Cache-Control for Offline-First Document Editors: Lessons From LibreOffice Users

Why offline-first document editors break standard caching assumptions (and what keeps you up at night)

If your users edit documents offline and then sync back to a server, you already know the pain: stale reads, wasted bandwidth from full downloads, and complex invalidation when multiple devices race to update the same file. As an engineer or infra lead, you need patterns that reduce sync latency, lower hosting costs, and preserve authoritative correctness — even when a device has been offline for days.

The 2026 context: what changed and why this matters now

Late 2025 and early 2026 brought three practical shifts that affect offline-first editors:

• Edge compute and serverless functions at the CDN edge are now mainstream, enabling lightweight authoritative checks without round trips to a central origin.
• CRDT libraries and operation-based sync improved performance and determinism — making real-time, offline-friendly collaboration more practical for structured and binary-backed docs.
• Browsers hardened background sync and storage persistence APIs; HTTP/3 and QUIC reduced per-request cost, changing how you weigh revalidation vs full fetch costs.

High-level patterns: optimistic updates, stale-while-revalidate, and authoritative bypass

For offline-first editors, three patterns form the backbone of a pragmatic cache and sync design:

• Optimistic updates – apply edits locally immediately and queue network synchronization.
• Stale-while-revalidate – serve a cached document instantly while refreshing in the background.
• Authoritative bypass – skip caches or force revalidation when you need the single source of truth (conflict resolution points, final save confirmations, or permission checks).

Why these three? (Short answer)

Optimistic updates preserve UX during offline periods. Stale-while-revalidate provides fast reads and cost savings. Bypassing caches at authoritative moments prevents merge mistakes and ensures legal/audit correctness.

Practical header rules: what to set at the origin and edge

Use Cache-Control directives intentionally. Below are concrete, pragmatic examples you can adopt for editors in 2026.

Static editor assets (JS/CSS/worker)

Cache-Control: public, max-age=31536000, immutable

These never change between releases; set a long max-age and use content-hash build outputs. This reduces cold-start cost for offline devices that update infrequently.

Document GETs (read-mostly, stale-while-revalidate)

Cache-Control: public, max-age=60, stale-while-revalidate=86400, stale-if-error=604800
ETag: '"v1.2.345"'
Vary: Accept-Encoding
s-maxage: 60

Explanation:

• max-age=60 keeps the document fresh for a short period.
• stale-while-revalidate (1 day): allows edges and service workers to serve a stale response while they refresh the content in the background — ideal for offline editors that want instant load with eventual reconciliation.
• stale-if-error: if origin is unreachable, serve stale versions for an emergency window.
• s-maxage for CDN control separate from browser TTLs.

Authoritative endpoints (document save, lock, merge checks)

Cache-Control: no-store
Pragma: no-cache
Vary: Authorization

Write endpoints must never be cached. Responses containing the post-save authoritative state should also be returned with no-store so intermediaries and browsers don’t keep an incorrect canonical copy.

Conditional revalidation (to reduce payloads)

GET /doc/123
If-None-Match: '"v1.2.345"'

# 304 Not Modified when the server confirms no change

Use ETags (or Oxford-style version tokens) to perform cheap validation checks instead of full downloads. This is vital for big binary-backed documents where payload cost matters.

Service worker strategy: local cache + IndexDB + sync queue

A service worker should be the local orchestrator: serve from the in-memory/local cache for instant UX, persist edits to IndexedDB, and manage the sync queue to the server. Below is a robust fetch handler template you can adopt.

self.addEventListener('fetch', (event) => {
  const url = new URL(event.request.url);

  // Requests for doc content
  if (url.pathname.startsWith('/api/doc/')) {
    event.respondWith(handleDocRequest(event.request));
    return;
  }

  // Fallback to network for other requests
  event.respondWith(fetch(event.request));
});

async function handleDocRequest(req) {
  // Try IndexedDB first (fast, offline)
  const local = await localDb.getDoc(getDocId(req.url));
  if (local) {
    // Return local copy immediately and revalidate in background
    revalidateInBackground(req);
    return new Response(JSON.stringify(local), { headers: { 'Content-Type': 'application/json' } });
  }

  // No local copy, fetch network (will be cached by worker based on headers)
  return fetch(req);
}

Key behaviors:

• Return local copy immediately for instant UX.
• Start a background revalidation that performs conditional requests with ETag/If-None-Match.
• Queue outgoing edits in IndexedDB with retry/backoff; sync when connectivity returns.

Optimistic updates and the sync queue

Optimistic updates keep users productive: apply the edit locally and show it as confirmed, even while the network attempt is pending. But you must track promises, retries, and failure semantics.

Sync queue semantics (recommended)

1. Client writes are assigned a local operation id and a causal metadata token (vector clock or Lamport timestamp).
2. Operation is appended to the local queue (IndexedDB) and applied to local state (optimistic).
3. Background sync attempts to push operations to the sync endpoint in order; on success, server responds with authoritative sequence or merge token.
4. On conflict, server responds with a merge response or conflict marker; client uses CRDT merge where possible or presents a UI merge for manual resolution.

// Example queue item (JSON stored in IndexedDB)
{
  'opId': 'local-1670-abc',
  'docId': '123',
  'type': 'insert',
  'payload': { 'pos': 48, 'text': 'new paragraph' },
  'clock': { 'deviceA': 42 },
  'retries': 0,
  'status': 'queued'
}

Use a small exponential backoff for retries, and escalate to a user-visible error if an operation fails repeatedly (e.g., 5 attempts). Preserve the queue across restarts so that users do not lose edits.

Conflict resolution: real-world patterns

Conflicts are inevitable. The goal is to resolve automatically where possible and surface to the user only when required.

Prefer CRDTs for collaborative, granular edits

If your editor is collaborative or needs per-character concurrency, use a CRDT (state-based or op-based). In 2026, CRDT implementations are faster and more memory-efficient; many teams use Yjs or Automerge-inspired approaches for structured docs.

Fallback: LWW with vector clocks for simple document models

For simpler use-cases (single-author heavy edits), last-writer-wins (LWW) with vector timestamps is pragmatic. It’s simple and predictable: the server keeps a version token and rejects writes that don't match unless the client asks for a merge.

Server responses and merge flow

POST /api/doc/123/sync
Authorization: Bearer ...
Content-Type: application/json

{
  'ops': [ ... ],
  'baseVersion': 'v1.2.345'
}

# Server responses
// On success
200 OK
{ 'newVersion': 'v1.2.346', 'appliedOps': [ ... ] }

// On conflict
409 Conflict
{ 'serverVersion': 'v1.2.350', 'mergeNeeded': true, 'mergeHints': [ ... ] }

When the server returns 409, the client must fetch the authoritative state (bypass caches — see below), attempt an automatic CRDT merge, or present a conflict UI.

When to bypass caches for authoritative state

Caches are great for reads, but you must bypass or force revalidation in these cases:

• Conflict detection and merge — always revalidate the authoritative document before performing a potentially destructive merge.
• Final save confirmations — when a user explicitly publishes or versions a document for legal/audit reasons.
• Permissions/ownership changes — when ACLs or visibility change, cached copies can leak access assumptions.
• Large binary blobs — when you need the canonical bytes (use conditional requests or force no-cache depending on staleness tolerance).

Mechanically, ask for authoritative state with:

fetch('/api/doc/123?authoritative=1', {
  method: 'GET',
  headers: { 'Cache-Control': 'no-cache' }
});

On the server, return short TTL or no-store for the authoritative path. Use ETags to still allow conditional checks without always transferring payloads.

Edge invalidation and cost control

Invalidating CDN caches per-edit is expensive. Use these techniques to balance consistency and cost:

• Keep default reads with short TTL + stale-while-revalidate. This often eliminates the need for immediate invalidation.
• Invalidate by content hash or version token rather than by document ID where possible (push the new version token, let edges expire old ones).
• For high-value documents, use targeted purge APIs for the few times you need instant propagation.
• Use s-maxage for edge-vs-browser control: let the CDN keep a copy with a different TTL than browsers.

Observability: metrics that matter for caching + sync

Track these metrics to understand cache effectiveness and user pain:

• Client cache hit ratio — percent of doc reads served from local cache/IndexedDB or service worker without network.
• Edge hit ratio — percent of requests served by CDN without origin hits.
• Sync latency — time between local commit and server acknowledgement (median and p95).
• Divergence size — average number of conflicting ops per doc per day.
• Bandwidth saved — bytes avoided by conditional revalidation or SWR vs full fetches.
• Retry / queue length — the number of outstanding queued ops per device (helps identify unreliable users/devices).

Log events from both client and server to reconcile missed merges and audit client-side merges. Include version tokens, op ids, and device ids in telemetry to reconstruct sequences when debugging.

Security and privacy considerations (brief)

Cached documents on devices may be sensitive. Use device-level encryption or the platform-provided encrypted storage. Ensure that any cacheable responses for protected documents include appropriate Vary headers and the server sets Cache-Control: private where intermediaries must not store copies.

Real-world example: lessons from LibreOffice-style users

LibreOffice users taught the community an important lesson: offline-first workflows are common and users expect local control over files. Many LibreOffice-style workflows store entire documents locally and only occasionally interact with cloud services. When you add sync to a local-first editor, follow these lessons:

• Assume long offline windows. Keep a persistent sync queue and prioritize minimizing data transfer when reconnecting.
• Don't force uploads on every minor save. Use autosave locally and batch pushes intelligently (e.g., debounce or push on significant changes).
• Offer both automatic merges (CRDT) and explicit versioned uploads for users who need file-level control like in traditional desktop editors.
• Provide clear UX for conflict resolution — users who are used to file-based editors expect visible merge steps, not silent overwrites.

"Offline-first users want speed, predictability, and control. Your caching and sync strategy must prioritize these in that order."

Checklist: Proven configuration and operational steps

1. Use long-cache, immutable assets for the editor runtime.
2. Serve document reads with short max-age + stale-while-revalidate.
3. Implement optimistic local updates and a persistent IndexedDB sync queue.
4. Use ETag/If-None-Match conditional requests for revalidation to save bandwidth.
5. Keep write endpoints no-store and require conditional checks for merges.
6. Prefer CRDTs for collaborative editors; fall back to vector clocks/LWW for simpler models.
7. Track cache hit ratios, sync latency, divergence rates, and queue lengths.
8. Only purge CDN caches for a small set of critical documents; otherwise rely on SWR and versioned responses.

Predictions and trends to watch (2026+)

Expect these trends to accelerate over the next 12–24 months:

• Edge functions will increasingly perform lightweight authoritative checks (e.g., validate a merge token without origin round trips).
• CRDT adoption will continue for document editors, supplemented by hybrid models that use server-side operational transforms for heavy binary merges.
• Background sync and periodic sync APIs will get more predictable scheduling, improving reliability for queued pushes on mobile devices.
• WebTransport and WebRTC will be used more for peer-to-peer sync in local networks, reducing origin load for LAN-based collaboration scenarios.

Final takeaways — what to implement this quarter

• Ship optimistic local commits with a durable sync queue this sprint.
• Deploy stale-while-revalidate for document GETs and measure bandwidth savings within 30 days.
• Make write endpoints explicitly non-cacheable and instrument conflict responses (409) for monitoring and alerts.
• Plan a CRDT proof-of-concept for your most collaborative doc flows within the next two quarters.

Call to action

Want a tailored audit of your editor's caching and sync architecture? Get a short checklist and CDN + service-worker configuration review from caching.website. We’ll map where to safely apply stale-while-revalidate, where to force authoritative revalidation, and how to lower sync costs without compromising correctness.

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