Limit the maximum numbers of open sockets

[mcollina]

One of the first issues with "fetch failed" is that we don't limit the number of outgoing sockets, and the process runs out of them, triggering a timeout error.

I think we should add a limit of 127 per pool by default (or any other magic number)

[metcoder95]

The global dispatcher or the overall default Client/Agent settings?

[mcollina]

Both are good ideas.

[JoshMock]

The Elasticsearch client library (of which I am the current maintainer) currently has an open request from the Kibana team to support limiting the maximum open sockets and max idle sockets when keepalive is enabled. If this functionality is added to Undici, it would be helpful if it exposed options to limit the max open and max total sockets, both globally and per upstream target, as well as max free sockets for keepalive use cases, to provide parity with http.Agent's options:

• maxFreeSockets: max free sockets when keepalive is enabled
• maxSockets: max concurrent sockets per origin
• maxTotalSockets: max concurrent sockets globally

I've not contributed to this library before, but would love to help add this functionality.

[metcoder95]

Overall, I think this should be already doable (unless I'm overlooking at something).

By using Pool you can limit the number of connections (Client instance) that the pool can have. As a Client is mapped to a single Socket.

Ref: https://undici.nodejs.org/#/docs/api/Pool?id=parameter-pooloptions

The options listed should be ported to the Agent, as it can manage these options at a higher level, at the end, depending on the number of connections passed, can be a Pool of connections or a single connection per origin.

maxFreeSockets - maybe maxIdleConnections? Here the Pool and/or the Agent can be preemptive and destroy idle connections before waiting for more request to come.

maxTotalSockets - these should be ported to Agent I believe, as the Pool already has an option for that.

Would you like to send a PR for it?

[mcollina]

@JoshMock this is already possible at Pool level.