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L3 Patterns-in-Code Document — HELEP (Template)

~3 pages. Each pattern entry needs a code citation (file:line). No citation = no marks.

Part A — Pre-implemented patterns

Identify and explain each pattern below as it appears in the starter code.

A.1 Choreographed Saga

  • Where: trace events from sos-service/app/main.py → dispatch-service/app/main.py → notification-service/app/main.py
  • Compensation step: where, and what state is rolled back?
  • What event is the rollback trigger?

A.2 Pub/Sub via Apache Kafka

  • Where: app/events.py in every service (aiokafka producer + consumer)
  • Consumer group semantics: explain at-least-once + how manual await consumer.commit() AFTER handler success preserves the invariant (auto-commit is disabled)
  • Partition keying: explain why publish(..., key=incident_id) matters for the saga (same-key events → same partition → ordering)

A.3 Repository

  • Where: app/db.py in every service
  • Why: what would break if we let route handlers query SQLite directly?

A.4 Strategy

  • Where: dispatch-service/app/matching.py
  • How to switch: environment variable MATCHER
  • Add a third strategy (e.g. round-robin) and cite the lines you added

A.5 Outbox-lite

  • Where: sos-service/app/main.py trigger()
  • Why is this "lite"? What would a real Outbox add?

A.6 Circuit Breaker (stub → complete it)

  • Where: events.py class CircuitBreaker
  • Task: complete the allow() state machine (CLOSED → OPEN → HALF_OPEN) and cite the lines you added.
  • Explain what triggers state transitions in your impl.

Part B — Patterns you added (minimum 2)

For each pattern:

  • Pattern name (GoF, EAA, or Cloud-Native catalogue)
  • Where you added it (file:line)
  • Problem it solves in HELEP
  • Trade-off vs alternative pattern

B.1 your first pattern

B.2 your second pattern

Part C — Anti-patterns avoided

Briefly call out one anti-pattern the architecture explicitly avoids (e.g. shared database across services, distributed monolith, synchronous fan-out) and cite the file that demonstrates the avoidance.

Submission

Submit as patterns.pdf. Keep code excerpts ≤ 10 lines.