# Workflow Patterns

## Image Processing Pipeline

```typescript
export class ImageProcessingWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event, step) {
    const imageData = await step.do('fetch', async () => (await this.env.BUCKET.get(event.params.imageKey)).arrayBuffer());
    const description = await step.do('generate description', async () => 
      await this.env.AI.run('@cf/llava-hf/llava-1.5-7b-hf', {image: Array.from(new Uint8Array(imageData)), prompt: 'Describe this image', max_tokens: 50})
    );
    await step.waitForEvent('await approval', { event: 'approved', timeout: '24h' });
    await step.do('publish', async () => await this.env.BUCKET.put(`public/${event.params.imageKey}`, imageData));
  }
}
```

## User Lifecycle

```typescript
export class UserLifecycleWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event, step) {
    await step.do('welcome email', async () => await sendEmail(event.params.email, 'Welcome!'));
    await step.sleep('trial period', '7 days');
    const hasConverted = await step.do('check conversion', async () => {
      const user = await this.env.DB.prepare('SELECT subscription_status FROM users WHERE id = ?').bind(event.params.userId).first();
      return user.subscription_status === 'active';
    });
    if (!hasConverted) await step.do('trial expiration email', async () => await sendEmail(event.params.email, 'Trial ending'));
  }
}
```

## Data Pipeline

```typescript
export class DataPipelineWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event, step) {
    const rawData = await step.do('extract', {retries: { limit: 10, delay: '30s', backoff: 'exponential' }}, async () => {
      const res = await fetch(event.params.sourceUrl);
      if (!res.ok) throw new Error('Fetch failed');
      return res.json();
    });
    const transformed = await step.do('transform', async () => 
      rawData.map(item => ({ id: item.id, normalized: normalizeData(item) }))
    );
    const dataRef = await step.do('store', async () => {
      const key = `processed/${Date.now()}.json`;
      await this.env.BUCKET.put(key, JSON.stringify(transformed));
      return { key };
    });
    await step.do('load', async () => {
      const data = await (await this.env.BUCKET.get(dataRef.key)).json();
      for (let i = 0; i < data.length; i += 100) {
        await this.env.DB.batch(data.slice(i, i + 100).map(item => 
          this.env.DB.prepare('INSERT INTO records VALUES (?, ?)').bind(item.id, item.normalized)
        ));
      }
    });
  }
}
```

## Human-in-the-Loop Approval

```typescript
export class ApprovalWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event, step) {
    await step.do('create approval', async () => await this.env.DB.prepare('INSERT INTO approvals (id, user_id, status) VALUES (?, ?, ?)').bind(event.instanceId, event.params.userId, 'pending').run());
    try {
      const approval = await step.waitForEvent<{ approved: boolean }>('wait for approval', { event: 'approval-response', timeout: '48h' });
      if (approval.approved) { await step.do('process approval', async () => {}); } 
      else { await step.do('handle rejection', async () => {}); }
    } catch (e) {
      await step.do('auto reject', async () => await this.env.DB.prepare('UPDATE approvals SET status = ? WHERE id = ?').bind('auto-rejected', event.instanceId).run());
    }
  }
}
```

## Testing Workflows

### Setup

```typescript
// vitest.config.ts
import { defineWorkersConfig } from '@cloudflare/vitest-pool-workers/config';

export default defineWorkersConfig({
  test: {
    poolOptions: {
      workers: {
        wrangler: { configPath: './wrangler.jsonc' }
      }
    }
  }
});
```

### Introspection API

```typescript
import { introspectWorkflowInstance } from 'cloudflare:test';

const instance = await env.MY_WORKFLOW.create({ params: { userId: '123' } });
const introspector = await introspectWorkflowInstance(env.MY_WORKFLOW, instance.id);

// Wait for step completion
const result = await introspector.waitForStepResult({ name: 'fetch user', index: 0 });

// Mock step behavior
await introspector.modify(async (m) => {
  await m.mockStepResult({ name: 'api call' }, { mocked: true });
});
```

## Best Practices

### ✅ DO

1. **Granular steps**: One API call per step (unless proving idempotency)
2. **Idempotency**: Check-then-execute; use idempotency keys
3. **Deterministic names**: Use static or step-output-based names
4. **Return state**: Persist via step returns, not variables
5. **Always await**: `await step.do()`, avoid dangling promises
6. **Deterministic conditionals**: Base on `event.payload` or step outputs
7. **Store large data externally**: R2/KV for >1 MiB, return refs
8. **Batch creation**: `createBatch()` for multiple instances

### ❌ DON'T

1. **One giant step**: Breaks durability & retry control
2. **State outside steps**: Lost on hibernation
3. **Mutate events**: Events immutable, return new state
4. **Non-deterministic logic outside steps**: `Math.random()`, `Date.now()` must be in steps
5. **Side effects outside steps**: May duplicate on restart
6. **Non-deterministic step names**: Prevents caching
7. **Ignore timeouts**: `waitForEvent` throws, use try-catch
8. **Reuse instance IDs**: Must be unique within retention

## Orchestration Patterns

### Fan-Out (Parallel Processing)
```typescript
const files = await step.do('list', async () => this.env.BUCKET.list());
await Promise.all(files.objects.map((file, i) => step.do(`process ${i}`, async () => processFile(await (await this.env.BUCKET.get(file.key)).arrayBuffer()))));
```

### Parent-Child Workflows
```typescript
const child = await step.do('start child', async () => await this.env.CHILD_WORKFLOW.create({id: `child-${event.instanceId}`, params: { data: result.data }}));
await step.do('other work', async () => console.log(`Child started: ${child.id}`));
```

### Race Pattern
```typescript
const winner = await Promise.race([
  step.do('option A', async () => slowOperation()),
  step.do('option B', async () => fastOperation())
]);
```

### Scheduled Workflow Chain
```typescript
export default { async scheduled(event, env) { await env.DAILY_WORKFLOW.create({id: `daily-${event.scheduledTime}`, params: { timestamp: event.scheduledTime }}); }};
export class DailyWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event, step) {
    await step.do('daily task', async () => {});
    await step.sleep('wait 7 days', '7 days');
    await step.do('weekly followup', async () => {});
  }
}
```

See: [configuration.md](./configuration.md), [api.md](./api.md), [gotchas.md](./gotchas.md)