The core issue is a concurrency conflict, where multiple processes attempt to update the same record simultaneously, causing errors. Recommendation A directly addresses this by introducing a coordinating middleware layer. This layer can manage the delivery of requests using patterns like message queuing with exclusive consumers, ensuring only one process handles a specific payment request at a time, thus eliminating update conflicts.

To improve the SLA, the system must be resilient. Recommendation C introduces idempotency, a critical principle for reliable systems. By ensuring the Payment System processes a unique request only once, it allows for safe retries (a common technique to handle transient errors) without the risk of duplicate payments. This combination of coordination and idempotency creates a robust and reliable system.

1. Hohpe

G.

& Woolf

B. (2003). Enterprise Integration Patterns: Designing

Building

and Deploying Messaging Solutions. Addison-Wesley.

For A (Coordination): The "Message Broker" pattern describes a central component that decouples destinations from each other

enabling coordination and routing of messages to appropriate consumers. (Chapter 5: Messaging Routing

pp. 136-141).

For C (Idempotency): The "Idempotent Receiver" pattern is explicitly designed to handle duplicate messages safely

which is essential for systems that use retries to achieve reliability. The pattern states the receiver must be able to "safely receive the same message multiple times." (Chapter 10: Messaging Endpoints

pp. 433-436).

2. Kleppmann

M. (2017). Designing Data-Intensive Applications: The Big Ideas Behind Reliable

Scalable

and Maintainable Systems. O'Reilly Media.

For A (Coordination): Chapter 9

"Consistency and Consensus

" discusses the necessity of coordination mechanisms (e.g.

locking

transactions) to prevent data corruption from concurrent writes

which is the exact problem described.

For C (Idempotency): Chapter 11

"Stream Processing

" in the section on "Fault Tolerance

" explains that idempotency is a fundamental requirement for achieving exactly-once processing semantics

which is necessary for reliable systems like payment processors to meet their SLAs.