(refactor) Pick First LB state machine - #2784
Open
nathanielford wants to merge 6 commits into
Open
Conversation
… ready during resolver update.
nathanielford
marked this pull request as ready for review
July 30, 2026 22:33
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Motivation
The initial implementation of the PickFirst LB was a little tangled in how it implemented the Happy Eyeballs requirements. This meant it was handling some phases with special state and others without. Overall it made it hard to reason about.
Further, there was abehavior gaps between the original implementation and the spec: the original discards unselected addresses upon entering READY, causing subsequent reconnections from IDLE to only attempt the single failed address instead of sweeping all resolved backends.
Solution
This implementation is fairly different, using a state machine to hold state for each of the four states of Happy Eyeballs (Idle, FirstPass, SteadyState and Ready), and being clear about state transitions between them. The construction ensures the retention of appropriate information (i.e. addresses), and makes things easier to reason about.
As a bonus, this pass also eliminates a bunch of allocations that were unneeded. Gemini estimates about a 70% smaller memory footprint. As a double plus bonus, it now passes linting checks.