# Lifecycle hooks

> onStart, onSuccess, onError and onFinish, for logging and metrics.

`@trpc12/trpc/middleware` has lifecycle helpers, each an ordinary middleware:

```ts
import { onError, onFinish } from '@trpc12/trpc/middleware';

const observed = t.procedure
  .use(onFinish((result, opts) => metrics.count(opts.path, result.ok)))
  .use(onError((err, opts) => log.warn(opts.path, err.code)));
```

`onStart` runs before the rest of the chain, `onSuccess` with the result when the rest succeeds (for a subscription, once the stream is set up), `onError` with the error when it fails, and `onFinish` with the outcome either way. Put them on a base procedure to cover every procedure built from it ([Middleware](https://v12.trpc.io/docs/server/middleware.md)).

For spans rather than logs, put a tracer in the handler's `layer`: every call already runs in a `trpc.<path>` span ([Tracing](https://v12.trpc.io/docs/server/effect.md#tracing)).