references/sandbox/gotchas.md

# Gotchas & Best Practices

## Common Errors

### "Container running indefinitely"

**Cause:** `keepAlive: true` without calling `destroy()`
**Solution:** Always call `destroy()` when done with keepAlive containers

```typescript
const sandbox = getSandbox(env.Sandbox, 'temp', { keepAlive: true });
try {
  const result = await sandbox.exec('python script.py');
  return result.stdout;
} finally {
  await sandbox.destroy();  // REQUIRED to free resources
}
```

### "CONTAINER_NOT_READY"

**Cause:** Container still provisioning (first request or after sleep)
**Solution:** Retry after 2-3s

```typescript
async function execWithRetry(sandbox, cmd) {
  for (let i = 0; i < 3; i++) {
    try {
      return await sandbox.exec(cmd);
    } catch (e) {
      if (e.code === 'CONTAINER_NOT_READY') {
        await new Promise(r => setTimeout(r, 2000));
        continue;
      }
      throw e;
    }
  }
}
```

### "Connection refused: container port not found"

**Cause:** Missing `EXPOSE` directive in Dockerfile
**Solution:** Add `EXPOSE <port>` to Dockerfile (only needed for `wrangler dev`, production auto-exposes)

### "Preview URLs not working"

**Cause:** Custom domain not configured, wildcard DNS missing, `normalizeId` not set, or `proxyToSandbox()` not called
**Solution:** Check:
1. Custom domain configured? (not `.workers.dev`)
2. Wildcard DNS set up? (`*.domain.com → worker.domain.com`)
3. `normalizeId: true` in getSandbox?
4. `proxyToSandbox()` called first in fetch?

### "Slow first request"

**Cause:** Cold start (container provisioning)
**Solution:**
- Use `sleepAfter` instead of creating new sandboxes
- Pre-warm with cron triggers
- Set `keepAlive: true` for critical sandboxes

### "File not persisting"

**Cause:** Files in `/tmp` or other ephemeral paths
**Solution:** Use `/workspace` for persistent files

### "Bucket mounting doesn't work locally"

**Cause:** Bucket mounting requires FUSE, not available in `wrangler dev`
**Solution:** Test bucket mounting in production only. Use mock data locally.

### "Different normalizeId = different sandbox"

**Cause:** Changing `normalizeId` option changes Durable Object ID
**Solution:** Set `normalizeId` consistently. `normalizeId: true` lowercases the ID.

```typescript
// These create DIFFERENT sandboxes:
getSandbox(env.Sandbox, 'MyApp');              // DO ID: hash('MyApp')
getSandbox(env.Sandbox, 'MyApp', { normalizeId: true });  // DO ID: hash('myapp')
```

### "Code context variables disappeared"

**Cause:** Container restart clears code context state
**Solution:** Code contexts are ephemeral. Recreate context after container sleep/wake.

## Performance Optimization

### Sandbox ID Strategy

```typescript
// ❌ BAD: New sandbox every time (slow)
const sandbox = getSandbox(env.Sandbox, `user-${Date.now()}`);

// ✅ GOOD: Reuse per user
const sandbox = getSandbox(env.Sandbox, `user-${userId}`);
```

### Sleep & Traffic Config

```typescript
// Cost-optimized
getSandbox(env.Sandbox, 'id', { sleepAfter: '30m', keepAlive: false });

// Always-on (requires destroy())
getSandbox(env.Sandbox, 'id', { keepAlive: true });
```

```jsonc
// High traffic: increase max_instances
{ "containers": [{ "class_name": "Sandbox", "max_instances": 50 }] }
```

## Security Best Practices

### Sandbox Isolation
- Each sandbox = isolated container (filesystem, network, processes)
- Use unique sandbox IDs per tenant for multi-tenant apps
- Sandboxes cannot communicate directly

### Input Validation

```typescript
// ❌ DANGEROUS: Command injection
const result = await sandbox.exec(`python3 -c "${userCode}"`);

// ✅ SAFE: Write to file, execute file
await sandbox.writeFile('/workspace/user_code.py', userCode);
const result = await sandbox.exec('python3 /workspace/user_code.py');
```

### Resource Limits

```typescript
// Timeout long-running commands
const result = await sandbox.exec('python3 script.py', {
  timeout: 30000  // 30 seconds
});
```

### Secrets Management

```typescript
// ❌ NEVER hardcode secrets
const token = 'ghp_abc123';

// ✅ Use environment secrets
const token = env.GITHUB_TOKEN;

// Pass to sandbox via exec env
const result = await sandbox.exec('git clone ...', {
  env: { GIT_TOKEN: token }
});
```

### Preview URL Security
Preview URLs include auto-generated tokens:
```
https://8080-sandbox-abc123def456.yourdomain.com
```
Token changes on each expose operation, preventing unauthorized access.

## Limits

| Resource | Lite | Standard | Heavy |
|----------|------|----------|-------|
| RAM | 256MB | 512MB | 1GB |
| vCPU | 0.5 | 1 | 2 |

| Operation | Default Timeout | Override |
|-----------|----------------|----------|
| Container provisioning | 30s | `SANDBOX_INSTANCE_TIMEOUT_MS` |
| Port readiness | 90s | `SANDBOX_PORT_TIMEOUT_MS` |
| exec() | 120s | `timeout` option |
| sleepAfter | 10m | `sleepAfter` option |

**Performance**:
- **First deploy**: 2-3 min for container build
- **Cold start**: 2-3s when waking from sleep
- **Bucket mounting**: Production only (FUSE not in dev)

## Production Guide

See: https://developers.cloudflare.com/sandbox/guides/production-deployment/

## Resources

- [Official Docs](https://developers.cloudflare.com/sandbox/)
- [API Reference](https://developers.cloudflare.com/sandbox/api/)
- [Examples](https://github.com/cloudflare/sandbox-sdk/tree/main/examples)
- [npm Package](https://www.npmjs.com/package/@cloudflare/sandbox)
- [Discord Support](https://discord.cloudflare.com)