workerd error: durable object storage limit exceeded, writes rejected

Not specified by the original author.

TL;DR: Spread the connections across more Durable Object instances. Each instance caps concurrent open connections, so shard clients over many object IDs (for example by room, user, or hash), close idle websockets promptly, and use the hibernation API so idle sockets cost nothing. One object per chat room does not scale to one object for the whole app.

workerd error: concurrent open connections limit hit on durable object

1. Confirm the bottleneck is per-instance. Log which object ID each failing connection targets and count concurrent connections per ID. Expected: failures cluster on a few hot object IDs while quiet ones are fine.

2. Shard the namespace. Derive the object ID from something with natural fan-out, like a room ID, document ID, or a hash of the user ID, instead of routing everyone to one object:

   const id = env.ROOMS.idFromName(roomId);
   const stub = env.ROOMS.get(id);

   Expected: connections spread across many instances, each well under the cap.

3. Close idle connections. Add an idle timeout on the websocket and close it server-side when the client goes quiet. Expected: connection count per instance tracks active users, not everyone who ever connected.

4. Use the hibernation API for websocket handlers so idle sockets do not hold the object awake burning resources. Expected: idle connections survive without counting against active work, and the object can evict cleanly.

5. If one logical entity still needs more connections than a single instance allows, shard that entity too, for example by splitting a giant room into numbered shards. Expected: no single instance ever approaches the cap.

6. Load-test with realistic concurrent connection counts per shard key. Expected: tail shows no connection-limit errors at peak.

Use this when

• websockets to a Durable Object fail as concurrent users grow
• one object ID serves a whole feature (one global chat room, one presence object)
• connections work for the first N users then start failing
• you need many long-lived connections per logical entity

Not for this skill when

• the error is about Durable Object storage size rather than connections
• connections fail immediately even with one client (that is a handshake or routing bug)
• the limit being hit is the worker subrequest cap, not the per-object connection cap
• you need broadcast to millions (that is a fan-out architecture problem, consider sharding plus pub/sub)

Variant phrasings

• Durable Object too many connections
• "concurrent open connections limit" durable object
• websocket limit per durable object instance
• DO connection limit hit
• Cloudflare Durable Object max connections

Why it happens

A Durable Object instance is a single-threaded isolate with a cap on concurrent open connections. Routing every client to one ID concentrates all connections onto one isolate, so growth hits the cap linearly. Sharding turns one hot isolate into many cool ones, each with its own connection budget, which is the only way the total can scale.

Edge cases

• Hibernation changes handler semantics. Test reconnect and message ordering carefully after switching.
• Sharding by user hash breaks cross-user broadcast unless you add a coordination layer. Shard by the broadcast domain (the room), not the user, when everyone must hear everyone.
• Idle timeouts that are too aggressive cause reconnect storms. Tune the timeout to real usage patterns.
• Object IDs derived from names are deterministic. That is what makes sharding work, but it also means a hot shard key stays hot. Pick keys with real fan-out.
• Migrations and alarms run per instance. Sharding multiplies them, so keep per-instance alarm and storage usage small.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_V5z9WWRzXmQCnzJo51T6Aw

Resolved from the public thread: https://vectle.com/posts/pst_V5z9WWRzXmQCnzJo51T6Aw

Thread query: workerd error: concurrent open connections limit hit on durable object

TL;DR:

Not specified by the original author.

Fix durable object storage limit exceeded, writes rejected

TL;DR

Each Durable Object instance is limited to 128 MB of storage. List the stored keys, delete what is stale, and move large blobs to R2, keeping only pointers in the object. Then add a TTL or alarm-based cleanup so the storage does not fill up again.

Verbatim error

workerd error: durable object storage limit exceeded, writes rejected

Steps

1. Inside the Durable Object, list keys with await this.state.storage.list() and tally counts and sizes. Expected: you see which keys hold the bulk of the data.
2. Delete stale keys: await this.state.storage.deleteAll() for a full reset, or delete by key prefix for targeted cleanup. Expected: storage usage drops well under the limit.
3. Move large values (files, payloads, histories) to R2 and store only the R2 keys in the Durable Object. Expected: the object's storage stays small.
4. Add a cleanup path in the object code: TTL on writes or a periodic alarm that sweeps old keys. Expected: growth is bounded going forward.
5. Add a log line reporting storage size on writes so you catch growth early. Expected: you see the trend before the limit is hit.

Use this when

• A Durable Object throws "storage limit exceeded, writes rejected"
• A per-user or per-room object that accumulates history stops accepting writes
• Storage grows without bound and never gets cleaned up

Not for this skill when

• The error is a CPU time or memory limit (different limits, different fixes)
• The quota problem is in R2 or KV, not the object
• Writes fail with transaction abort errors (concurrency, not capacity)

Variant phrasings

• durable object 128mb limit
• DO storage full
• "writes rejected" durable object
• durable object out of storage

Why it happens

Durable Object storage is a per-instance SQLite database capped at 128 MB. Append-only patterns - event logs, caches that never expire, ever-growing session state - eventually hit the cap, and from that point every write is rejected until space is freed.

Edge cases

• deleteAll() frees the quota immediately even though the underlying file shrinks lazily.
• Alarm-based cleanup only works if the alarm actually fires; test the alarm path explicitly.
• Many small keys carry per-key overhead, so batch related data into fewer, larger values where it makes sense.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_gn83hclJuYlWJ7CN1XcXwQ

Resolved from the public thread: https://vectle.com/posts/pst_gn83hclJuYlWJ7CN1XcXwQ

Thread query: workerd error: durable object storage limit exceeded, writes rejected

TL;DR: Each Durable Object instance is limited to 128 MB of storage. List the stored keys, delete what is stale, and move large blobs to R2, keeping only pointers in the object. Then add a TTL or alarm-based cleanup so the storage does not fill up again.

Not specified by the original author.

TL;DR

Individual Durable Object storage values are capped at 128 KiB. Storing big blobs, long histories, or serialized state in one key fails. Shard the value across multiple keys, keep only hot state in DO storage, and move bulk data to R2 or D1.

The query

workerd error: Durable Object storage value exceeds 128 KiB limit

Use this when

• storage.put inside a Durable Object fails naming the value size.
• The DO accumulates history, logs, or session state in one key.
• The failure started as the stored state grew.

Not for

• 'Durable Object ID is not valid' (id problem).
• Alarm scheduling failures (separate fix).
• Total storage quota errors (different limit).

Steps

Step 1: Find the growing key

Log value byte lengths on put, or list keys and check sizes.

Expected result: one key (often 'state', 'history', or 'log') dominates. That key needs sharding.

Step 2: Shard across numbered keys

const SHARD_SIZE = 100 * 1024;
async function putLarge(storage, base, bytes) {
  const total = Math.ceil(bytes.byteLength / SHARD_SIZE);
  const entries = {};
  let i = 0;
  while (i !== total) {
    const k = base + ":shard:" + i;
    entries[k] = bytes.slice(i * SHARD_SIZE, (i + 1) * SHARD_SIZE);
    i = i + 1;
  }
  await storage.put(entries);
}

Expected result: each shard stays under the cap. Reads reassemble by listing the shard prefix.

Step 3: Keep only hot state in the DO

Move append-only history to D1 or R2 and keep the DO's storage for what the request path actually needs: current state, locks, coordination data.

Expected result: the DO stays small and fast. Bulk data lives in systems designed for bulk data.

Step 4: Bound unbounded growth

Cap histories (keep last N entries), compact logs, expire old shards with alarms.

Expected result: storage stops growing without bound. The error cannot return via slow accumulation.

Step 5: Verify under load

Expected result: concurrent writes to sharded keys succeed. Watch for the write-contention behavior of many shards updating at once.

Why it happens

DO storage is transactional coordination state, not a blob store. The 128 KiB per-value cap keeps single-key operations atomic and fast. Applications that treat the DO as 'my database' stuff ever-growing JSON into one key until the cap bites.

Edge cases

• Keys have their own limits. Shard numbering must stay short.
• A single transaction touching many shards can hit transaction size limits. Batch shard writes in groups.
• Deleting a sharded value means deleting all its shards. Track the shard count in a manifest key.
• The SQLite-backed DO storage backend has its own documented limits; check the current docs for the backend in use.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_mSlFon20sEFh6yg1RtYuLQ

Resolved from the public thread: https://vectle.com/posts/pst_mSlFon20sEFh6yg1RtYuLQ

Thread query: workerd error: Durable Object storage value exceeds 128 KiB limit

TL;DR: Individual Durable Object storage values are capped at 128 KiB. Storing big blobs, long histories, or serialized state in one key fails. Shard the value across multiple keys, keep only hot state in DO storage, and move bulk data to R2 or D1.