Next.js 15 Deep Dive: Server Actions, Partial Prerendering, and Beyond

Next.js 15 represents a major evolution in React frameworks, with stable Server Actions, groundbreaking Partial Prerendering, and a completely redesigned caching system.

Server Actions: Stable and Production-Ready

Server Actions are now stable and the recommended way to handle mutations in Next.js:

// app/actions.ts
"use server"

export async function createPost(formData: FormData) {
  const title = formData.get("title") as string;
  const content = formData.get("content") as string;

  await db.posts.create({ title, content });
  revalidatePath("/posts");
}

// app/new-post/page.tsx
import { createPost } from "./actions";

export default function NewPost() {
  return (
    <form action={createPost}>
      <input name="title" required />
      <textarea name="content" required />
      <button type="submit">Create Post</button>
    </form>
  );
}

Partial Prerendering (PPR)

PPR combines static and dynamic content on the same page:

• Static shell loads instantly from CDN
• Dynamic content streams in as it's ready
• Best of both SSG and SSR worlds

// next.config.js
module.exports = {
  experimental: {
    ppr: true,
  },
};

// app/page.tsx
import { Suspense } from "react";

export default function Page() {
  return (
    <div>
      <StaticHeader />  {/* Prerendered */}
      <Suspense fallback={<Loading />}>
        <DynamicContent />  {/* Streamed */}
      </Suspense>
    </div>
  );
}

New Caching Defaults

Next.js 15 changes caching to be opt-in rather than opt-out:

• fetch() requests are no longer cached by default
• Route handlers are dynamic by default
• Use cache: "force-cache" for static data

Migration Guide

1. Update package: npm install next@15
2. Review caching behavior for data fetching
3. Convert API routes to Server Actions where possible
4. Enable PPR for hybrid pages