Dotnet Background Processing

Dotnet Background Processing (Azure Functions)

This document details asynchronous background processing in PluckIt.Functions.

Documentation metadata

• Audience: external contributors
• Last reviewed: 2026-03-16
• Scope: queue and timer contracts only

Queue worker: ProcessImageJob

• Trigger: ProcessImageJob queue function
• Queue name: image-processing-jobs
• Input message: ImageProcessingMessage
• Workflow:
  1. Idempotency check (DraftStatus == Processing)
  2. Download raw blob
  3. Optional segmentation path:
     • If skip_segmentation is true, upload archive directly
     • Else call Python processor endpoint /api/process-image
  4. Parse processor result and persist draft terminal state
  5. Extract clothing metadata from processed image
  6. Write DraftStatus.Ready or Failed with error text

Timer worker: CleanupAbandonedDrafts

• Trigger: 0 0 3 * * * (03:00 UTC)
• Behavior:
  • Mark stale Processing drafts older than 2 hours as Failed
  • Purge old Failed drafts older than 7 days
  • Also performs related blob cleanup

Operational notes: queue and timer failure behavior

• Failure mode: ProcessImageJob error before final draft write
  • Expected outcome: draft transitions to DraftStatus.Failed with ErrorText populated, or remains retriable on re-invocation
  • Retry/alert: call POST /api/wardrobe/drafts/{id}/retry; track retry counts and alarm on sustained Failed spikes
• Failure mode: queue message consumed while host restarts mid-processing
  • Expected outcome: message can be retried; idempotency guard prevents duplicate finalization when DraftStatus != Processing
  • Retry/alert: check queue depth and dead-letter activity
• Failure mode: CleanupAbandonedDrafts miss at schedule window
  • Expected outcome: stale Processing and stale Failed records remain until next successful timer run
  • Retry/alert: alert on cron drift and LastSuccess lag > 25h
• Failure mode: timer blocked by partial failures (blob/API cleanup errors)
  • Expected outcome: cleanup of terminal states may be incomplete while visible to queries
  • Retry/alert: rerun timer manually after dependency recovery and track failed blob deletion counts

Issue update log

• 2026-03-16: Implemented issue #59 in ClothingMetadataService so invalid LLM category values from metadata extraction are now validated against an allowlist, with fallback to Uncategorised and warning logging.