[feature][kv_offload] Self-describing KV events for OffloadingConnector

[Change72]

What

This PR makes native OffloadingConnector CPU-offload KV-cache events self-describing behind an
explicit opt-in:

kv_connector_extra_config={
    "self_describing_kv_events": True,
}

The flag is inert unless vLLM KV cache events are also enabled. With the flag off, the connector
keeps the legacy placeholder payload (token_ids=[], block_size=0, parent_block_hash=None);
note that stored events are now emitted one-per-offload-key rather than one-per-batch.

The implementation isolates the event payload logic in events.py, derives
OffloadingKVEventsConfig once from the vLLM KV-event config plus connector extra config, and
carries per-group KV-cache metadata through OffloadingEventGroupSpec. The opt-in is documented in
docs/features/kv_offloading_usage.md.

For CPU offload stores, the connector now records the request metadata while it still has access
to the request and KV-cache-group context, then emits BlockStored events with:

• block_hashes
• parent_block_hash
• token_ids
• per-block block_size
• LoRA metadata
• group_idx and cache-spec metadata

For chunked offloading (kv_connector_extra_config["block_size"] > --block-size), one offloaded
CPU chunk is emitted as one BlockStored carrying all constituent per-block hashes plus the whole
chunk token span. block_size remains the GPU/hash block size, not the whole chunk size. Removal
events fan out the same constituent hashes.

Why

The legacy connector events were useful for observability but not sufficient for external routers
or lower-tier indexers. They often carried placeholder payloads (token_ids=[], block_size=0,
parent_block_hash=None), so consumers such as Dynamo could not reconstruct their own block keys
or parent chain and had to drop the CPU-tier events.

Making the producer self-describing keeps the event contract local to the connector: a consumer
does not need to join CPU events with a separate GPU event stream or depend on event ordering.

Chunk overlap semantics

The producer intentionally uses plain fan-out. In chunk mode, if a shared prefix is not aligned to
the offloaded chunk size, two sibling chunks can legitimately list the same constituent per-block
hash. In that case duplicate store/remove announcements are expected on the wire.

Consumers that index at per-block granularity must ref-count or deduplicate those duplicate
announcements. Dynamo's standard worker publisher path already does this with EventDedupFilter.
Filter-less consumers may conservatively under-credit CPU-tier cache after overlapping chunk
evictions, but this does not create data corruption.

Scope

• Native OffloadingConnector.
• CPUOffloadingSpec / single-tier CPU offload.
• Full-attention groups.
• By-block mode and chunk mode.
• Sliding-window / SSM groups keep the legacy placeholder payload.
• TieringOffloadingSpec rejects self_describing_kv_events for now; multi-tier support needs a
  follow-up design that preserves event metadata across tier transitions.
• extra_keys-heavy workloads such as multimodal/cache-salt/prompt-embedding paths should be
  validated separately before relying on the new payload for routing.

Tests

Unit coverage is split between a focused event-tracker test file and scheduler-level integration
coverage.

tests/v1/kv_connector/unit/offloading_connector/test_events.py covers:

• by-block self-describing BlockStored
• chunked store/remove with multiple constituent hashes
• cross-batch parent chaining
• order-independent store emission
• opt-out placeholder behavior
• sliding-window fallback behavior
• multi-group removal grouping
• store -> remove -> re-store after eviction
• reset behavior
• TieringOffloadingSpec scope guard

tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py keeps the scheduler-level
integration coverage.

Validated on the workstation:

python -m pytest tests/v1/kv_connector/unit/offloading_connector/test_events.py -q
# 9 passed

python -m pytest tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py -q
# 70 passed

The latest GitHub pre-commit check also passes on the PR head.

End-to-end validation

Validated with Dynamo PR #10368 on a real single-GPU L4 stack:

• model: Qwen/Qwen3-0.6B
• vLLM block size: 16
• offloaded chunk size: 48 (factor=3)
• CPU pool: 128 MiB, intentionally small to force real CPU LRU evictions
• explicit ZMQ KV events enabled
• self_describing_kv_events=true
• Dynamo worker publisher EventDedupFilter in the path

Result:

• CPU BlockStored: 354 events, zero placeholders, every CPU chunk store has n_hashes=3
• CPU BlockRemoved: 24 events from real CPU evictions
• GPU BlockStored: 331 events
• router metric kv_cache_events_applied: stored ok = 685, removed ok = 24
• zero lower-tier warnings / BlockNotFound

Duplication check

Not a duplicate. gh pr list --repo vllm-project/vllm --state open --search "self_describing_kv_events"
returns only this PR (#43468). Nearby open OffloadingConnector / KV-offload PRs address different
concerns, for example #43946 (eviction-triggered store), #44295 (request convoy on shared loading
blocks), #42992 (DSV4 crash fix), #45693 (hugepage tiering), and #44865 (reshape per-group transfer
data model). #44865 is the closest in surface area (it touches per-group offload specs/alignment)
but is orthogonal to self-describing KV events.

AI assistance

This change was developed with AI assistance. The human submitter has reviewed every changed line,
runs the tests, and owns the change end-to-end.

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[gemini-code-assist]

Code Review

This pull request enhances the OffloadingConnectorScheduler to support self-describing, routable KV cache events for 1:1 full-attention configurations. It introduces a side-table (_pending_event_metadata) to capture block metadata—including token IDs, parent hashes, and LoRA information—during the store job creation phase. This metadata is then used to populate BlockStored events when they are drained via take_events, while other configurations fall back to placeholder payloads. Additionally, the PR includes comprehensive unit tests covering event emission, parent chaining, and cache resets. I have no feedback to provide.

[orozery]

The current KVEvents are sufficient, at least from an llm-d consumer point of view.
Not sure about Dynamo, but I don't think they integrate with the offloading connector as they have their own connector.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Change72.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[Change72]

Thanks @orozery for the comment!

This PR is driven by a concrete integration requirement from the Dynamo KVBM team, which consumes vLLM's KVEvents to maintain a router-side prefix index across both GPU and CPU tiers. With the current block_size=0 / empty token_ids payload, their wire decoder drops every CPU offload event, so the CPU pool is invisible to KV-aware routing.

The architecture matches the SGLang + HiCache pattern — let the framework own the offload, and let the router-side block manager consume the events rather than re-implement the pool. That requires the events to be self-describing.

Backwards-compatible: consumers that only read block_hashes + medium (which seems to cover llm-d's current usage) keep working unchanged. The change also closes the asymmetry with block_pool.py, which already emits the full payload on the GPU tier.

[Change72]

@orozery I looked at the llm-d event path a bit more. My read is that empty-token CPU offload BlockStored events are handled as a location/device-tier update:

• normal BlockStored with token_ids computes canonical request keys from parent_hash + token_ids
• empty-token CPU offload events call GetRequestKey(engineKey) for each emitted hash, then add a CPU-tier PodEntry for the resolved request keys

That explains why the current placeholder payload can work when the matching GPU BlockStored has already created the engineKey -> requestKey mapping.

I still wanted to check two concrete edge cases:

1. block_size_factor > 1 / hash_block_size_factor > 1

   In the offloading connector, one OffloadKey can represent a grouped offload block rather than a single vLLM GPU/hash block. For hash_block_size_factor > 1, RequestOffloadState.update_offload_keys() samples every hash_block_size_factor-th request block hash, effectively using the last hash in the group to construct the offload key.

   For a normal BlockStored that carries token_ids, llm-d can infer the mapping from the ratio of len(engineKeys) to the canonical request keys it recomputes.

   But for the current CPU offload placeholder, token_ids=[] and block_size=0, so llm-d cannot recompute the canonical keys for the grouped range. It can only resolve the emitted hash through the existing engineKey -> requestKey table.

   In that case, is the intended llm-d behavior that the CPU offload event updates only the request key mapped from the emitted last hash, or is there an expectation that it should update all request/canonical blocks covered by that grouped offload range? If the latter, where is that range recovered from without token_ids / block_size?

2. GPU BlockRemoved before CPU BlockStored

   The offload copy is async, so it seems possible for a GPU BlockRemoved(X) event and the later CPU BlockStored(X) event to be observed in an order where the GPU removal is processed first.

   In llm-d, the empty-token CPU update path appears to require:

   rk, err := p.index.GetRequestKey(ctx, engineKey)

   If the GPU removal was the last tier entry for that request key, does the engineKey -> requestKey mapping remain available for the later CPU store? Or is there an ordering guarantee that CPU BlockStored is always processed before the corresponding GPU BlockRemoved?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Change72.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Change72.

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[orozery]

@Change72 Thanks for taking on integrating the offloading connector with dynamo!
I'm good with introducing it, but I want to keep the offloading connector scheduler.py code neat.
My suggestion is that we move all logic into a new file (e.g. events.py).
A new class will be responsible for populating fields of KV events (maintaining _pending_event_metadata).
This class will be used in OffloadingConnectorScheduler.take_events, as well as updated in _build_store_jobs.
I would also make this an opt-in feature.

[orozery]

Can we add a comment here explaining this field?
Also in kv_offloading_usage.md.

[Change72]

Done.

• Added comments on OffloadingKVEventsConfig explaining both fields.
• Added self_describing_kv_events to kv_offloading_usage.md.

The docs call out that this is currently single-tier only, inert unless global KV events are enabled, and rejected by TieringOffloadingSpec.

[orozery]

I suggest we remove the default (it makes no sense).
This means moving this field up above sliding_window_size_in_blocks.

[Change72]

Done.

Removed the default from kv_event_group_spec and moved it before the defaulted fields in GroupOffloadConfig.

The field is now required for every group, which matches how it is constructed from the corresponding KVCacheGroupSpec.

[orozery]

None returns remain only for real fallback cases:

1. a constituent block hash is unavailable
2. the parent hash is unavailable

Can you elaborate on the flows which make them possible?

My Claude could not verify it:

1. Block hash within the chunk is None — req.block_hashes is typed as list[BlockHash] (where BlockHash = NewType("BlockHash", bytes)). It's only ever populated by hash_block_tokens() which always returns a real BlockHash. The list grows by appending valid hashes; it never inserts None. Additionally, the existing code in update_offload_keys() passes req.block_hashes entries directly to make_offload_key() which would crash on None — proving the invariant is relied upon elsewhere without guards.
2. Parent block hash is None — Same thing. If block_hashes[first_hash_idx] is valid (which it must be, since we just iterated over the chunk), then block_hashes[first_hash_idx - 1] is also a valid BlockHash for any first_hash_idx > 0, because the list only contains real hashes up to its length.

[orozery]

Missing test according to Claude:

No test for re-store after eviction (the overwrite case)
record_store does self._pending_event_metadata[offload_key] = meta — if the same prefix is re-offloaded after eviction (eviction pops the entry, then the same blocks are offloaded again), this correctly writes a fresh entry. But there's no test covering this cycle (store → evict → re-store → emit).

[Change72]

Done.

Added test_take_events_supports_restore_after_eviction.

It covers:

1. store and emit metadata
2. remove and pop the side-table entry
3. re-record the same offload key
4. emit again with fresh metadata

This verifies the overwrite/re-store cycle after eviction.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Change72.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

Documentation preview: https://vllm--43468.org.readthedocs.build/en/43468/

[mergify]

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[mergify]

Hi @Change72, the pre-commit checks have failed. Please run:

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Then, commit the changes and push to your branch.

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[orozery]

Thanks for the hard work @Change72 !

[Change72]

Thanks again for the careful review @orozery! I really appreciate the guidance throughout this PR.