[PD-Disagg] Support query dp rank from bootstrap server.

[hnyls2002]

Motivation

Based on PR #14726 by @changhuaixin. Allows prefill servers to use load balance methods other than follow_bootstrap_room (e.g., round_robin) in PD-disaggregation mode, which significantly reduces TTFT under load (see benchmarks in #14726).

When prefill uses non-deterministic load balancing, the decode server cannot infer which prefill DP rank processed a request. This PR adds per-request dp_rank synchronization through the bootstrap server.

Modifications

• Bootstrap server: New POST /register_dp_rank and POST /query_dp_ranks endpoints with TTL-based cleanup. Parallel info now includes follow_bootstrap_room flag so decode knows the prefill's LB strategy.
• Prefill sender: Registers dp_rank per request when not using follow_bootstrap_room.
• Decode side: Resolves dp_rank before creating the KV receiver. Requests whose dp_rank is not yet available are held in a pending_reqs queue and batch-resolved each scheduler loop.
• Connection-level init: Extracted ensure_parallel_info() on CommonKVManager to decouple parallel info fetching from receiver creation, shared by both DecodePreallocQueue and CommonKVReceiver.
• Naming cleanup: Removed redundant target_dp_group alias; unified to prefill_dp_rank.

Design difference from #14726

dp_rank is resolved before receiver creation, not inside init(). This means:

• No half-initialized receiver state or should_notify_dp_rank flag
• init() signature unchanged — zero changes to mooncake/nixl/fake backends
• pop_preallocated loop has no dp_rank awareness
• When using follow_bootstrap_room, dp_rank is computed locally (bootstrap_room % dp_size) with no HTTP overhead — the original PR always queries the bootstrap server

[gemini-code-assist]

Summary of Changes

Hello @hnyls2002, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the PD-Disaggregation feature by introducing dynamic data parallel (DP) rank resolution capabilities. It allows decode instances to actively communicate with the bootstrap server to register and query DP ranks, moving beyond static assignments. The bootstrap server itself has been upgraded with new API endpoints and a crucial cleanup mechanism to maintain the integrity of registered entries. These changes aim to improve the robustness and flexibility of distributed KV cache management, particularly in scenarios involving complex load balancing strategies.

Highlights

• Dynamic DP Rank Resolution: Implemented a mechanism for decode instances to query and register their data parallel (DP) ranks with the bootstrap server, allowing for more flexible load balancing beyond simple bootstrap_room modulo.
• Bootstrap Server Enhancements: The bootstrap server now supports new API endpoints (/register_dp_rank, /query_dp_ranks) to facilitate DP rank management and includes a periodic cleanup task for expired entries.
• Configuration and Documentation: Introduced a new environment variable (SGLANG_DISAGGREGATION_BOOTSTRAP_ENTRY_CLEANUP_INTERVAL) to control the cleanup frequency, and updated documentation.
• Code Refactoring: Streamlined the KVSender and KVReceiver classes by making register_to_bootstrap a public abstract method and adjusting internal logic for parallel information retrieval.

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Changelog

• docs/advanced_features/pd_disaggregation.md
  • Added documentation for the new SGLANG_DISAGGREGATION_BOOTSTRAP_ENTRY_CLEANUP_INTERVAL environment variable.
• python/sglang/srt/disaggregation/base/conn.py
  • Added an abstract method register_to_bootstrap to BaseKVSender.
• python/sglang/srt/disaggregation/common/conn.py
  • Imported time and envs modules.
  • Added server_args to KVSender initialization.
  • Renamed _register_to_bootstrap to register_to_bootstrap and made it a public method.
  • Included load_balance_method in the payload when registering to the bootstrap server.
  • Added follow_bootstrap_room_table to KVSender to store load balancing information.
  • Introduced _register_prefill_dp_rank method in KVReceiver to register the prefill DP rank with the bootstrap server.
  • Modified _get_prefill_parallel_info_from_server to return the follow_bootstrap_room status.
  • Refactored prefill_dp_rank assignment logic and introduced _setup_bootstrap_infos in KVReceiver.
  • Updated the bootstrap_key generation to use prefill_dp_rank instead of target_dp_group.
  • Modified calls to _get_bootstrap_info_from_server to use prefill_dp_rank.
  • Updated CommonKVBootstrapServer constructor to accept dp_size.
  • Added follow_bootstrap_room, room_to_dp_rank, and entry_cleanup_interval attributes to CommonKVBootstrapServer.
  • Added new routes /register_dp_rank and /query_dp_ranks to the bootstrap server.
  • Implemented _handle_register_dp_rank and _handle_query_dp_ranks handlers for the new routes.
  • Included follow_bootstrap_room in the response from the /route endpoint.
  • Implemented _cleanup_expired_entries as an asynchronous task for periodic cleanup of room_to_dp_rank entries.
  • Added a static method query_prefill_dp_ranks to CommonKVReceiver for batch querying DP ranks.
• python/sglang/srt/disaggregation/decode.py
  • Added a pending_reqs list to DecodeScheduler to hold requests awaiting DP rank resolution.
  • Introduced _resolve_dp_rank to determine the DP rank for a request.
  • Created _create_receiver_and_enqueue to encapsulate receiver creation and request enqueuing.
  • Modified the add method to use the new DP rank resolution and queuing logic.
  • Added _resolve_pending_reqs to batch-resolve DP ranks for pending requests.
  • Called _resolve_pending_reqs in pop_preallocated to process pending requests.
• python/sglang/srt/environ.py
  • Added SGLANG_DISAGGREGATION_BOOTSTRAP_ENTRY_CLEANUP_INTERVAL environment variable with a default value of 120 seconds.
• python/sglang/srt/managers/disagg_service.py
  • Passed the dp_size argument to the kv_bootstrap_server_class constructor.
• python/sglang/srt/server_args.py
  • Removed the backward compatibility warning for the round_robin load balancing method in PD prefill mode.

Activity

• The pull request is currently marked as 'WIP' (Work In Progress), indicating that the author is still actively developing and refining the changes.

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[gemini-code-assist]

Code Review

This pull request adds support for querying the data parallelism (DP) rank from the bootstrap server, which is a key feature for enabling more flexible load balancing strategies like round_robin in a prefill/decode disaggregated setup. The changes include adding new API endpoints to the bootstrap server for registering and querying DP ranks, along with a cleanup mechanism for stale entries. The decode scheduler is updated to handle requests that need to query the DP rank by queueing them and processing them in batches. The changes are well-structured, but there are a couple of issues regarding a new dp_size parameter that is introduced but not correctly handled in the class hierarchy and is unused in the constructor.

[hnyls2002]

/tag-and-rerun-ci

[ShangmingCai]

CI looks good. Failed amd tests are irrelevant.