Investigate High Memory Usage

You are on call as an engineer for a delivery platform.

System context

• Couriers accept and complete deliveries through a mobile app.
• The mobile app invokes a public gateway service (Dasher Service), which in turn calls a Payment Card Integration Service.
• At certain merchants, the courier has to pay in person with a prepaid debit card.
• The card is loaded programmatically at checkout via a third-party payment card provider.
• The integration service also depends on Redis to cache card and account information.
• The company is currently migrating from a monolith to microservices.

High-level flow: Courier App -> Dasher Service -> Payment Card Integration Service -> Third-Party Card Provider Payment Card Integration Service <-> Redis cache

Incident

At 4:30 PM Pacific, during a busy window, you receive a page because the Payment Card Integration Service is reporting memory utilization far above normal.

Describe how you would approach this on-call investigation. Your response should address:

1. How you would evaluate severity and business impact.
2. Which metrics, dashboards, and logs you would inspect first.
3. The most probable reasons for high memory usage in this architecture.
4. How you would decide whether the cause is traffic, a recent deploy, Redis behavior, retries, or the third-party provider.
5. Immediate mitigation actions you would consider.
6. How you would communicate while the incident is ongoing.
7. What long-term fixes or follow-up actions you would suggest after recovery.

Overview: This question assesses incident-response and operational debugging abilities, centered on diagnosing high memory usage in a distributed microservice architecture with caching and third-party integrations.