Fintech
Payment API Rebuild
Client name withheld under NDA · Fintech payments platform
Overview
The engagement redesigned a payment processing API that handled transaction initiation, provider communication, status tracking, and webhook processing. The prior system ran as a single synchronous flow with limited retry behavior and limited auditability.
System Architecture
Client App
React SPA
API Gateway
Rate limiting + auth
Webhook Receiver
External events
Payment Service
Event-driven · Idempotent
PostgreSQL
Transactions
Event Bus
Async processing
Audit Log
Compliance trail
Payment Provider
Stripe / ACH
Notification Service
Email + Slack
The Problem
Peak-load failures were difficult to contain because the synchronous pipeline lacked queueing, backpressure, and dependable retry behavior.
Webhook handling was tightly coupled to the main transaction flow, which made out-of-order provider events harder to process safely.
Investigation depended on unstructured application logs rather than a searchable audit trail.
Compliance review required stronger traceability for transaction state changes.
Technical Approach
Mapped the existing transaction lifecycle
We traced payment states and transitions to identify race conditions and failure points before changing architecture.
Separated the pipeline into discrete events
The process was broken into independent events such as initiation, provider submission, confirmation, and settlement, each with its own handler.
Added idempotency and retry logic
Handlers were designed to be idempotent. Failed events retry with backoff and move to a dead-letter path for review after repeated failure.
Built a compliance-oriented audit log
State transitions, external API calls, and webhook receipts were recorded with timestamps and payload context for end-to-end investigation.
Rebuilt the webhook handler as a separate path
Incoming webhooks are validated, logged, and queued instead of being processed inline with the primary transaction request.
Tech Stack
backend
data
integrations
infrastructure
Engagement Details
Timeline
Approximately 10 weeks (subject to verification)
Team
Senior Axionvex Tech engineer with client engineering counterparts
Outcomes
Design and control changes are listed below. Numeric claims stay unpublished until measurement is verified.
Load resilience
Design changeSynchronous path with limited failure isolation
Queued, idempotent processing with retry and dead-letter handling
Traceability
Design changeUnstructured logs for incident reconstruction
Structured audit history for transaction investigation
Measured production results
Pending verificationBaseline metrics not yet published
Numeric outcomes withheld pending verification
Architecture, controls, and implementation notes are published. Measured before/after figures and client attribution remain pending verification.