,
  keepAliveMsecs: 1000,
  maxSockets: 50,
  maxFreeSockets: 10,
  timeout: 60000
});

// 2. Create an Axios instance using the custom agents
const apiClient = axios.create({
  httpAgent,
  httpsAgent,
  baseURL: 'https://api.example.com'
});

// 3. Make requests using the configured instance
async function fetchData() {
  try {
    const response = await apiClient.get('/data');
    console.log(response.data);
  } catch (error) {
    console.error('Request failed:', error.message);
  }
}

fetchData();

Key Configuration Options

When defining custom agents, adjust the following parameters based on your application's concurrency and workload:

• keepAlive (boolean): Must be set to true to reuse sockets.
• keepAliveMsecs (number): The initial delay in milliseconds for TCP Keep-Alive packets.
• maxSockets (number): The maximum number of concurrent open sockets allowed per origin. Defaults to Infinity in Node.js.
• maxFreeSockets (number): The maximum number of sockets left open in an idle state waiting for new requests. Defaults to 256.
• timeout (number): Socket inactivity timeout in milliseconds before an idle socket is destroyed.

Global Axios Configuration

If you prefer applying keep-alive globally across all Axios requests instead of creating an instance, assign the agents to axios.defaults:

axios.defaults.httpAgent = httpAgent;
axios.defaults.httpsAgent = httpsAgent;

Using custom agents with keep-alive significantly reduces connection latency, minimizes DNS lookups, and enhances throughput for high-volume Node.js services.