feat(kv-router): approximate, remote, and hint-aware indexing in SelectionService

[ishandhanani]

PR 2 of 9 implementing ai-dynamo#14310 (one SelectionCore for the frontend, the standalone selection service, and the EPP). Base: idhanani/up-1-host-hooks (PR 1 of this stack).

Fork staging copy of the upstream PR, rebased onto ai-dynamo/main. Replaces fork #14 to #16, which stay open for review history.

Summary

Adds approximate, remote and hint-aware KV indexing to the standalone selection core. Booked requests can contribute cache knowledge when worker events are disabled, while remote-index and transfer-hint paths use the current typed protocols.

Implementation

• IndexerPolicy chooses an event-driven primary, an event-driven primary with a prediction side index, or an approximate TTL primary from router configuration.
• Successful bookings record routing decisions into the approximate index; query-only selection records nothing.
• An HTTP remote primary exposes tiered overlap through POST /query_tiered_by_hash and replaces local KV-event subscriptions.
• Worker metadata exposes backend role and per-rank control endpoints; booked responses carry a typed kv_hint when a usable source has a longer prefix than the selected worker.
• Python selection-service bindings accept remote_indexer_url, including the feature-gated lifecycle-test constructor at this boundary.

Walkthrough

flowchart LR
  Config[Router configuration] --> Policy[IndexerPolicy]
  Policy --> Index[Local or remote primary]
  Request[Request and worker metadata] --> Core[SelectionCore]
  Index --> Core
  Core --> Booking[Booking and optional KV hint]
  Booking --> Recording[Approximate decision recording]

Loading

Index selection and writes

When use_kv_events=false, the primary is populated from admitted routing decisions instead of remaining empty. select_and_reserve, cached booking replay and explicit reservations carrying tokens or hashes record their decisions. Recording happens after booking, so a recording error is logged rather than reported as a failed booking.

A remote primary needs neither worker KV-event endpoints nor local listeners or peer recovery. It is rejected with events disabled because the remote service has no routing-decision write API. Capacity-bounded approximate retention still falls back to TTL at this boundary until request-lease support is shared in #51.

Transfer hints and boundaries

The core retains matched chains only when the partition can consume hints. Source selection checks the backend role, control endpoint, cached-prefix advantage and the current transfer constraints; the response field is SelectResponse::kv_hint, not the prototype router_hint field. Query-only selection emits no transfer hint.

This adds indexing and selection metadata. It does not add the separate DisaggCoordinator piece or claim a disaggregated end-to-end test.

Validation

Current stack boundary: 8dfeecbcc5305219333dd881af32e80c378fdc69; the series is based on Dynamo main ab58558c2dff28f4ff1ea4e219030eac5da3f48f.

• Passed cargo check -p dynamo-kv-router -p dynamo-llm -p dynamo-ext-proc --no-default-features --features dynamo-kv-router/standalone-selection,dynamo-kv-router/standalone-slot-tracker --all-targets for this stack boundary.
• Python bindings passed cargo check --manifest-path lib/bindings/python/Cargo.toml --features select-service --all-targets at this boundary.
• The complete series was tested at feat(kv-router): session affinity for the standalone service and EPP #53; its description records the Rust/Python suites, aggregated GPU correctness probes and paired main/final benchmarks. Those end-to-end results apply to the final stack, not to a separate deployment of this intermediate PR.

GitHub CI is not green across this series. Recurring failures are the missing fork Fastchecker runner group, helm-tester image and approval-bot token, plus published-docs composition and an unchanged external documentation link. The local compilation and final-stack validation above are recorded separately from those CI failures.

Stack

#	Branch	Title	Fork PRs it replaces
1	idhanani/up-1-host-hooks	feat(kv-router): host-supplied inputs for SelectionService parity with the frontend scheduler	#8 to #12
2 (this PR)	idhanani/up-2-core-indexing	feat(kv-router): approximate, remote, and hint-aware indexing in SelectionService	#14 to #16
3	idhanani/up-3-embed-core	feat(router): re-host KvRouter scheduling on an embedded selection partition	#18, #35
4	idhanani/up-4-delete-kvscheduler	refactor(router): delete the runtime-bound KV router scheduler	#36
5	idhanani/up-5-policy-source	refactor(router): selection policy source, SelectionHost, and dead-flag removal	#37
6	idhanani/up-6-membership	feat(kv-router): one worker-membership path for frontend and EPP	#38
7	idhanani/up-7-kv-index-source	refactor(kv-router): one Indexer type and one KV-event ingress for every selection host	#39, #41, #43
8	idhanani/up-8-affinity-table	refactor(kv-router): one session-affinity implementation for every selection host	#42
9	idhanani/up-9-epp-affinity	feat(kv-router): session affinity for the standalone service and EPP	#44