Inngest Flow Control

# Inngest Flow Control

Master Inngest flow control mechanisms to manage resources, prevent overloading systems, and ensure application reliability. This skill covers all flow control options with prescriptive guidance on when and how to use each.

> **These skills are focused on TypeScript.** For Python or Go, refer to the [Inngest documentation](https://www.inngest.com/llms.txt) for language-specific guidance. Core concepts apply across all languages.

## Quick Decision Guide

- **"Limit how many run at once"** → Concurrency
- **"Spread runs over time"** → Throttling
- **"Block after N runs in a period"** → Rate Limiting
- **"Wait for activity to stop, then run once"** → Debounce
- **"Only one run at a time for this key"** → Singleton
- **"Process events in groups"** → Batching
- **"Some runs are more important"** → Priority

## Concurrency

**When to use:** Limit the number of executing steps (not function runs) to manage computing resources and prevent system overwhelm.

**Key insight:** Concurrency limits active code execution, not function runs. A function waiting on `step.sleep()` or `step.waitForEvent()` doesn't count against the limit.

### Basic Concurrency

```typescript
inngest.createFunction(
  {
    id: "process-images",
    concurrency: 5,
    triggers: [{ event: "media/image.uploaded" }]
  },
  async ({ event, step }) => {
    // Only 5 steps can execute simultaneously
    await step.run("resize", () => resizeImage(event.data.imageUrl));
  }
);
```

### Concurrency with Keys (Multi-tenant)

Use `key` parameter to apply limit per unique value of the key.

```typescript
inngest.createFunction(
  {
    id: "user-sync",
    concurrency: [
      {
        key: "event.data.user_id",
        limit: 1
      }
    ],
    triggers: [{ event: "user/profile.updated" }]
  },
  async ({ event, step }) => {
    // Only 1 step per user can execute at once
    // Prevents race conditions in user-specific operations
  }
);
```

### Account-level Shared Limits

```typescript
inngest.createFunction(
  {
    id: "ai-summary",
    concurrency: [
      {
        scope: "account",
        key: `"openai"`,
        limit: 60
      }
    ],
    triggers: [{ event: "ai/summary.requested" }]
  },
  async ({ event, step }) => {
    // Share 60 concurrent OpenAI calls across all functions
  }
);
```

**When to use each:**

- Basic: Protect databases or limit general capacity
- Keyed: Multi-tenant fairness, prevent "noisy neighbor" issues
- Account-level: Share quotas across multiple functions (API limits)

## Throttling

**When to use:** Control the rate of function starts over time to work around API rate limits or smooth traffic spikes.

**Key difference from concurrency:** Throttling limits function run starts; concurrency limits step execution.

```typescript
inngest.createFunction(
  {
    id: "sync-crm-data",
    throttle: {
      limit: 10, // 10 function starts
      period: "60s", // per minute
      burst: 5, // plus 5 immediate bursts
      key: "event.data.customer_id" // per customer
    },
    triggers: [{ event: "crm/contact.updated" }]
  },
  async ({ event, step }) => {
    // Respects CRM API rate limits: 10 calls/min per customer
    await step.run("sync", () => crmApi.updateContact(event.data));
  }
);
```

**Configuration:**

- `limit`: Functions that can start per period
- `period`: Time window (1s to 7d)
- `burst`: Extra immediate starts allowed
- `key`: Apply limits per unique key value

## Rate Limiting

**When to use:** Hard limit to prevent abuse or skip excessive duplicate events.

**Key difference from throttling:** Rate limiting discards events; throttling delays them.

```typescript
inngest.createFunction(
  {
    id: "webhook-processor",
    rateLimit: {
      limit: 1,
      period: "4h",
      key: "ev