Cloudflare Durable Objects Knowledge Patch

Cloudflare Durable Objects

Use this skill when designing, configuring, deploying, testing, or debugging Cloudflare Durable Objects. Start with the quick references below, then load the topic file that matches the task. Treat project configuration, installed package types, runtime behavior, and tests as the final authority for this rolling service surface.

How to use this skill

1. Identify the namespace's storage backend and whether the Worker uses legacy migrations or the declarative exports lifecycle map.
2. Preserve already-provisioned namespace state. A code export alone does not create a namespace, storage backends cannot be changed in place, and lifecycle operations need deliberately ordered deployments.
3. Check the Worker's compatibility date before relying on date-gated behavior.
4. Read the relevant reference file before changing concurrency, storage, placement, WebSockets, lifecycle declarations, alarms, or test isolation.
5. Exercise restart, eviction, and mixed-version behavior where correctness depends on in-memory state or a coordinated rollout.

Reference index

Deployment and storage decisions

Prefer SQLite for new namespaces

Create new classes with SQLite storage. In legacy lifecycle configuration, use new_sqlite_classes; in declarative lifecycle configuration, set "storage": "sqlite" on each live class.

[[migrations]]
tag = "v1"
new_sqlite_classes = ["ChatRoom"]

SQLite-backed objects provide SQL, point-in-time recovery, and synchronous KV access. Existing KV-backed namespaces remain usable, but there is no in-place migration from their storage to SQLite. New KV namespace creation is restricted, so do not choose new_classes for a new deployment.

Choose one lifecycle system

Wrangler accepts either the ordered migrations array or the declarative exports map, never both. Under exports, every desired or existing namespace needs an entry; live entries need storage, and lifecycle tombstones must omit it. A live configured class missing from Worker code causes deployment to fail.

{
  "exports": {
    "ChatRoom": {
      "type": "durable-object",
      "storage": "sqlite"
    }
  }
}

Migration-to-exports conversion preserves namespace data when every active class is declared with its original backend (sqlite or legacy-kv), but the switch is one-way. Read the lifecycle reference before converting, renaming, deleting, or transferring a class.

Stage lifecycle operations

• For a rename, first deploy the new implementation under both class names; then deploy the old-name renamed tombstone and new live entry; remove the code alias only after the rollout.
• For a cross-Worker transfer, deploy expecting-transfer on the target first, commit with transferred on the source, then make the target entry live and change bindings last.
• Keep tombstones until reconciliation explicitly reports them safe to remove. Deletion is blocked while code exports the class or any Worker still binds to its namespace.
• Apply lifecycle changes only with a full wrangler deploy. Upload-only, gradual deployment, and rollback across a lifecycle change are unsupported.

SQL and transaction safety

Materialize cursors before yielding

A SqlStorageCursor is not a stable snapshot across await. Convert results with toArray() or otherwise exhaust the cursor synchronously before external I/O; resumed iteration can observe later writes, even from a transaction that eventually rolls back.

const rows = this.ctx.storage.sql.exec("SELECT * FROM users").toArray();
await sendRows(rows);

Within a multi-statement exec(), bindings and the returned cursor belong only to the final statement. Cursor iteration and raw() share one position, and one() requires exactly one returned row.

Use storage transaction APIs

Do not issue BEGIN, SAVEPOINT, or other transaction-control SQL through sql.exec(). Use transactionSync() for synchronous SQL or KV work; its callback cannot return a promise, passes its return value through, and rolls back when it throws.

this.ctx.storage.transactionSync(() => {
  this.ctx.storage.sql.exec(
    "UPDATE counters SET value = value + 1 WHERE id = ?",
    counterId,
  );
});

On SQLite-backed objects, direct storage operations participate in transaction(); do not retain the obsolete txn wrapper. For schema changes, track versions in an ordinary table—PRAGMA user_version is unsupported—and run constructor migrations inside blockConcurrencyWhile().

Concurrency and lifecycle safety

Storage awaits receive input-gate protection; external fetch(), R2, and other non-storage awaits do not. After such I/O, re-read or revalidate any version or precondition before committing dependent state.

Pending storage writes hold outgoing responses and network requests behind the output gate. Consecutive writes without an intervening await are coalesced into one atomic implicit transaction; an await ends that boundary.

Do not depend on a shutdown callback—none exists for eviction, deployment, or runtime replacement. Persist checkpoints incrementally. Also keep Worker-to- object request and RPC contracts compatible across adjacent releases because rollouts can temporarily pair new Worker code with an older object instance.

Alarms are non-recurring and can run more than once. Schedule the next alarm explicitly, make handlers idempotent, and use retryCount when deciding whether to install a fresh alarm before retries are exhausted.

Identity and placement

Use namespace.jurisdiction() for residency constraints. Jurisdiction scopes have distinct ID spaces: an ID belongs to its scope, although an unscoped namespace can resolve a restricted ID. The same name therefore maps to different IDs in eu, us, and fedramp.

ctx.id.name exists only when the object was reached by name, subject to the documented length and lookup limitations. ctx.id.jurisdiction survives string round-trips. Placement locationHint values affect only first placement and do not move an existing object.

WebSocket hibernation

Enable hibernation with ctx.acceptWebSocket(server) and handle messages and closes in Durable Object class methods. Calling the standard ws.accept() does not enable hibernation, and outbound WebSockets cannot hibernate.

Persist per-connection state with serializeAttachment() within its 16,384-byte limit, and write longer-lived or larger state to Durable Object storage. Use tags for bounded grouping, auto-responses for wake-free request/reply pairs, and the hibernatable event timeout when a handler needs an explicit runtime cap.

Testing essentials

Workers Vitest 4 uses the cloudflareTest() Vite plugin and a Wrangler config that declares the bindings and lifecycle configuration. Type test env through ProvidedEnv. Exercise the Worker's public HTTP route with exports.default.fetch() and call a binding from env for direct stub tests.

Eviction helpers reset instances while preserving storage. Use evictDurableObject() for a named stub, its { webSockets: "close" } option for the non-hibernating socket path, and evictAllDurableObjects() for all running instances. runDurableObjectAlarm() reports whether it found and ran an alarm.

Pre-deployment checklist

• Confirm every live namespace and retained tombstone is represented in the chosen lifecycle configuration.
• Verify storage backend declarations against the namespaces that already exist; never infer them from the current class implementation.
• Audit all await boundaries between reads and dependent writes.
• Verify alarms are idempotent and reschedule themselves when recurring work is intended.
• Test a fresh construction, hibernation wake-up, explicit eviction, and any WebSocket close path the application relies on.
• Keep lifecycle tombstones and compatibility shims through the entire rollout, then remove them only when reconciliation or deployment sequencing permits.