Discrepancies Between Dev Mode and Build Behavior in Next.js

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1

When testing the following Next.js code, there is a noticeable difference between development (npm run dev --turbo) and production (npm run build && npm run start) modes:

import Link from "next/link";

async function getRandomNumber() {
  const res = await fetch('http://localhost:3020/random-number');
  const data = await res.json();
  return data.numero;
}

export default function Home() {
  return (
    <div>
      <p>{getRandomNumber()}</p>
    </div>
  );
}

In development mode, the fetch request is triggered every time the page is loaded, navigated to, or revisited, and the updated data is rendered.
In production mode, the data appears to be cached, and the page does not fetch fresh data as expected, rendering a fixed value.
Adding export const dynamic = 'force-dynamic' resolves the issue, ensuring that the data is fetched dynamically in both modes.

Step-by-Step Guide to Reproduce

  1. Set up the test environment:
  • Ensure you have a Next.js application initialized (npx create-next-app).
  • Add the provided code to a pages/index.js or app/page.js file.
  • Start a simple server at http://localhost:3020/numero-aleatorio that returns a random number (like the Express example provided earlier).
  1. Run in Development Mode:
  • Start the development server with the Turbo engine enabled:

bash

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npm run dev --turbo
  • Open http://localhost:3000 in your browser.
  • Refresh the page or navigate away and return to it.
    3.Expected behavior:* The fetch request is triggered every time, and the random number updates.
  1. Run in Production Mode:
  • Build the application:

bash

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npm run build
  • Start the production server:

bash

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npm run start
  • Open http://localhost:3000 in your browser.
  • Refresh the page or navigate away and return to it.
    5.Expected behavior:* Without dynamic: 'force-dynamic', the fetched data does not update.

Is it normal for Next.js to behave differently in development and production modes regarding fetch requests and caching, requiring explicit configurations like dynamic: 'force-dynamic' to align the behaviors?

2

If you’re having trouble deploying an Express app, this guide can help.

You can also ask v0 for suggestions tailored to your own project setup.

3

Development mode does work differently from a deployed app since it serves a different purpose.

Dev mode needs to be fast and always update with the latest changes, so caching and optimizing aren’t really something you would want in that environment. A deployed site, on the other hand, does benefit from optimization strategies to speed up site loading and reduce resource usage.

Next attempts to make this easy with some default settings that suit most apps. But there are ways configure different behavior, as you did with force-dynamic. You can find more info about the default settings in the Next.js docs: Building Your Application: Caching | Next.js

I hope that helps! Please let me know if you have any other questions

4

I understand the goal, but I can say that I personally prefer to have the same local experience as I do in the production environment.

The gain of a few milliseconds compared to a possible cache that could affect the experience and even pose risks in some scenarios seems a bit risky to me! In the end, every application focuses on serving the client, and I see this as more important than serving the developer.

Nowadays, with so much technology, I feel that people rely on documentation to find solutions when problems occur, usually locally. For a beginner, I think it’s difficult to notice these kinds of issues.

Again, it’s just my opinion, and thank you for the clarification!

5

Thanks for coming back to elaborate. I agree, it’s very important to make sure everything works as expected before deploying to the production environment. Preview deployments are perfect for that. But I hear what you’re saying about the local environment and the potential for beginner confusion.

I appreciate you sharing your experience. This kind of feedback helps us make deployment even easier with Vercel :smile: