feat(mocker): publish native KV events from the vLLM gRPC server

[jthomson04]

Summary

The vLLM mock gRPC server currently advertises no KV event sources, so sidecar runs cannot populate the router's KV index. Publish native KV events when prefix caching is enabled and the serving role is not decode, using the regular mocker's event generation and ZMQ publisher.

Details

• Complete publisher setup before serving discovery, then expose the bound endpoint through GetKvEventSources.
• Preserve the synchronous VllmMockerService::new API and its existing active-Tokio-runtime requirement. Bind sockets and read endpoints synchronously, then spawn the publisher on the current runtime. The existing async ZmqKvEventSink::new remains compatible.
• Select a free event port by default and honor explicit event and replay ports. Keep existing fixed-port sink callers compatible.
• Reuse event payloads, batching, replay, and cleanup. Log publisher setup failures and continue serving without an event source.
• Document the settings in the vLLM mock-server README. SGLang, AISimulate, and sidecar implementation code are unchanged.

Validation

Run outside the sandbox against the existing published AISimulate dependency:

• cargo test --locked -p dynamo-vllm-mocker — 19 tests passed. Includes publication conditions and failure handling, plus native source discovery through the real vLLM sidecar and stored/removed events reaching the router index through the shared ZMQ relay.
• cargo test --locked -p dynamo-mocker --features zmq-events services::zmq_events --lib — 6 tests passed, including automatic/fixed ports, replay, socket cleanup, and an error when no Tokio runtime is active.
• Changed Rust files pass rustfmt --check; git diff --check passes.

Additional validation of the synchronous-construction follow-up used the isolated upgrade candidate from #14744, pinned to unchanged AISimulate PR #188 at 3e5b5214556e255cd5940fdf9155a9d31d651470 with the documented Dynamo compatibility adapters. Both mock-server suites passed (30 tests). Four full HTTP smoke cases passed (68 completion requests): vLLM and SGLang, each with automatic and fixed event ports. The real sidecar relays accepted stored and removed events, and the router reported 0% -> 100% -> 0% hits for a measured prompt before caching, after caching, and after eviction.

Where should the reviewer start?

Start with lib/mocker/servers/vllm/src/server.rs, then lib/mocker/src/services/zmq_events.rs and the sidecar integration test.

Related Issues

• Confirmed — no related issue

Summary by CodeRabbit

• New Features

  • Added support for publishing vLLM KV-cache events through a sidecar.
  • Added KV-cache event source discovery, including stored-block and eviction events.
  • Added automatic event-port allocation with replay endpoint support.
  • Added end-to-end validation of cache indexing, cache hits, and evictions.

• Documentation

  • Documented KV-cache event configuration, sidecar discovery, replay ports, and behavior when caching or event setup is unavailable.

• Bug Fixes

  • Improved handling of event publisher startup failures so services continue operating without event sources.

[coderabbitai]

Walkthrough

The vLLM mocker now publishes KV-cache events through dynamically bound ZMQ endpoints. It exposes event source metadata, supports sidecar relay, and adds unit and end-to-end coverage for publication, cache hits, and evictions.

Changes

vLLM KV event publication

Layer / File(s)	Summary
Dynamic ZMQ event binding lib/mocker/src/services/zmq_events.rs	ZmqKvEventSink can allocate PUB ports automatically, reserve replay ports, expose resolved endpoints, and validate replay behavior.
vLLM publisher integration lib/mocker/servers/vllm/Cargo.toml, lib/mocker/servers/vllm/src/server.rs, lib/mocker/servers/vllm/src/main.rs, lib/mocker/servers/vllm/tests/sidecar.rs, lib/mocker/servers/vllm/README.md	VllmMockerService::new is asynchronous. Supported modes configure ZMQ KV publishers and expose their endpoints. Documentation describes configuration and failure behavior.
KV event and sidecar validation lib/mocker/servers/vllm/src/server_tests.rs, lib/mocker/servers/vllm/tests/common/mod.rs, lib/mocker/servers/vllm/tests/sidecar.rs	Tests cover event-source discovery, publisher failures, native event relay, cache hits, and eviction handling.

Priority: ⬇️ Low

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟡 Moderate · up to 6b358

Following the documented separate-container setup leaves the sidecar unable to reach KV events. Correct the deployment guidance or support configurable bind and advertised addresses before merge.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 27 functions across 6 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The description covers the change, implementation areas, validation, reviewer starting points, and no-related-issue confirmation. However, it uses "Summary" instead of the required "Overview" heading …	Rename "Summary" to "Overview" and update the implementation details to describe the asynchronous VllmMockerService::new API and awaited construction. Ensure the description matches the committed changes.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary change: publishing native KV events from the vLLM gRPC server.

Full details: Docstring Coverage

Explanation

Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 27 functions across 6 files. (2 skipped: 2 unsupported.)

Full details: Description check

Explanation

The description covers the change, implementation areas, validation, reviewer starting points, and no-related-issue confirmation. However, it uses "Summary" instead of the required "Overview" heading and states that VllmMockerService::new remains synchronous, while the changes make this method asynchronous.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@lib/mocker/servers/vllm/README.md`:
- Around line 57-58: Update the cross-container guidance in the README: require
the sidecar to share the vLLM server’s network namespace, or first implement
configurable bind and advertised addresses before documenting separate-container
or host deployment. Remove the claim that exposing a fixed port alone makes the
discovered 127.0.0.1 endpoint reachable.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between d8dfcd1 and 6b3585b.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (8)

• lib/mocker/servers/vllm/Cargo.toml
• lib/mocker/servers/vllm/README.md
• lib/mocker/servers/vllm/src/main.rs
• lib/mocker/servers/vllm/src/server.rs
• lib/mocker/servers/vllm/src/server_tests.rs
• lib/mocker/servers/vllm/tests/common/mod.rs
• lib/mocker/servers/vllm/tests/sidecar.rs
• lib/mocker/src/services/zmq_events.rs

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[dmitry-tokarev-nv]

Approving. Reviewed the whole diff, verified by execution on macOS ARM64 at head 27b9f9c with a scratch CARGO_TARGET_DIR. No P0/P1/P2/P3 findings. I have replied on the three existing threads with independent verification rather than opening duplicates.

Because the mocker is a test oracle, I treated event fidelity as the decisive question — not "does it publish events" but "are they the same events a real vLLM engine emits".

Event fidelity

The published events are consumed by the same decoder real vLLM's ZMQ events go through — dynamo_kv_router::zmq_wire::decode_event_batch → convert_event, reached via KvEventSourceConfig::Zmq. There is no mocker-specific decode path, so any divergence would surface as a decode error or a filtered event, not as a silently different index.

I traced dynamo_kv_router::zmq_wire::convert and dynamo_llm::kv_router::publisher through the new sidecar test. For the measured prompt [11,22,33,44,55,66,77,88] at block_size = 4:

Creating stored block: external_block_hash=4117275816416024698, tokens_hash=4117275816416024698, token_ids=[11, 22, 33, 44]
Creating stored block: external_block_hash=5947898228081096063, tokens_hash=6081346995852963768, token_ids=[55, 66, 77, 88]

• Block hashes are sequence-aware (parent-chained), as vLLM's are — not position-independent. The root block's chained hash equals its local hash; the second block's does not. That was the divergence I was most worried about, because a position-independent external hash would let one eviction remove the wrong radix node and leave the router over-reporting a cache it does not have. It does not happen.
• tokens_hash is recomputed by the decoder from the published token_ids and matches the router's own compute_block_hash_for_seq over the request tokens. That is the property KV-aware routing actually depends on, and the test asserts it.
• Store/remove semantics line up. Both blocks arrive in one batch in sequence order with parent_hash: None at the prompt root, and eviction reports the chained hashes leaf-first: Removed([5947898228081096063, 4117275816416024698]) — child before parent, which is the only safe order for a radix tree.
• Across the full run there were no Failed to decode, no filtered-event, and no "Block not published" warnings — so nothing is being silently dropped at the consumer.

One structural point in the encoder's favour: KvEventData in the upstream simulation crate has only Stored and Removed variants, so the Cleared => vec![] arm in convert_to_zmq_events is unreachable from this producer. There is no path where a cache clear is swallowed and leaves the index stale.

Mutation testing — 5 mutations, 5 kills

mutation	test killed
XOR 1 into every published stored block_hash	sidecar_relays_stored_and_evicted_blocks (eviction phase)
suppress BlockRemoved entirely	sidecar_relays_stored_and_evicted_blocks (eviction phase)
+1 every published token_id	sidecar_relays_stored_and_evicted_blocks (store phase)
fall back to an automatic port when the configured bind fails	failed_kv_publisher_is_not_advertised
drop the config.mode != ServerMode::Decode guard	kv_event_discovery_follows_regular_mocker_rules

The new tests pin what they claim. I also flipped admitting_args() back to enable_prefix_caching(true): all 15 lib tests still pass, so that fixture change is pure socket isolation and is not masking a failure.

Default path, and only one event path

MockEngineArgs::default() has enable_prefix_caching = true, so a server started with no --extra-engine-args now binds an ephemeral ZMQ PUB socket and advertises one source where it previously advertised none. That is the intended change and the README says so; the two things I checked are that it cannot surprise a non-KV deployment, and that it cannot double-publish:

• Nothing subscribes unless KV routing is on — lib/backend-common/src/worker.rs returns before kv_event_sources() when !config.enable_kv_routing, and returns early again when the source list is empty, so a failed bind degrades to exactly the pre-PR behaviour.
• KvEventPublishers::new(None, sink) installs a raw sink only, with no in-process relay, and GetKvEventSources returns exactly the one bound endpoint. There is no configuration where the ZMQ path and an event-plane path are both live.
• Token-id retention is wired correctly: raw_enabled() drives emit_kv_token_ids in grouped_scheduler.rs, so the encoder's assert_eq!(token_ids.len(), block_hashes.len() * block_size) cannot be reached with empty token ids through this path.

Coverage

rust-tests (.) runs cargo test --locked --all-targets from the workspace root, and lib/mocker/servers/vllm is a workspace member with no default-members narrowing, so the new unit tests and the tests/sidecar.rs integration target genuinely execute in pre-merge — not skipped.

Where verification stopped

• Not compared against a live vLLM process. No vLLM is installed here. "Matches real vLLM" above means: matches the decoder this repo uses for real vLLM events, and matches the schema pinned by components/src/dynamo/vllm/tests/test_vllm_kv_events_api.py.
• Disaggregated publishing is only unit-tested at the discovery level. (Prefill, true, 1) and (Decode, true, 0) assert source counts; no test runs a prefill/decode pair and checks that the prefill leg's events reach an index.
• No container run, so the README's cross-container port-mapping instructions are verified by reading zmq_connect_endpoint and by the observed wildcard-to-host rewrite, not by running two containers.

One local-environment note, not a finding

On this Mac sidecar_relays_stored_and_evicted_blocks fails at "KV relay did not become ready" unless DYN_EVENT_PLANE_HOST=127.0.0.1 is set: dynamo-runtime's event plane advertises the host's LAN IP and the loopback SUB never connects. Every hop this PR owns completes in the failing run — ZMQ receive, decode, convert, and "Forwarded KV event batch to router event plane" all appear — so it is event-plane behaviour, not this change, and CI is unaffected. Recording it only so the next person on macOS does not chase it.