A115: disable priority LB child policy retention cache

A115-remove-priority-lb-child-policy-cache.md

@@ -0,0 +1,83 @@
+A115: disable Priority LB policy child policy retention cache
+----
+* Author(s): @apolcyn, @markdroth
+* Approvers: @markdroth, @ejona86, @dfawley, @easwars
+* Status: {Draft}
+* Implemented in: C-core, Java, Go, Node
+* Last updated: 2026-03-17
+* Discussion at: <google group thread> (filled after thread exists)
+
+## Abstract
+
+[A56](A56-priority-lb-policy.md) describes
+[mechanisms](A56-priority-lb-policy.md#child-lifetime-management)
+whereby priority LB child policies are cached. There are two cases:
+
+1) When a higher priority child becomes reachable, we deactive
+the lower-priority children, and remove them only after an expiry.
+
+2) When a child is removed from the LB policy config.
+
+This proposal removes the usage of a cache for case 2.
+
+## Background
+
+The priority LB child policy retention cache consumes excessive memory under the
+right circumstances (depending on the rate and pattern of locality updates).
+
+This is especially the case when a locality is flapping between failover and primary
+priorities. For example, notice how priority LB child names increase in the following
+sequence of locality updates. On each child name update, previous policies are added
+to the retention cache.
+
+For example, consider the following sequence of updates:
+1. P0=[AA, BB], P1=[CC, DD]: P0 will be assigned "child0", P1 will be assigned "child1".
+2. P0=[CC], P1=[DD, EE]: P0 will be assigned "child1" (reusing the child name containing "CC" from the previous update), P1 will be assigned "child2" (new child number).
+3. P0=[AA, BB], P1=[CC, DD]: P0 will be assigned "child3" (new child number), P1 will be assigned "child1" (reusing the child name containing "CC" from the previous update).
+
+Additionally, priority LB child names are generated with strictly increasing numbers
+(once a priority LB child name is unconfigured, it will never be configured again). As such,
+the cache is not providing us value.
+
+## Proposal
+
+Priority LB should disable the child policy retention cache, when a
+child is removed from its config (i.e., case 2 only).
+
+Note this should be done for Java and Go only.
+
+For C-core, we are actually potentially getting benefit from this
+behavior due to subchannel pooling, so we're not planning to drop
+it there until we have the longer-term solution ready.
+
+### Temporary environment variable protection
+
+Implementations should provide an environment variable to revert
+to the previous behavior (child policy cache enabled with 15-minute timer).
+
+This should be kept around for a few releases, and then removed.
+
+Env var name: `GRPC_EXPERIMENTAL_ENABLE_PRIORITY_LB_CHILD_POLICY_CACHE`.
+
+## Rationale
+
+- Caching the child when it gets removed from the config does not actually
+  accomplish anything useful in the case of choosing a priority within an
+  xDS cluster (which is the primary case where this policy is used), because
+  the hueristic in the cds policy that assigns the child names will never
+  reuse a child name once it has been removed from the config.
+
+- We have seen cases where retaining the children has used up a lot of memory
+  and file descriptors, which has caused problems for users.
+
+- In the long run, we want a better solution involving a separate layer for
+  caching subchannels rather than LB policies, but that will be a separate
+  project to be undertaken later.
+
+## Implementation
+
+N/A
+
+## Open issues (if applicable)
+
+N/A

A56-priority-lb-policy.md

@@ -6,6 +6,7 @@ A56: `priority_experimental` LB policy
 * Implemented in: C-core, Java, Go, Node
 * Last updated: 2025-08-18
 * Discussion at: <google group thread> (filled after thread exists)
+* Updated by: [gRFC A115][A115])
 
 ## Abstract
 
@@ -76,6 +77,7 @@ today, which has evolved somewhat since it was originally introduced.
 [A28]: A28-xds-traffic-splitting-and-routing.md
 [A37]: A37-xds-aggregate-and-logical-dns-clusters.md
 [A42]: A42-xds-ring-hash-lb-policy.md
+[A115]: A56-update-remove-priority-lb-child-policy-cache.md
 
 ## Proposal