The Hidden Problems of Offline-First Sync: Idempotency, Retry Storms, and Dead Letters

Offline-first is easy to romanticize. Making it trustworthy under replay, reconnection, and permanent failure is the real engineering challenge. IndexGrid / offline-first-sync-queue Production-style offline-first synchronization architecture: local IndexedDB queue, batched POS sync, idempotent backend (externalId), retry/backoff, endpoint grouping, payload limits, and dead-letter handling. Built with Next.js, TypeScript, NestJS, and PostgreSQL. Offline-First POS Monorepo Deterministic, testable, and operationally credible reference implementation of an offline-first Point of Sale (POS) system with batch synchronization. 🏗️ Repository Topology This project is organized as a monorepo using NPM Workspaces: apps/api : NestJS backend service with Prisma and PostgreSQL. apps/web : Next.js frontend application with IndexedDB. packages/sync-contract : Shared Zod schemas and TypeScript types (the "Single Source of Truth"). infra : Docker orchestration and database configuration. docs : Architec