[disagg] Fix KV-event publisher port collision under pure data parallelism (#29211)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

python/sglang/srt/disaggregation/kv_events.py

@@ -36,6 +36,28 @@ from pydantic import BaseModel
 logger = logging.getLogger(__name__)
 
 
+def select_kv_publisher_dp_rank(
+    attn_dp_size: int, attn_dp_rank: int, dp_rank: Optional[int]
+) -> int:
+    """Index used to offset this scheduler's KV-event publisher port.
+
+    Each independent KV cache must publish on its own port so a consumer can
+    subscribe per replica. There are always ``dp_size`` such publishers; which
+    rank distinguishes them depends on the parallelism mode:
+
+    - DP-attention (``attn_dp_size > 1``): each attention-DP rank owns a KV
+      cache shard, so distinguish by ``attn_dp_rank``.
+    - Pure DP (``attn_dp_size == 1``): every worker has ``attn_dp_rank == 0``,
+      so distinguish by ``dp_rank`` (the data-parallel replica index).
+
+    Both span ``0..dp_size-1``, matching the ``dp_size`` advertised in
+    ``/server_info`` and the per-rank ports the router subscribes to.
+    """
+    if attn_dp_size > 1:
+        return attn_dp_rank
+    return dp_rank or 0
+
+
 class EventBatch(
     msgspec.Struct,
     array_like=True,  # type: ignore[call-arg]

python/sglang/srt/managers/scheduler_components/kv_events_publisher.py

@@ -15,6 +15,7 @@ import zmq
 from sglang.srt.disaggregation.kv_events import (
     EventPublisherFactory,
     KVEventBatch,
+    select_kv_publisher_dp_rank,
 )
 from sglang.srt.managers.io_struct import hook_custom_types, sock_send
 
@@ -69,7 +70,10 @@ class SchedulerKvEventsPublisher:
 
         if self.enable_kv_cache_events:
             self.kv_event_publisher = EventPublisherFactory.create(
-                kv_events_config, self.ps.attn_dp_rank
+                kv_events_config,
+                select_kv_publisher_dp_rank(
+                    self.ps.attn_dp_size, self.ps.attn_dp_rank, self.ps.dp_rank
+                ),
             )
 
     def emit_kv_metrics(self):