A113: pick_first: Weighted Random Shuffling#535
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easwars
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Jan 27, 2026
ejona86
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Jan 27, 2026
| also for Ring Hash | ||
| * Using fixed point Q31 format has predictable bounds on precision, and allows us to continue representing | ||
| weights as integers. Note our math assumes the sum of weights within a grouping does not exceed max uint32, | ||
| which is mandated in the XDS protocol. |
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Somewhere we should say we are preserving the "sum is a uint32" property for the internal weights (after the multiplication). That can be in the CDS section, or in this section. But we should call it out as a thing we are doing on purpose and desire. (It could also be used as an explanation for why we are using unsigned UQ1.31 instead of UQ32)
| This "fix" will not require any changes within Ring Hash LB itself. | ||
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| We can continue to represent weights as integers if we represent their normalized values in | ||
| fixed point Q1.31 format. Math as follows (citation due for @ejona): |
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If you're going to use this notation, then it is UQ1.31. Q1.31 is signed.
ejona86
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Jan 27, 2026
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| CDS LB policy and Pick First LB policy behavior changes will be guarded by `GRPC_EXPERIMENTAL_PF_WEIGHTED_SHUFFLING`. | ||
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| Barring unexpected issues, this should be enabled by default. |
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"After testing"? (Although unit testing isn't enough...) Or "after the implementation is shown to be stable"? I feel like we've written something like that before, but I don't recall where.
easwars
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Jan 28, 2026
dfawley
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Jan 30, 2026
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This PR implements the currently in-review gRFC A113: grpc/proposal#535. I've split the PR into logically separate commits to help with the review process. Summary of changes: - Commit 1: simplify the implementation of `groupLocalitiesByPriority` - Change the implementation to use newly added methods in the stdlib `maps` and `slices` package to significantly simplify the implementation (and get rid of an unnecessary test) - Commit 2: Remove code that handles localities and endpoints of weight 0 - Remove unnecessary checks for locality and endpoint weights of `0` in `cluster_resolver`. The xDS client already guarantees that these weights will never be set to `0`. - Commit 3: add the env var GRPC_EXPERIMENTAL_PF_WEIGHTED_SHUFFLING - Commit 4: Weight computation changes in cluster_resolver LB policy - This performs the weight normalization and fixed-point arithmetic specified in A113 - The change here is guarded by the above env var - Ended up duplicating the tests that verify the weight computation behavior. This will make it easier to delete the old tests when the env var is removed. - Commit 5: Fix a broken test in ring_hash due to the new weight computation - Commit 6: Weighted shuffling in pick_first - Contains the changes specified in A113 for the pick_first LB policy - Changes are guarded by the env var RELEASE NOTES: - pickfirst: Add support for weighted random shuffling of endpoints, as described in gRFC A113
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