@balena-io/eye-patch

This package patches http(s).Agent to implement "happy eyeballs" (rfc8305), a standard published by the IETF.

It improves client performance and reliability by trying concurrently trying multiple ip addresses for a given host name. See Explanation for more detail.

Installation

npm i --save-dev @balena/eye-patch

Quick Usage

To use this library, simply import the module, and the prototype of http.Agent.createConnection and https.Agent.createConnection will be replaced.

import '@balena/eye-patch';

Explicit Usage

If you want to be more explicit, you can explicitly patch the http(s).Agent:

import { patch } from '@balena/eye-patch';
import { Agent as HttpAgent } from 'http';
import { Agent as HttpsAgent } from 'https';

patch(HttpAgent);
patch(HttpsAgent);

Although, this is exactly what the import '@balena/eye-patch/explicit'; does anyway.

You could also implement your own agent and replace the createConnection method:

import { createConnection } from '@balena/eye-patch/explicit';
import { Agent as HttpAgent } from 'http';
import { Agent as HttpsAgent } from 'https';

export class MyHttpAgent extends HttpAgent {
  createConnection = createConnection;
}

export class MyHttpsAgent extends HttpsAgent {
  createConnection = createConnection;
}

This does basically the same thing as the previous examples though.

Explanation

Essentially, the algorithm amounts to this:

1. Receive a hostname
2. Have we connected to this host before?

• If yes:
  • Try the family (Ipv4 or IPv6) of last successful connection if we have connected to this host before
  • If connection is not successful within 300ms, proceed to next step
• If no: proceed to next step.

3. Did the DNS lookup return both IPv4 and IPv6 addresses?

• If yes: try, both addresses concurrently until all have been tried
• If no: just try the address of the existent family

4. Proceed with each address in chain, trying both families concurrently until either: *. A connection is made *. All connection attempts time out *. All connection attempt fail
5. If no connection was successful, return error of the first connection attempt or a "time out" error if all connections timed out