tasks/INC8_cascading_timeout/spec.md

INC8: Cascading Timeout

Goal

A 3-service request chain has timeout misconfigurations that cause cascading failures, plus retry policies that amplify the problem. Fix the timeout cascade and the 2 harmful retry configurations while preserving the 2 correct retries.

Requirements

1. Fix the timeout cascade: upstream timeouts must be >= downstream timeouts
2. Fix the 2 harmful retry configurations
3. Preserve the 2 correct retry configurations (documented in RETRY_POLICY.md)
4. All tests must pass after changes: pytest tests/

Architecture

Three services form a synchronous request chain:

gateway.py  -->  order_service.py  -->  inventory_service.py

Each service has configurable timeout and retry settings.

Timeout Cascade Bug

The gateway timeout (5s) is SHORTER than the order_service timeout (10s). When the order_service takes 6-10 seconds to respond, the gateway times out first, returning an error to the client. Meanwhile, the order_service continues processing (zombie request), potentially completing a state-changing operation that the client thinks failed.

Fix: gateway timeout must be >= order_service timeout + buffer. Similarly, order_service timeout must be >= inventory_service timeout + buffer.

Retry Configurations (4 total)

Correct Retries (preserve these)

1. Idempotent GET retry: GET /inventory/check retries on timeout are safe because the operation is read-only. Documented in RETRY_POLICY.md.

2. Health check retry: /health endpoint retries are safe because health checks are idempotent and stateless. Documented in RETRY_POLICY.md.

Harmful Retries (fix these)

3. Non-idempotent POST retry: POST /orders/create retries on timeout. If the first request succeeded but the response was lost (timeout), the retry creates a duplicate order and double-charges the customer. Fix: disable retry for non-idempotent operations, or require idempotency keys.

4. Retry storm amplification: When inventory_service is slow, order_service retries 3 times. Each retry causes gateway to also retry, creating 3x3=9 total requests to inventory_service from a single client request. Fix: add retry budget or circuit breaker to prevent amplification.

Supporting Files

• gateway.py — API gateway with timeout and retry config
• order_service.py — order processing service
• inventory_service.py — inventory checking service
• INCIDENT_REPORT.md — documents a past cascading timeout incident
• RETRY_POLICY.md — documents which operations are idempotent
• config.py — service configuration (timeouts, retry counts)
• tests/test_services.py — service tests (must all pass)