A113: pick_first: Weighted Random Shuffling

Author: apolcyn

A113-pick-first-weighted-shuffling.md

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+A113: pick_first: Weighted Random Shuffling
+----
+* Author(s): Alex Polcyn (@apolcyn)
+* Approver: Mark Roth (@markdroth), Eric Anderson (@ejona86), Doug Fawley (@dfawley), Easwar Swaminathan (@easwars)
+* Status: Draft
+* Implemented in: <language, ...>
+* Last updated: Jan 26, 2026
+* Discussion at: <google group thread> (filled after thread exists)
+
+## Abstract
+
+Support weighted random shuffling in the pick first LB policy.
+
+## Background
+
+The pick first LB policy currently supports random shuffling. A primary intention of the feature
+is for load balancing, however it does not take (possibly present) locality or endpoint weights
+into account. Naturally this can lead to skewed load distribution and hotspots, when the load
+balancing control plane delivers varied weights and expects them to be followed.
+
+
+### Related Proposals: 
+* [A62](https://github.com/grpc/proposal/blob/master/A62-pick-first.md): pick_first: sticky TRANSIENT_FAILURE and address order randomization
+* [A42](https://github.com/grpc/proposal/blob/master/A42-xds-ring-hash-lb-policy.md) xDS Ring Hash LB Policy
+
+## Proposal
+
+Modify behavior of pick_first when the `shuffle_address_list` option is set, and
+perform a weighted random sort *based on per-endpoint weights*:
+* Use the [Weighted Random Sampling](https://utopia.duth.gr/~pefraimi/research/data/2007EncOfAlg.pdf) algorithm
+proposed by Efraimidis, Spirakis.
+* Set the weight of each endpoint to `u ^ (1 / weight)`, where `u` is a uniform random number in `(0, 1)` and weight
+is the weight of the endpoint (as present in a weight attribute). Default to 1 if no weight attribute is present.
+
+Note in XDS, we have a notion of both locality and endpoint weights. The expectation of the load balancing
+control plane is to *first* pick locality and *second* pick endpoint. The total probability distribution
+reflected by per-endpoint weights must reflect this. As such, we need to normalize locality weights within
+each priority and endpoint weights within locality; the final weight provided to `pick_first` should be a
+product of the two normalized weights (i.e. a logical AND of the two selection events).
+
+The CDS LB policy currently calculates per-endpoint weight attributes. We can continue with this however
+we need to modify CDS LB to compute the final per-endpoint weights as a product of normalized locality
+and endpoint weights rather than their product outright. Note: as a side effect this will fix per-endpoint
+weights in Ring Hash LB.
+
+We can continue to represent weights as integers if we represent their normalized values in
+fixed point Q31 format (citation due for @ejona):
+
+1) Normalize a weight within a `weight_sum` as follows: `uint32_t normalized = ((uint64_t)weight * 2 ^ 31) / weight_sum`.
+
+2) Multiply two normalized weights as follows: `weight = ((uint64_t) weight1 * weight2) >> 31`
+
+3) Zero weights should be rounded up to 1.
+
+### Temporary environment variable protection
+
+CDS LB policy and Pick First LB policy behavior changes will be guarded by `GRPC_EXPERIMENTAL_PF_WEIGHTED_SHUFFLING`.
+
+## Rationale
+
+* CDS LB policy changes are needed to generate correct weight distributions, not only for Pick First but
+  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.
+
+## Implementation
+
+TBD
+