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name: inngest-durable-functions description: Create and configure Inngest durable functions. Covers triggers (events, cron, invoke), step execution and memoization, idempotency, cancellation, error handling, retries, logging, and observability.

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
    retries: 4, // Default: 4 retries per step
    concurrency: 10 // Max concurrent executions
  },
  { event: "order/created" }, // Trigger
  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

// Single event trigger
{ event: "user/signup" }

// Event with conditional filter
{
  event: "user/action",
  if: 'event.data.action == "purchase" && event.data.amount > 100'
}

// Multiple triggers (up to 10)
[
  { event: "user/signup" },
  { event: "user/login", if: 'event.data.firstLogin == true' },
  { cron: "0 9 * * *" } // Daily at 9 AM
]

Cron Triggers

// Basic cron
{
  cron: "0 */6 * * *";
} // Every 6 hours

// With timezone
{
  cron: "TZ=Europe/Paris 0 12 * * 5";
} // Fridays at noon Paris time

// Combine with events
[
  { event: "manual/report.requested" },
  { cron: "0 0 * * 0" } // Weekly on Sunday
];

Function Invocation

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

// Use returned data
await step.run("process-report", () => {
  return processReport(result);
});

Idempotency Strategies

Event-Level Idempotency (Producer Side)

// Prevent duplicate events with custom ID
await inngest.send({
  id: `checkout-completed-${cartId}`, // 24-hour deduplication
  name: "cart/checkout.completed",
  data: { cartId, email: "user@example.com" }
});

Function-Level Idempotency (Consumer Side)

const sendEmail = inngest.createFunction(
  {
    id: "send-checkout-email",
    // Only run once per cartId per 24 hours
    idempotency: "event.data.cartId"
  },
  { event: "cart/checkout.completed" },
  async ({ event, step }) => {
    // This function won't run twice for same cartId
  }
);

// Complex idempotency keys
const processUserAction = inngest.createFunction(
  {
    id: "process-user-action",
    // Unique per user + organization combination
    idempotency: 'event.data.userId + "-" + event.data.organizationId'
  },
  { event: "user/action.performed" },
  async ({ event, step }) => {
    /* ... */
  }
);

Cancellation Patterns

Event-Based Cancellation

In expressions, event = the original triggering event, async = the new event being matched. See Expression Syntax Reference for full details.

const processOrder = inngest.createFunction(
  {
    id: "process-order",
    cancelOn: [
      {
        event: "order/cancelled",
        if: "event.data.orderId == async.data.orderId"
      }
    ]
  },
  { event: "order/created" },
  async ({ event, step }) => {
    await step.sleepUntil("wait-for-payment", event.data.paymentDue);
    // Will be cancelled if order/cancelled event received
    await step.run("charge-payment", () => processPayment(event.data));
  }
);

Timeout Cancellation

const processWithTimeout = inngest.createFunction(
  {
    id: "process-with-timeout",
    timeouts: {
      start: "5m", // Cancel if not started within 5 minutes
      finish: "30m" // Cancel if not finished within 30 minutes
    }
  },
  { event: "long/process.requested" },
  async ({ event, step }) => {
    /* ... */
  }
);

Handling Cancellation Cleanup

// Listen for cancellation events
const cleanupCancelled = inngest.createFunction(
  { id: "cleanup-cancelled-process" },
  { event: "inngest/function.cancelled" },
  async ({ event, step }) => {
    if (event.data.function_id === "process-order") {
      await step.run("cleanup-resources", () => {
        return cleanupOrderResources(event.data.run_id);
      });
    }
  }
);

Error Handling and Retries

Default Retry Behavior

• 5 total attempts (1 initial + 4 retries) per step
• Exponential backoff with jitter
• Independent retry counters per step

Custom Retry Configuration

const reliableFunction = inngest.createFunction(
  {
    id: "reliable-function",
    retries: 10 // Up to 10 retries per step
  },
  { event: "critical/task" },
  async ({ event, step, attempt }) => {
    // `attempt` is the function-level attempt counter (0-indexed)
    // It tracks retries for the currently executing step, not the overall function
    if (attempt > 5) {
      // Different logic for later attempts of the current step
    }
  }
);

Non-Retriable Errors

Prevent retries for code that won't succeed upon retry.

import { NonRetriableError } from "inngest";

const processUser = inngest.createFunction(
  { id: "process-user" },
  { event: "user/process.requested" },
  async ({ event, step }) => {
    const user = await step.run("fetch-user", async () => {
      const user = await db.users.findOne(event.data.userId);

      if (!user) {
        // Don't retry - user doesn't exist
        throw new NonRetriableError("User not found, stopping execution");
      }

      return user;
    });

    // Continue processing...
  }
);

Custom Retry Timing

import { RetryAfterError } from "inngest";

const respectRateLimit = inngest.createFunction(
  { id: "api-call" },
  { event: "api/call.requested" },
  async ({ event, step }) => {
    await step.run("call-api", async () => {
      const response = await externalAPI.call(event.data);

      if (response.status === 429) {
        // Retry after specific time from API
        const retryAfter = response.headers["retry-after"];
        throw new RetryAfterError("Rate limited", `${retryAfter}s`);
      }

      return response.data;
    });
  }
);

Logging Best Practices

Proper Logging Setup

import winston from "winston";

// Configure logger
const logger = winston.createLogger({
  level: "info",
  format: winston.format.json(),
  transports: [new winston.transports.Console()]
});

const inngest = new Inngest({
  id: "my-app",
  logger // Pass logger to client
});

Function Logging Patterns

const processData = inngest.createFunction(
  { id: "process-data" },
  { event: "data/process.requested" },
  async ({ event, step, logger }) => {
    // ✅ GOOD: Log inside steps to avoid duplicates
    const result = await step.run("fetch-data", async () => {
      logger.info("Fetching data for user", { userId: event.data.userId });
      return await fetchUserData(event.data.userId);
    });

    // ❌ AVOID: Logging outside steps can duplicate
    // logger.info("Processing complete"); // This could run multiple times!

    await step.run("log-completion", async () => {
      logger.info("Processing complete", { resultCount: result.length });
    });
  }
);

Performance Optimization

Checkpointing

// Enable checkpointing for lower latency
const realTimeFunction = inngest.createFunction(
  {
    id: "real-time-function",
    checkpointing: {
      maxRuntime: "5m", // Max continuous execution time
      bufferedSteps: 2, // Buffer 2 steps before checkpointing
      maxInterval: "10s" // Max wait before checkpoint
    }
  },
  { event: "realtime/process" },
  async ({ event, step }) => {
    // Steps execute immediately with periodic checkpointing
    const result1 = await step.run("step-1", () => process1(event.data));
    const result2 = await step.run("step-2", () => process2(result1));
    return { result2 };
  }
);

Advanced Patterns

Conditional Step Execution

const conditionalProcess = inngest.createFunction(
  { id: "conditional-process" },
  { event: "process/conditional" },
  async ({ event, step }) => {
    const userData = await step.run("fetch-user", () => {
      return getUserData(event.data.userId);
    });

    // Conditional step execution
    if (userData.isPremium) {
      await step.run("premium-processing", () => {
        return processPremiumFeatures(userData);
      });
    }

    // Always runs
    await step.run("standard-processing", () => {
      return processStandardFeatures(userData);
    });
  }
);

Error Recovery Patterns

const robustProcess = inngest.createFunction(
  { id: "robust-process" },
  { event: "process/robust" },
  async ({ event, step }) => {
    let primaryResult;

    try {
      primaryResult = await step.run("primary-service", () => {
        return callPrimaryService(event.data);
      });
    } catch (error) {
      // Fallback to secondary service
      primaryResult = await step.run("fallback-service", () => {
        return callSecondaryService(event.data);
      });
    }

    return { result: primaryResult };
  }
);

Common Mistakes to Avoid

1. ❌ Non-deterministic code outside steps
2. ❌ Database calls outside steps
3. ❌ Logging outside steps (causes duplicates)
4. ❌ Changing step IDs after deployment
5. ❌ Not handling NonRetriableError cases
6. ❌ Ignoring idempotency for critical functions

Next Steps

• See inngest-steps for detailed step method reference
• See references/step-execution.md for detailed step patterns
• See references/error-handling.md for comprehensive error strategies
• See references/observability.md for monitoring and tracing setup
• See references/checkpointing.md for performance optimization details

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This skill covers Inngest's durable function patterns. For event sending and webhook handling, see the inngest-events skill.