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# Server Functions

Server Functions are async functions that run on the server for form submissions and mutations.

## Example: createPost

From `data/actions/post-actions.ts`:

```tsx
'use server';

export async function createPost(formData: FormData): Promise<ActionResult> {
  const rawData = { title: formData.get('title'), ... };

const result = postSchema.safeParse(rawData);
  if (!result.success) {
    return { error: result.error.issues[0].message, formData: rawData, success: false };
  }

await prisma.post.create({ data: result.data });
  updateTag('posts');
  return { success: true };
}
```

## Returning Errors

```tsx
export type ActionResult =
  | { success: true }
  | { success: false; error: string; formData?: FormValues };
```

Return submitted data on errors so forms can repopulate.

## Cache Invalidation

```tsx
export async function updatePost(slug: string, formData: FormData) {
  // ... validate and update
  updateTag('posts');        // Invalidate the list
  updateTag(`post-${slug}`); // Invalidate this post
}
```

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