# Merging Routers

From https://trpc.io/docs/server/merging-routers

Writing all API-code in the same file can become unwieldy. It's easy to merge routers together in order to break them up.

## Merging with child routers

```ts no-check title="routers/user.ts"
import { router, publicProcedure } from '../trpc';
import { z } from 'zod';

export const userRouter = router({
  list: publicProcedure.query(() => {
    // [..]
    return [];
  }),
});
```

```ts no-check title="routers/post.ts"
import { router, publicProcedure } from '../trpc';
import { z } from 'zod';

export const postRouter = router({
  create: publicProcedure
    .input(
      z.object({
        title: z.string(),
      }),
    )
    .mutation((opts) => {
      const { input } = opts;
      // [...]
    }),
  list: publicProcedure.query(() => {
    // ...
    return [];
  }),
});
```

```ts no-check title="routers/_app.ts"
import { router } from '../trpc';

import { userRouter } from './user';
import { postRouter } from './post';

const appRouter = router({
  user: userRouter,
  post: postRouter,
});

appRouter.user

appRouter.post

export type AppRouter = typeof appRouter;
```

## Merging with `t.mergeRouters`

If you prefer having all procedures flat in one single namespace, you can instead use `t.mergeRouters`. It throws if two routers define the same procedure path, so keep the names unique. Merge routers from the same [`t`](https://trpc.io/docs/server/routers#initialize-trpc): it also throws if they come from separate tRPC instances that each set a transformer or error formatter.

```ts no-check title="routers/user.ts"
import { router, publicProcedure } from '../trpc';
import { z } from 'zod';

export const userRouter = router({
  userList: publicProcedure.query(() => {
    // [..]
    return [];
  }),
});
```

```ts no-check title="routers/post.ts"
import { router, publicProcedure } from '../trpc';
import { z } from 'zod';

export const postRouter = router({
  postCreate: publicProcedure
    .input(
      z.object({
        title: z.string(),
      }),
    )
    .mutation((opts) => {
      const { input } = opts;
      // [...]
    }),
  postList: publicProcedure.query(() => {
    // ...
    return [];
  }),
});
```

```ts no-check title="routers/_app.ts"
import { mergeRouters } from '../trpc';

import { userRouter } from './user';
import { postRouter } from './post';

const appRouter = mergeRouters(userRouter, postRouter);

export type AppRouter = typeof appRouter;
```

## Dynamically load routers

You can use the `lazy` function to dynamically load your routers. This can be useful to reduce cold starts of your application. There's no difference in how you use the router after it's been lazy loaded vs. how you use a normal router.

```ts no-check title="routers/greeting.ts"
import { router, publicProcedure } from '../trpc';

export const greetingRouter = router({
  hello: publicProcedure.query(() => 'world'),
});
```

```ts no-check title="routers/user.ts"
import { router, publicProcedure } from '../trpc';

export const userRouter = router({
  list: publicProcedure.query(() => ['John', 'Jane', 'Jim']),
});
```

```ts no-check title="routers/_app.ts"
import { lazy } from '@trpc/server';
import { router } from '../trpc';

export const appRouter = router({
  // Option 1: Short-hand when the module has exactly 1 router exported
  greeting: lazy(() => import('./greeting.js')),
  // Option 2: if exporting more than 1 router
  user: lazy(() => import('./user.js').then((m) => m.userRouter)),
});
export type AppRouter = typeof appRouter;
```