@balena/eye-patch

@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.

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

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

`ts
import '@balena/eye-patch';
`

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

`ts
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:

`ts
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.

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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