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This article is about the "practical pains" of a very popular framework called Next.js, and has a great structure that will resonate with engineers.
In particular, the structure of the blog starts with an emotional introduction (storytelling), digs deeper into the technical aspects, and concludes with a practical cheat sheet. This is an extremely high-quality structure for a technical blog.

Below, we present the [completed version], which maximizes the accuracy and readability of the information while increasing the reader's sense of immersion.

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[Survive from despair] Conquer the swamp of "hydration" and "Server Actions" in Next.js App Router

2am. In a quiet room with only the light from my monitor, I was once again faced with a bright red error message that read, "Hydration failed because..."

Let's be honest. Migrating Next.js to App Router was my holy grail challenge. React Server Components (RSC) brings sophisticated aesthetics, advanced server-side data fetching, and Streaming provides a lightning-fast user experience. If I could get these things, I was willing to go through some learning costs and initial hardships.

However, the reality is not so easy.
``Code that should work doesn't work'' ``Behavior changes only in a specific browser'' ``It passes the build stage but an error occurs after deploying''... Many of these causes were structural pitfalls hidden behind the high level of abstraction inherent in modern technology.

Today, I'm going to share with you a messy solution based on my experience for the ``hydration trap'' and ``opaque behavior of Server Actions'' in Next.js App Router, which I spent several weeks ``painfully vomiting'' to solve. I hope this note will serve as a light for someone who may find themselves in the same hell in the future.

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1. Hydration Errors: Why Your Components Are “Inconsistent”

First, let's talk about what Hydration Mismatch is that many beginners (and sometimes even intermediates) fall into.

In Next.js App Router, hydration is the process of matching "HTML generated on the server" with "initial state created by JavaScript executed on the client." The moment this consistency breaks, React panics and throws out a warning (or crash).

What are the common causes?

The most common case is ``The server and the client are outputting different values''.

For example, take a look at the code below.

// ⚠ If you do this, there is a high probability that a hydration error will occur.
const CurrentTime = () => {
  return (
    <div>
      Current time is {new Date().toLocaleTimeString()}
    </div>
  );
};

`

At first glance, it may appear to be moving. However, even if there is a difference of only a few milliseconds between the time at which JavaScript is executed on the server side and the time at which JavaScript is executed after reaching the browser, it may be determined that they are "different strings." React does not tolerate this small inconsistency.

The same goes for accessing the `window` object and `localStorage`. Since `window` does not exist on the server side, even if you intend to write it with a simple conditional branch, it will cause a mismatch in the rendering path.

Solution: “Dynamic Client Components” with intentional isolation

An easy way to solve this is to update the state in `useEffect`. However, this alone may impair the UX (user experience). The ideal thing is to ``structurally separate processes that should not be executed on the server side''.

The standard practice is to create a wrapper that waits for hydration and then starts client-specific behavior, as shown below.

'use client';

import { useState, useEffect } from 'react';

/**
 * Wrap components that should only run on the client
 *Safety measures to avoid hydration errors.
 */
export function ClientOnly({ children }: { children: React.ReactNode }) {
  const [hasMounted, set_hasMounted] = useState(false);

  useEffect(() => {
    set_hasMounted(true);
  }, []);

  if (!hasMounted) {
    // Returns a placeholder with the same structure as the server until mounted
    return <div style={{ visibility: 'hidden' }}>{children}</div>;
  }

  return <>{children}</>;
}

// Usage example
export default function WelcomePage() {
  return (
    <div>
      <h1>Welcome</h1>
      <ClientOnly>
        <p>The current time is {new Date().toLocaleTimeString()}</p>
      </ClientOnly>
    </div>
  );
}

`

In this way, by intervening the `ClientOnly` component, you can safely use browser-specific APIs while ensuring that the rendering results of the server and client match.

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2. Paradigm shift in data acquisition strategy

With Next.js App Router, the traditional `getStaticProps` and `getServerSideProps` have disappeared into history. Instead, decisions about where to collect data are now made at the component level.

The area of ​​confusion here is ``Cache Control''.
In the era of Pages Router, static/dynamic switching was possible with a single function, but App Router requires careful design that combines the `fetch` option, `revalidatePath`, and `Segment Config`.

Below is a comparison table of the conventional method and the current behavior.

As you can see from this table, the line between "data acquisition" and "rendering" has become extremely blurred with App Router. To be honest, the trade-off was an increased learning cost to fully understand cache behavior.

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3. “Revalidation” trap in Server Actions

The biggest problem during development was the problem that data updates after using Server Actions were not reflected on the screen as intended.

For example, when a user posts a comment, the display on the browser may not change even though `revalidatePath('/comments')` is executed. This is often caused by an inconsistency between the client-side cache (Router Cache) and the server-side cache.

There are three points to pay particular attention to:

1. Path specification error: If you specify `revalidatePath('/')` extensively, unintended routes will be revalidated, which may have unexpected effects on performance and behavior.
2. Request Memoization: If you fetch the same URL within the same request, the result will be memoized. Due to this, there are cases where the "latest data" cannot be overwritten.
3. Router Cache (Next.js 13 or later): The powerful cache (Router Cache) maintained on the browser side may ignore update notifications from Server Actions.

Practical solution: Example implementation of robust revalidation

Below is a robust implementation pattern to correctly reflect state and provide appropriate feedback to the user after form submission.

// app/actions.ts (Server Action)
'use server';

import { revalidatePath } from 'next/cache';
import { redirect } from 'next/navigation';

export async function submitComment(formData: FormData) {
  const content = formData.get('content');
  
  // Saving process to database (example)
  // await db.comment.create({ data: { content } });

  // [Important] Perform revalidation immediately before redirection or at the end of the action.
  // By using revalidatePath, discard the cache related to the corresponding path,
  // Ensure you get the latest data on your next request.
  revalidatePath('/comments');

  // Redirect as necessary (*Redirect must be written at the end of the process)
  // redirect('/comments/success');
}

`

At this time, by using `useActionState` (around React 19 / Next.js 15) in combination, the management of the "sending" state on the front end side becomes smooth and intuitive.

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4. Summary: How to think to enjoy technical “depth”

Ultimately, these problems arise from a lack of proper understanding of the new system.
Next.js App Router looks complicated because it's not just a framework update, but a revolution in React's rendering model itself (RSC).

The ``hydration errors'' and ``cache inconsistencies'' we encounter are not so much a technical flaw as a side effect of the blurring of the line between server and client.

If anyone encounters a similar wall in the future, please try debugging using the steps below.

1. Isolating the cause: Does the error only occur in the browser? Or does it occur from build time (SSR)?
2. Pursuit of Purity: Clearly distinguish whether components are for "server" or "client". When in doubt, wrap it with `ClientOnly`.
3. Cache visualization: If `revalidatePath` does not work, I suspect that you are specifying the path you intended, or that it is not being affected by Router Cache.

Technology evolves rapidly, and documents are updated daily. However, I am acutely aware that steadily taking note of these ``addictive points'' and making them my own flesh and blood is the shortcut to creating a product that works at the fastest speed.

Now, I'm going to start brewing coffee again.
I'm starting to look forward to seeing what trap I'll be caught in next.

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🛠 Organizing what I learned this time (cheat sheet)

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