Browser Storage Failure Modes

Browser offline storage is durable enough for serious apps only when you design for quotas, eviction, upgrades, and multiple tabs.

Failure Table

Failure	Symptom	Mitigation
quota exceeded	writes fail	catch errors, show cleanup/export UX, compress or prune caches
best-effort eviction	data disappears under pressure	request persistent storage and keep backup/export path
blocked upgrade	new app version cannot open DB	detect blocked upgrade and ask user to close other tabs
versionchange in old tab	old tab keeps using stale schema	listen for version changes and reload/close old connection
private mode	data clears when session ends	warn users before storing durable-only data
large export	UI freezes	use worker-backed export/import
background sync unavailable	queued writes stay pending	retry on foreground app open

export async function requestPersistentStorage() {
  if (!navigator.storage?.persist) return false;
  return navigator.storage.persist();
}

Persistence is not a backup. It only changes eviction behavior when the browser grants it.

db.on("blocked", () => {
  showReloadOtherTabsMessage();
});

Test this with two tabs open: one old tab holding a connection, one new tab trying to upgrade.