Inngest Durable Functions

Master Inngest's durable execution model for building fault-tolerant, long-running workflows. This skill covers the complete lifecycle from triggers to error handling.

These skills are focused on TypeScript. For Python or Go, refer to the Inngest documentation for language-specific guidance. Core concepts apply across all languages.

Core Concepts You Need to Know

Durable Execution Model

• Each step should encapsulate side-effects and non-deterministic code
• Memoization prevents re-execution of completed steps
• State persistence survives infrastructure failures
• Automatic retries with configurable retry count

Step Execution Flow

// ❌ BAD: Non-deterministic logic outside steps
async ({ event, step }) => {
  const timestamp = Date.now(); // This runs multiple times!

  const result = await step.run("process-data", () => {
    return processData(event.data);
  });
};

// ✅ GOOD: All non-deterministic logic in steps
async ({ event, step }) => {
  const result = await step.run("process-with-timestamp", () => {
    const timestamp = Date.now(); // Only runs once
    return processData(event.data, timestamp);
  });
};

Function Limits

Every Inngest function has these hard limits:

• Maximum 1,000 steps per function run
• Maximum 4MB returned data for each step
• Maximum 32MB combined function run state including, event data, step output, and function output
• Each step = separate HTTP request (~50-100ms overhead)

If you're hitting these limits, break your function into smaller functions connected via step.invoke() or step.sendEvent().

When to Use Steps

Always wrap in step.run():

• API calls and network requests
• Database reads and writes
• File I/O operations
• Any non-deterministic operation
• Anything you want retried independently on failure

Never wrap in step.run():

• Pure calculations and data transformations
• Simple validation logic
• Deterministic operations with no side effects
• Logging (use outside steps)

Function Creation

Basic Function Structure

const processOrder = inngest.createFunction(
  {
    id: "process-order", // Unique, never change this
    triggers: [{ event: "order/created" }],
    retries: 4, // Default: 4 retries per step
    concurrency: 10 // Max concurrent executions
  },
  async ({ event, step }) => {
    // Your durable workflow
  }
);

Step IDs and Memoization

// Step IDs can be reused - Inngest handles counters automatically
const data = await step.run("fetch-data", () => fetchUserData());
const more = await step.run("fetch-data", () => fetchOrderData()); // Different execution

// Use descriptive IDs for clarity
await step.run("validate-payment", () => validatePayment(event.data.paymentId));
await step.run("charge-customer", () => chargeCustomer(event.data));
await step.run("send-confirmation", () => sendEmail(event.data.email));

Triggers and Events

Event Triggers

Triggers are defined in the triggers array in the first argument of createFunction:

// Single event trigger
inngest.createFunction(
  { id: "my-fn", triggers: [{ event: "user/signup" }] },
  async ({ event }) => { /* ... */ }
);

// Event with conditional filter
inngest.createFunction(
  { id: "my-fn", triggers: [{ event: "user/action", if: 'event.data.action == "purchase" && event.data.amount > 100' }] },
  async ({ event }) => { /* ... */ }
);

// Multiple triggers (up to 10)
inngest.createFunction(
  {
    id: "my-fn",
    triggers: [
      { event: "user/signup" },
      { event: "user/login", if: 'event.data.firstLogin == true' },
      { cron: "0 9 * * *" } // Daily at 9 AM
    ]
  },
  async ({ event }) => { /* ... */ }
);

Cron Triggers

// Basic cron
inngest.createFunction(
  { id: "my-fn", triggers: [{ cron: "0 */6 * * *" }] }, // Every 6 hours
  async ({ step }) => { /* ... */ }
);

// With timezone
inngest.createFunction(
  { id: "my-fn", triggers: [{ cron: "TZ=Europe/Paris 0 12 * * 5" }] }, // Fridays at noon Paris time
  async ({ step }) => { /* ... */ }
);

// Combine with events
inngest.createFunction(
  {
    id: "my-fn",
    triggers: [
      { event: "manual/report.requested" },
      { cron: "0 0 * * 0" } // Weekly on Sunday
    ]
  },
  async ({ event, step }) => { /* ... */ }
);

Function Invocation

// Invoke another function as a step
const result = await step.invoke("generate-report", {
  function: generateReportFunction,
  data: { userId: event.data