Conflict Resolution — Mobile API Best Practices

Two devices edit the same record offline; who wins? Choose per data type:

Last-writer-wins (LWW) — simple; loses data; fine for settings.
Field-level merge — merge non-overlapping field changes; conflict only on the same field.
Optimistic concurrency — mutation carries base_version; server rejects stale edits (409/412) and the client rebases or asks the user.
Domain rules — e.g. inventory decrements are commutative operations, not overwrites.
CRDTs — data types that merge automatically (counters, sets, text) for collaborative editing (Automerge, Yjs).

Conflict strategies

Criterion	LWW	Field merge	Version check + rebase	CRDT
Data loss	Yes	Only same-field	No (user/rule decides)	No
Complexity	Low	Medium	Medium	High
Needs user prompt	No	Rarely	Sometimes	No
Server role	Pick newest	Merge fields	Authority on versions	Relay/persist ops
Best for	Preferences, status	Profiles, forms	Business records	Collaborative docs, lists

function mergeTask(base: Task, mine: Task, theirs: Task): { merged: Task; conflicts: string[] } {
  const merged = { ...theirs };
  const conflicts: string[] = [];
  for (const f of ["title", "notes", "dueDate", "status"] as const) {
    const iChanged = mine[f] !== base[f];
    const theyChanged = theirs[f] !== base[f];
    if (iChanged && !theyChanged) merged[f] = mine[f];
    else if (iChanged && theyChanged && mine[f] !== theirs[f]) conflicts.push(f);
  }
  return { merged, conflicts };
}

PATCH /v1/tasks/01J9…
If-Match: "v14"
{"status": "done"}

412 Precondition Failed      # server is at v15 → client fetches v15, rebases, retries

Common pitfalls

Device timestamps deciding LWW (skewed clocks).
One strategy for all data.
Silently dropping the losing edit.

Key points

Pick a strategy per data type
Server-assigned versions, not device clocks
CRDTs for real collaboration

Further reading: crdt.tech: CRDT resources

Practice questions

When is last-writer-wins acceptable?

Answer

For data where losing a concurrent edit is harmless and rare — e.g. user preferences or a toggle — and when 'last' is decided by the server, not device clocks.

How does a three-way merge decide whether a field conflicts?

Answer

It compares both edited versions with their common base: if only one side changed the field, take it; if both changed it to different values, it's a conflict.