Inngest Middleware
Master Inngest middleware to handle cross-cutting concerns like logging, error tracking, dependency injection, and data transformation. Middleware runs at key points in the function lifecycle, enabling powerful patterns for observability and shared functionality.
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.
Note: The middleware system was significantly rewritten in v4. The lifecycle hooks documented here reflect the v4 API. If migrating from v3, consult the migration guide for details on breaking changes.
⚠ For Realtime use the `inngest-realtime` skill, NOT this one. Inngest v3 used realtimeMiddleware() from @inngest/realtime to inject a publish arg into function handlers. v4 ships realtime natively — step.realtime.publish is built-in, no middleware required. Do NOT install @inngest/realtime on a v4 project (it's a v3-era package and produces TypeError: Cls is not a constructor at runtime). See the inngest-realtime skill for the v4 pattern.
What is Middleware?
Middleware allows code to run at various points in an Inngest client's lifecycle - during function execution, event sending, and more. Think of middleware as hooks into the Inngest execution pipeline.
When to use middleware:
- Observability: Add logging, tracing, or metrics
- Dependency injection: Share client instances across functions
- Data transformation: Encrypt/decrypt, validate, or enrich data
- Error handling: Custom error tracking and alerting
- Authentication: Validate user context or permissions
Middleware Lifecycle
Middleware can be registered at client-level (affects all functions) or function-level (affects specific functions).
Execution Order
const inngest = new Inngest({
id: "my-app",
middleware: [
loggingMiddleware, // Runs 1st
errorMiddleware // Runs 2nd
]
});
inngest.createFunction(
{
id: "example",
middleware: [
authMiddleware, // Runs 3rd
metricsMiddleware // Runs 4th
],
triggers: [{ event: "test" }]
},
async () => {
/* function code */
}
);
Order matters: Client middleware runs first, then function middleware, in the order specified.
Creating Custom Middleware
Basic Middleware Structure
import { InngestMiddleware } from "inngest";
const loggingMiddleware = new InngestMiddleware({
name: "Logging Middleware",
init() {
// Setup phase - runs when client initializes
const logger = setupLogger();
return {
// Function execution lifecycle
// Note: `fn` is loosely typed in middleware generics; fn.id works at runtime
onFunctionRun({ ctx, fn }) {
return {
beforeExecution() {
logger.info("Function starting", {
functionId: fn.id,
eventName: ctx.event.name,
runId: ctx.runId
});
},
afterExecution() {
logger.info("Function completed", {
functionId: fn.id,
runId: ctx.runId
});
},
transformOutput({ result }) {
// Log function output
logger.debug("Function output", {
functionId: fn.id,
output: result.data
});
// Return unmodified result
return { result };
}
};
},
// Event sending lifecycle
onSendEvent() {
return {
transformInput({ payloads }) {
logger.info("Sending events", {
count: payloads.length,
events: payloads.map((p) => p.name)
});
// Spread to convert readonly array to mutable array
return { payloads: [...payloads] };
}
};
}
};
}
});
Python Implementation
Python middleware follows a similar pattern. See Dependency Injection Reference for complete Python examples.
## Dependency Injection
Share expensive or stateful clients across all functions. **See [Dependency Injection Reference](./references/dependency-injection.md) for detailed patterns.**
### Quick Example - Built-in DI
import { dependencyInjectionMiddleware } from "inngest";
const inngest = new Inngest({ id: 'my-app', middleware: [ dependencyInjectionMiddleware({ openai: new OpenAI(), db: new PrismaClient(), }), ], });
// Functions automatically get injected dependencies inngest.createFunction( { id: "ai-summary", triggers: [{ event: "document/uploaded" }] }, async ({ event, openai, db }) => { // Dependencies available in function context const summary = await openai.chat.completions.create({ messages: [{ role: "user", content: event.data.content }], model: "gpt-4", });
await db.document.update({ where: { id: event.data.documentId }, data: { summary: summary.choices[0].message.content } }); } );
## Middleware Packages
Beyond `dependencyInjectionMiddleware` (built-in, shown above), Inngest provides official middleware as **separate packages**. **See [Middleware Reference](./references/built-in-middleware.md) for complete details.**
### Encryption Middleware
npm install @inngest/middleware-encryption
import { encryptionMiddleware } from "@inngest/middleware-encryption";
const inngest = new Inngest({ id: "my-app", middleware: [ encryptionMiddleware({ key: process.env.ENCRYPTION_KEY }) ] });
Automatically encrypts all step data, function output, and event `data.encrypted` field. Supports key rotation via `fallbackDecryptionKeys`.
### Sentry Error Tracking
npm install @inngest/middleware-sentry
import * as Sentry from "@sentry/node"; import { sentryMiddleware } from "@inngest/middleware-sentry";
Sentry.init({ / your Sentry config / });
const inngest = new Inngest({ id: "my-app", middleware: [sentryMiddleware()] });
Captures exceptions, adds tracing to each function run, and includes function ID and event names as context. Requires `@sentry/*@>=8.0.0`.
## Common Middleware Patterns
### Metrics and Performance Tracking
const metricsMiddleware = new InngestMiddleware({ name: "Metrics Tracking", init() { return { onFunctionRun({ ctx, fn }) { let startTime: number;
return { beforeExecution() { startTime = Date.now(); metrics.increment("inngest.step.started", { function: fn.id, event: ctx.event.name }); },
afterExecution() { const duration = Date.now() - startTime; metrics.histogram("inngest.step.duration", duration, { function: fn.id, event: ctx.event.name }); },
transformOutput({ result }) { const status = result.error ? "error" : "success"; metrics.increment("inngest.step.completed", { function: fn.id, status: status });
return { result }; } }; } }; } });
### Advanced Patterns
**Authentication:** Validate tokens and inject user context
**Conditional logic:** Apply middleware based on event type or function
**Circuit breakers:** Prevent cascading failures from external services
### Configuration-Based Middleware
Create reusable middleware with configuration options for different environments and use cases. See reference documentation for complete examples.
## Best Practices
### Design Principles
1. **Keep middleware focused:** One concern per middleware
2. **Handle errors gracefully:** Don't let middleware crash functions
3. **Consider performance:** Middleware runs on every execution
4. **Use proper typing:** Let TypeScript infer middleware types
5. **Test thoroughly:** Middleware affects all functions that use it
### Common Use Cases to Implement
- **Retry logic** for transient failures
- **Circuit breakers** for external service calls
- **Request/response logging** for debugging
- **User context enrichment** from external sources
- **Feature flags** for gradual rollouts
- **Custom authentication** and authorization checks
### Error Handling in Middleware
const robustMiddleware = new InngestMiddleware({ name: "Robust Middleware", init() { return { onFunctionRun({ ctx, fn }) { return { transformOutput({ result }) { try { // Your middleware logic here return performTransformation(result); } catch (middlewareError) { // Log error but don't break the function console.error("Middleware error:", middlewareError);
// Return original result on middleware failure return { result }; } } }; } }; } });
### Testing Middleware
Use Inngest's testing utilities (`createMockContext`, `createMockFunction`) to unit test middleware behavior.
**For complete implementation examples and advanced patterns, see:**
- [Dependency Injection Reference](./references/dependency-injection.md)
- [Built-in Middleware Reference](./references/built-in-middleware.md)
