Summary

Completes the migration to OTEL-based journey tracing by removing all intermediate buffering and export mechanisms introduced in earlier PRs. This is the final PR in the 9-part journey tracing implementation series.

Changes:

• Remove journey event buffer dictionary and all buffering logic from scheduler
• Remove journey event flushing in update_from_output()
• Remove journey event export from output processor's do_tracing()
• Add direct timestamp capture (queued_ts, scheduled_ts) to Request object using time.monotonic()
• Propagate timestamps through EngineCoreOutput for Prometheus metrics
• Preserve backward compatibility with deprecated journey_events parameters

Key Design Decisions:

• Journey events now emit exclusively as OTEL spans (real-time)
• Prometheus metrics capture timestamps directly on Request objects (independent of tracing)
• Monotonic time domain used for all duration measurements
• Defensive guard ensures scheduled_ts set only once (never overwritten)

Test Plan

Added 16 comprehensive tests in tests/v1/core/test_pr9_no_buffering.py:

✅ TestNoBuffering (3 tests)

• Verify buffer dictionary doesn't exist
• Verify no buffering during request lifecycle
• Verify EngineCoreOutputs.journey_events always None

✅ TestSpanInfrastructure (2 tests)

• Verify span tracking infrastructure exists
• Verify span cleanup is safe and idempotent

✅ TestMetricsIndependence (3 tests)

• Verify metrics work with tracing disabled
• Verify monotonic time domain used
• Verify timestamps stored in Request object

✅ TestBackwardCompatibility (2 tests)

• Verify journey_events parameter accepted
• Verify EngineCoreOutputs.journey_events field exists

✅ TestTimestampCapture (4 tests)

• Verify queued_ts captured on add_request
• Verify scheduled_ts captured on first schedule
• Verify scheduled_ts not overwritten on subsequent schedules
• Verify timestamps not captured when log_stats=False

✅ TestZeroOverheadWhenDisabled (2 tests)

• Verify no tracing structures when disabled
• Verify metrics independent of tracing

Test Results: All 16 PR #9 tests pass ✓
Regression Check: All existing scheduler tests pass ✓

Dependencies

This PR depends on PRs #1-8 in the journey tracing series:

• PR [Feature] Add request journey event tracing to v1 scheduler #2: Scheduler span lifecycle management
• PR [Feature] Add CLI flag for journey tracing with OTEL integration #3: Journey state cleanup
• PR [Refactor] Use SpanAttributes constants for journey event attributes #4: Event emission to core spans
• PR [Bugfix] Fix prefill progress tracking for chunked prefill preemption #5: API span tracking infrastructure
• PR [Feature] Implement dual-stream journey tracing with OTEL spans #6: API parent span lifecycle
• PR [Bugfix] Remove legacy EngineCoreEvent system and restore Prometheus metrics #7: API↔Engine context propagation
• PR [Feature] Initialize OTEL tracer in scheduler for journey tracing (PR #1/9) #8: API lifecycle events and request attributes

Files Modified

• vllm/v1/core/sched/scheduler.py - Remove buffering, add direct timestamp capture
• vllm/v1/engine/output_processor.py - Remove export, propagate timestamps
• vllm/v1/engine/async_llm.py - Remove journey event distribution
• vllm/v1/request.py - Add queued_ts, scheduled_ts fields
• vllm/v1/engine/__init__.py - Add timestamp fields to EngineCoreOutput
• tests/v1/core/test_pr9_no_buffering.py - New comprehensive test suite
• JOURNEY_TRACING_PR_PLAN.md - Update PR [CI] Add Docker build and push workflow #9 design specification

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