Understanding Angular Hydration: How It Works and Its Benefits

Angular Hydration is a crucial process in server-side rendering (SSR) that enhances the performance and interactivity of your Angular applications. You can optimize your Angular applications for better performance and user experience by understanding how hydration works and its benefits. In this guide, we'll explore Angular Hydration, how it fits into the SSR process, the different types of rehydration, alternatives to rehydration, and the advantages it brings.

Introduction to Angular Hydration

Hydration in Angular refers to the process where the client-side, JavaScript code takes over the server-rendered HTML markup, making the page interactive. When using Angular Universal for SSR, the server initially renders the HTML and sends it to the client. Hydration is the step where Angular bootstraps on the client side, enabling dynamic behavior and interactivity.

How Hydration Works in Angular

Step-by-Step Process

1. Server-Side Rendering (SSR):

• The server receives a request for a page.

• Angular Universal renders the requested page on the server and generates the HTML.

• The server sends the fully rendered HTML to the client's browser.

2. Initial HTML Load:

• The browser receives the HTML and displays the content immediately.

• The user sees a fully rendered page without waiting for client-side JavaScript to load and execute.

3. Client-Side Hydration:

• Angular bootstraps the application on the client side.

• Angular compares the server-rendered HTML with the client-side application state.

• Event listeners and interactive behaviors are attached, making the page dynamic and fully interactive.

Example Scenario

Let's consider a simple Angular application with a Home component and an About component. Here's how hydration works:

• Home Page (/):

  • Server renders the HomeComponent and sends the HTML.

  • Browser displays the HomeComponent immediately.

  • Angular bootstraps on the client side, enabling interactivity such as button clicks and form submissions.

• About Page (/about):

  • Server renders the AboutComponent and sends the HTML.

  • Browser displays the AboutComponent immediately.

  • Angular bootstraps on the client side, enabling interactivity.

Benefits of Angular Hydration

• Improved Performance:

  Hydration allows the browser to display content immediately, reducing the time users wait to see the rendered page. This leads to faster-perceived load times and better performance, especially for users on slower networks or devices.

• Enhanced SEO:

  Search engines can easily crawl and index the server-rendered, HTML elements, improving your application's SEO. Hydration ensures that the interactive elements are added after the initial, HTML content is loaded, preserving SEO benefits while adding dynamic behavior.

• Better User Experience:

  Users experience a seamless transition from static content to interaction with an interactive application. The initial HTML load provides immediate and complete content visibility, while data hydration enables interactivity without noticeable delays.

Types of Rehydration

• Partial Rehydration

  Partial rehydration enables developers put interactive features on specific parts of the site while keeping the rest static. This approach reduces the amount of JavaScript needed to hydrate the page, improving load times and performance.

• Progressive Rehydration

  Progressive rehydration initializes individual pieces of functionality of a server-rendered application over time, rather than all at once. This method allows critical parts of the page to become interactive first, while less critical parts are hydrated later.

• Trisomorphic Rendering

  Trisomorphic rendering uses streaming server-side rendering for initial/non-JS navigations and then uses a service worker framework to take on the rendering of HTML for navigations after installation. This approach combines the benefits of SSR, client-side rendering, and service workers to provide a highly performant and interactive application.

Setting Up Angular Hydration

Step 1: Create an Angular Application

If you don’t already have created an Angular application, create one using the Angular CLI:

ng new angular-hydration-app
cd angular-hydration-app

Step 2: Add Angular Universal

Add Angular Universal to your project by running the following command:

ng add @nguniversal/express-engine

Step 3: Configure Server-Side Rendering

Ensure your server.ts file is correctly set up to handle SSR. This configuration is usually done automatically when you add Angular Universal.

Step 4: Create Components and Set Up Routing

Generate components and configure routing as needed. For example:

ng generate component home --module=app.module
ng generate component about --module=app.module

Update your app-routing.module.ts:

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { HomeComponent } from './home/home.component';
import { AboutComponent } from './about/about.component';

const routes: Routes = [
  { path: '', component: HomeComponent },
  { path: 'about', component: AboutComponent },
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

Update your app.component.html:

<nav>
  <a routerLink="/">Home</a>
  <a routerLink="/about">About</a>
</nav>
<router-outlet></router-outlet>

Step 5: Build and Serve the Application

Build the application for SSR:

npm run build:ssr

Serve the application code:

npm run serve:ssr

Navigate to http://localhost:4000 to see your server-rendered and hydrated Angular application.

Real-World Example

E-commerce Website Scenario

Consider an e-commerce website built with Angular. The website has multiple product pages, a shopping cart element, and a checkout process.

Without Hydration

• The server sends static HTML to the client.

• Users wait for client-side JavaScript to load and execute before the page becomes interactive.

• Slow initial load times on slower networks or devices.

With Hydration

• The server pre-renders HTML and sends it to the client.

• Users see content immediately as the HTML loads.

• Angular bootstraps on the client side, enabling interactivity like adding products to the cart and proceeding to checkout.

• Faster initial load times and improved user experience.

Benefits of This Scenario

• Faster Load Time: Users can see and interact with content immediately.

• Better SEO: Pre-rendered HTML is easily crawled and indexed by search engines.

• Enhanced User Experience: Smooth transition from static content to an interactive application.

Alternatives to Rehydration

Resumability

Resumability is a technique that avoids rehydration and its overhead. Instead of redoing the work that the server already did, resumability creates the necessary JavaScript lazily in response to user events. This approach can further improve performance by minimizing the amount of JavaScript executed during the initial load.

Conclusion

In this tutorial, we've explored Angular Hydration, its role in the server-side rendering process, and the benefits it brings to Angular applications. We also discussed the different types of rehydration, such as partial rehydration, progressive rehydration, and trisomorphic rendering, as well as alternatives like resumability. By pre-rendering HTML on the server and bootstrapping Angular on the client side, hydration improves performance, SEO, and user experience.

We walked through the steps to set up Angular Hydration and provided a real-world example to illustrate its advantages. By implementing Angular Hydration, you can optimize your Angular applications for better performance and user satisfaction. Feel free to experiment with hydration in your projects and see the benefits it brings firsthand.