Senior Backend Engineer - Webhook Idempotency, Concurrency Systems & API Integration

Project Overview We are a rapidly growing SaaS platform experiencing severe concurrency bottlenecks, duplicate database entries, and transaction failures at peak traffic. Our application integrates with multiple third-party APIs (payment gateways, real-time video providers, and scheduling APIs). Under high concurrent loads, our current backend suffers from classic Time-of-Check to Time-of-Use (TOCTOU) race conditions. Additionally, third-party webhook retries are causing duplicate data serialization, leading to compromised financial and operational ledgers. We are not looking for a standard CRUD developer. We need a specialized systems-level backend engineer who deeply understands database-level atomicity, asynchronous queuing, event-driven architectures, and distributed locking mechanisms to audit, redesign, and secure our integration pipelines. Core Problems You Will Solve Exactly-Once Webhook Processing: Our third-party API providers (such as Stripe and real-time streaming services) frequently retry webhooks due to network jitter or delayed HTTP 200 acknowledgments. We need an airtight idempotency layer that guarantees exactly-once execution under heavy concurrent traffic. Database Race Conditions and Double-Allocations: Concurrent requests are currently bypassing naive application-level checks, resulting in duplicate resource allocations and corrupted balance sheets. Out-of-Order Webhook Delivery: Webhooks from our providers are occasionally delivered out of chronological order (e.g., "session ended" is received or processed before "session started"). The backend must resolve these mismatches gracefully without corrupting database states. API Latency Blocking the Main Event Loop: Synchronous API calls to third-party endpoints during standard HTTP request cycles are causing database deadlocks and slow user-facing response times. Required Technical Skills Languages: Robust experience in Node.js/TypeScript (NestJS or Express) or Python (FastAPI). Databases: Advanced proficiency in PostgreSQL or MySQL. Must understand isolation levels, transactions, indexes, and database-level constraint optimization. Caching and Message Brokers: Direct experience utilizing Redis for state tracking, caching, or distributed lock management, and managing task queues (e.g., Celery, BullMQ, or Laravel Queues). Architecture Patterns: Masterful understanding of Event-Driven Architecture, Idempotent Automation, Strategy and Observer patterns, and asynchronous decoupled workers. Detailed Step-by-Step Implementation Timeline (4-Week Sprint) Phase 1: Architecture Audit, Log Analysis and Race Condition Replication (Days 1–5) Step 1.1 (Database Logging Audit): Inspect production error logs to isolate exact database transaction failures, tracking deadlock patterns, unhandled exceptions, and duplicate key violations. Step 1.2 (Race Condition Mocking): Set up a local containerized staging environment (via Docker Compose). Write localized scripting suites to simulate high-concurrency conditions (e.g., sending 100 concurrent HTTP requests inside a 20ms window targeting the same endpoint). Step 1.3 (Vulnerability Mapping): Document every application endpoint containing unsafe "read-then-write" query logic, mapping out exactly how duplicate database rows are passing current validation states. Phase 2: Atomic Idempotency and Webhook Standardization (Days 6–12) Step 2.1 (Idempotency Guard Implementation): Build a centralized, provider-neutral idempotency ledger database. Introduce strict database unique constraints (such as UNIQUE(provider_name, delivery_id)) to enforce atomic deduplication directly at the storage level. Step 2.2 (Decoupled Async Queues): Refactor the webhook controllers to return an immediate HTTP 200 (Success) response to the third-party provider upon signature verification, completely decoupling processing from the request cycle. Enqueue raw webhook payloads into a dedicated background worker queue (utilizing Redis or PostgreSQL-backed queues) to process database writes asynchronously. Step 2.3 (Provider-Agnostic Parsing): Implement the Strategy Pattern to dynamically route, validate, and normalize inbound payloads from disparate API providers, ensuring the core database schema is entirely insulated from external API changes. Phase 3: State Locking, Conflict Resolution and Session Handshakes (Days 13–20) Step 3.1 (Transactional Time-To-Live Locks): Implement a two-phase transactional lock for sensitive updates. Set up temporary Redis-backed or database-level reservation locks with strict Time-To-Live (TTL) limits to prevent dual-allocations while payments are being authorized. Step 3.2 (Chronological Out-of-Order Resolution): Design a multi-tier resolution fallback strategy. If a "terminate" webhook arrives before an "initialize" event, cache the terminating event state with an anchor key in a fast local cache, auto-resolving the transaction context as soon as the late-arrival initialization payload is ingested. Step 3.3 (Concurrency Safe State Management): Re-architect ledger increments (balances, seat counts, data tracking) using atomic SQL expressions (e.g., UPDATE table SET seats = seats - 1 WHERE id = :id AND seats :zero) instead of pulling state to memory and writing it back. Phase 4: Optimization, Backoff Policies and Stress Testing (Days 21–28) Step 4.1 (Asynchronous Error Handling and Retries): Configure custom queue listeners with robust retry mechanisms, integrating exponential-backoff intervals and dead-letter queue routing for critical external API timeouts. Step 4.2 (Load and Penetration Testing): Run high-volume stress testing across the newly decoupled routes. Validate that 100% of duplicate webhooks or redundant concurrent requests are successfully rejected by the idempotency database constraint with zero database corruption or leaks. Step 4.3 (Monitoring and Documentation): Add structured diagnostics and health checks. Provide a comprehensive system markdown file detailing the exact data flow pipelines, error-recovery loops, and configuration states for future handoff. Budget and Engagement Budget: $1,500 – $2,500 (Fixed Price) or $25 – $35/hr Timeline: 4 Weeks (28 Calendar Days)