Sync conflicts resolved with clear user-facing strategy

When a user edits data offline while the server receives a concurrent update from another session, a Last-Write-Wins strategy without field-level timestamps will silently discard one of the changes. CWE-362 (Race Condition) and CWE-694 are both implicated. ISO 25010:2011 reliability.recoverability requires that the system resolves conflicts without data loss. The failure mode is particularly damaging in collaborative tools or multi-device workflows: users believe their changes were saved, but the data they see after sync reflects a different device's state. Silent data loss is harder to detect than visible errors and more damaging to user trust.

Implement field-level timestamps on every mutable field and a deterministic merge function. Apply the merge during sync rather than overwriting the local or remote record wholesale.

interface Record {
  id: string;
  title: string;        titleAt: number;
  description: string;  descriptionAt: number;
}

function merge(local: Record, remote: Record): Record {
  return {
    id: local.id,
    title:       local.titleAt > remote.titleAt ? local.title : remote.title,
    titleAt:     Math.max(local.titleAt, remote.titleAt),
    description: local.descriptionAt > remote.descriptionAt ? local.description : remote.description,
    descriptionAt: Math.max(local.descriptionAt, remote.descriptionAt),
  };
}

async function syncRecord(id: string) {
  const [local, remote] = await Promise.all([getLocal(id), fetchRemote(id)]);
  if (local && remote) {
    const merged = merge(local, remote);
    await saveLocal(merged);
    await pushRemote(merged);
  }
}

For fields that cannot be field-merged (e.g., binary blobs), surface a conflict prompt to the user rather than silently picking a winner.