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Durable Objects Gotchas

Common Errors

"Hibernation Cleared My In-Memory State"

Problem: Variables lost after hibernation Cause: DO auto-hibernates when idle; in-memory state not persisted Solution: Use ctx.storage for critical data, ws.serializeAttachment() for per-connection metadata

// ❌ Wrong - lost on hibernation
private userCount = 0;
async webSocketMessage(ws: WebSocket, msg: string) {
  this.userCount++;  // Lost!
}

// ✅ Right - persisted
async webSocketMessage(ws: WebSocket, msg: string) {
  const count = this.ctx.storage.kv.get("userCount") || 0;
  this.ctx.storage.kv.put("userCount", count + 1);
}

"setTimeout Didn't Fire After Restart"

Problem: Scheduled work lost on eviction Cause: setTimeout in-memory only; eviction clears timers Solution: Use ctx.storage.setAlarm() for reliable scheduling

// ❌ Wrong - lost on eviction
setTimeout(() => this.cleanup(), 3600000);

// ✅ Right - survives eviction
await this.ctx.storage.setAlarm(Date.now() + 3600000);
async alarm() { await this.cleanup(); }

"Constructor Runs on Every Wake"

Problem: Expensive init logic slows all requests Cause: Constructor runs on every wake (first request after eviction OR after hibernation) Solution: Lazy initialization or cache in storage

Critical understanding: Constructor runs in two scenarios:

1. Cold start - DO evicted from memory, first request creates new instance

2. Wake from hibernation - DO with WebSockets hibernated, message/alarm wakes it

// ❌ Wrong - expensive on every wake
constructor(ctx: DurableObjectState, env: Env) {
  super(ctx, env);
  this.heavyData = this.loadExpensiveData();  // Slow!
}

// ✅ Right - lazy load
private heavyData?: HeavyData;
private getHeavyData() {
  if (!this.heavyData) this.heavyData = this.loadExpensiveData();
  return this.heavyData;
}

"Durable Object Overloaded (503 errors)"

Problem: 503 errors under load Cause: Single DO exceeding ~1K req/s throughput limit Solution: Shard across multiple DOs (see Patterns: Sharding)

"Storage Quota Exceeded (Write failures)"

Problem: Write operations failing Cause: DO storage exceeding 10GB limit or account quota Solution: Cleanup with alarms, use deleteAll() for old data, upgrade plan

"CPU Time Exceeded (Terminated)"

Problem: Request terminated mid-execution Cause: Processing exceeding 30s CPU time default limit Solution: Increase limits.cpu_ms in wrangler.jsonc (max 300s) or chunk work

"WebSockets Disconnect on Eviction"

Problem: Connections drop unexpectedly Cause: DO evicted from memory without hibernation API Solution: Use WebSocket hibernation handlers + client reconnection logic

"Migration Failed (Deploy error)"

Cause: Non-unique tags, non-sequential tags, or invalid class names in migration Solution: Check tag uniqueness/sequential ordering and verify class names are correct

"RPC Method Not Found"

Cause: compatibility_date < 2024-04-03 preventing RPC usage Solution: Update compatibility_date to >= 2024-04-03 or use fetch() instead of RPC

"Only One Alarm Allowed"

Cause: Need multiple scheduled tasks but only one alarm supported per DO Solution: Use event queue pattern to schedule multiple tasks with single alarm

"Race Condition Despite Single-Threading"

Problem: Concurrent requests see inconsistent state Cause: Async operations allow request interleaving (await = yield point) Solution: Use blockConcurrencyWhile() for critical sections or atomic storage ops

// ❌ Wrong - race condition
async incrementCounter() {
  const count = await this.ctx.storage.get("count") || 0;
  // ⚠️ Another request could execute here during await
  await this.ctx.storage.put("count", count + 1);
}

// ✅ Right - atomic operation
async incrementCounter() {
  return this.ctx.storage.sql.exec(
    "INSERT INTO counters (id, value) VALUES (1, 1) ON CONFLICT(id) DO UPDATE SET value = value + 1 RETURNING value"
  ).one().value;
}

// ✅ Right - explicit locking
async criticalOperation() {
  await this.ctx.blockConcurrencyWhile(async () => {
    const count = await this.ctx.storage.get("count") || 0;
    await this.ctx.storage.put("count", count + 1);
  });
}

"Migration Rollback Not Supported"

Cause: Attempting to rollback a migration after deployment Solution: Test with --dry-run before deploying; migrations cannot be rolled back

"deleted_classes Destroys Data"

Problem: Migration deleted all data Cause: deleted_classes migration immediately destroys all DO instances and data Solution: Test with --dry-run; use transferred_classes to preserve data during moves

"Cold Starts Are Slow"

Problem: First request after eviction takes longer Cause: DO constructor + initial storage access on cold start Solution: Expected behavior; optimize constructor, use connection pooling in clients, consider warming strategy for critical DOs

// Warming strategy (periodically ping critical DOs)
export default {
  async scheduled(event: ScheduledEvent, env: Env) {
    const criticalIds = ["auth", "sessions", "locks"];
    await Promise.all(criticalIds.map(name => {
      const id = env.MY_DO.idFromName(name);
      const stub = env.MY_DO.get(id);
      return stub.ping();  // Keep warm
    }));
  }
};

Limits

Hibernation Caveats

1. Memory cleared - All in-memory variables lost; reconstruct from storage or deserializeAttachment()

2. Constructor reruns - Runs on wake; avoid expensive operations, use lazy initialization

3. No guarantees - DO may evict instead of hibernate; design for both

4. Attachment limit - serializeAttachment() data must be JSON-serializable, keep small

5. Alarm wakes DO - Alarm prevents hibernation until handler completes

6. WebSocket state not automatic - Must explicitly persist with serializeAttachment() or storage

See Also

• Patterns - Workarounds for common limitations

• API - Storage limits and quotas

• Configuration - Setting CPU limits