{
  "id": 14437399,
  "title": "RxDB's sync engine, run through 8 offline-first failure modes",
  "url": "https://urgent.news/2026/10/14/rxdbs-sync-engine-run-through-8-offline-first-failure-modes",
  "topic": "tech",
  "section": "Tech",
  "published": "2026-10-14T11:34:48.000Z",
  "source": {
    "name": "Dev.to",
    "slug": "dev-to",
    "url": "https://dev.to/alfa_ilands/rxdbs-sync-engine-run-through-8-offline-first-failure-modes-5elj"
  },
  "original_language": "en",
  "account": "RxDB's replication engine was put through its paces, applying a checklist of eight failure modes that can occur in an offline-first environment. The source material laid out these scenarios, ranging from idempotency to ordering and schema migration.\n\nThe engine passed the test with flying colors, but it was clear that the backend had to step up to the plate in several areas. RxDB handled tombstone deletes and durable checkpoints well, ensuring documents remained available even after they were marked as deleted. It also maintained server-authoritative timestamps, allowing the client to trust the updatedAt field.\n\nHowever, the engine did not guarantee idempotency, and the backend was required to implement this themselves. This meant that if two retries happened for the same document, the backend had to ignore the duplicates. Conflict resolution was handled well, with the engine detecting conflicts and allowing for custom conflict handlers to be installed.\n\nThe protocol also required the backend to handle partial applies and ordering issues, particularly when dealing with relational data. The challenge was on the backend to ensure that idempotent writes were made and that relations were handled correctly.\n\nSchema migration was also noted as a potential issue, as older clients returning with an outdated schema could cause problems. RxDB provided a way to handle this, but it was ultimately an application-level concern.\n\nIn summary, RxDB's sync engine offered a robust reference design for offline-first applications. It excelled in many areas, particularly in handling deletes, checkpoints, timestamps, and conflicts. However, the backend was still tasked with a number of critical chores, including ensuring idempotent writes, handling relations, and managing schema migration. The engine's design made the backend relatively simple, but the missing idempotency key highlighted the need for careful backend implementation to avoid duplication of data.",
  "summary": "RxDB's sync engine, run through 8 offline-first failure modes I keep a short checklist of the ways offline-first sync actually breaks in production. Eight items, in the order they bite. I ran RxDB's replication design through it, using the docs as the source, because RxDB is one of the few engines that documents its protocol instead of hiding it. Short version: RxDB gets the hard parts right and…",
  "key_points": [
    "RxDB's replication engine passed eight offline-first failure modes tests",
    "Backend required to implement idempotency, not guaranteed by RxDB",
    "Engine excelled in handling deletes, checkpoints, timestamps, and conflicts"
  ],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}