Caching Strategies for Server-Side Rendered Apps

Caching is a critical technique for improving the performance of Server-Side Rendered (SSR) applications. By reducing redundant calculations and fetching, caching can make your application significantly faster, especially when handling data that doesn’t change often. In SSR applications, caching plays an essential role both on the server-side and in the client-side rendering process.

This guide explores various caching strategies to optimize performance in SSR applications, including HTTP caching, data caching, static caching, and edge caching.

1. Why Caching Is Important in SSR Apps

In Server-Side Rendered applications, the server generates the HTML on every request. Without caching, this can lead to high latency, unnecessary server processing, and degraded performance.

By introducing caching, we can:

Reduce load times for users by serving cached content instead of generating new content for every request.
Minimize server load by avoiding repeated data fetching or HTML rendering.
Improve scalability and reduce backend resource usage.

2. Types of Caching in SSR Apps

There are several types of caching strategies in SSR applications, which can be applied at different levels:

HTTP Caching
Static Caching
Edge Caching
Data Caching

Let’s take a closer look at these.

3. HTTP Caching

HTTP caching involves caching the entire response (HTML, CSS, JS, images) at the HTTP layer using HTTP headers like Cache-Control, ETag, and Last-Modified. This can be used to cache entire pages or resources at the server or client level.

Key HTTP Caching Headers:

Cache-Control: Defines caching behavior (e.g., max-age, s-maxage, public, private).
ETag: A unique identifier for the content that can be used to determine whether content has changed.
Last-Modified: Specifies the last modification date of a resource. The server can use it to check whether the client’s cached version is still valid.

How to Implement HTTP Caching:

On the server-side (e.g., Node.js with Express), you can configure HTTP caching headers for SSR content.

// In a Node.js (Express) SSR app
app.get('*', (req, res) => {
  res.setHeader('Cache-Control', 'public, max-age=60'); // Cache for 1 minute
  res.render('index');
});

For dynamic SSR pages, you may want to set shorter cache durations or use Cache-Control: no-cache to ensure fresh content for every request.
For static assets like images, JavaScript, and CSS, set longer cache expiration times, as these assets change infrequently.

4. Static Caching

Static Caching refers to caching the output of pages or assets that don’t change often (such as static pages or media files). These pages are rendered once and cached for subsequent requests.

For SSR, static pages can be cached at the server or CDN level to avoid regenerating the same content on every request.

Strategies for Static Caching:

Static Site Generation (SSG): Use a static site generation method (e.g., Next.js’ getStaticProps) to pre-render pages and serve them as static HTML. Static pages can be served via a CDN for lightning-fast load times.
Static Asset Caching: Ensure that static assets like images, stylesheets, and scripts are cached for long periods.

Example (Next.js Static Page Caching):

// In Next.js, using getStaticProps for static content
export async function getStaticProps() {
  return {
    props: {
      data: await fetchData(),
    },
    revalidate: 3600, // Rebuild the page after 1 hour
  };
}

CDN Caching: Serve static assets like images, fonts, and JavaScript via a CDN with a long cache lifetime (e.g., Cache-Control: public, max-age=31536000). This ensures that content is served as quickly as possible.

5. Edge Caching

Edge Caching involves caching content as close to the user as possible by leveraging CDNs and edge servers. With edge caching, you can store SSR pages or assets at multiple locations around the world, reducing latency by serving cached content from a location that is geographically closer to the user.

CDNs (Content Delivery Networks): Most CDN providers, like Cloudflare, AWS CloudFront, or Vercel, support edge caching for SSR apps. You can cache both static content and fully-rendered SSR pages at the edge.
Cache Invalidation: When content changes, you need to ensure that caches are invalidated or refreshed appropriately.

Example of Edge Caching in Next.js:

Next.js has built-in support for Incremental Static Regeneration (ISR), which allows static content to be cached on the CDN, but still keep the data fresh after the specified revalidation time.

// Next.js example for ISR
export async function getStaticProps() {
  return {
    props: {
      data: await fetchData(),
    },
    revalidate: 60, // Page will be regenerated after 60 seconds if necessary
  };
}

By leveraging edge caching, you can ensure that SSR pages are delivered extremely fast and efficiently.

6. Data Caching

Data caching involves caching the data that populates your pages, typically via an API or database query. This is crucial for SSR apps, as it avoids unnecessary data fetching on every page request. Caching API responses or database queries can reduce load times and reduce strain on your back-end.

Strategies for Data Caching:

In-memory Caching: Use in-memory caching tools like Redis to store database queries or API responses temporarily.
Server-Side API Caching: Cache API responses either in-memory or using a caching layer (e.g., Redis, Memcached).
Database Query Caching: For frequently accessed data, cache database query results on the server.

Example (Node.js with Redis):

// Node.js example using Redis for data caching
const redis = require('redis');
const client = redis.createClient();

// Middleware to check cache before querying the DB
app.get('/api/data', async (req, res) => {
  const cacheKey = 'data-cache-key';
  client.get(cacheKey, async (err, cachedData) => {
    if (cachedData) {
      return res.json(JSON.parse(cachedData));
    }

    const data = await fetchDataFromDb();  // fetch fresh data from DB
    client.setex(cacheKey, 3600, JSON.stringify(data));  // Cache for 1 hour
    res.json(data);
  });
});

Stale-While-Revalidate: Use strategies like stale-while-revalidate where you return cached data quickly but also fetch new data in the background to update the cache.

7. Cache Invalidation

Cache invalidation is one of the most challenging aspects of caching. As content is cached, you need to ensure that stale data is updated or purged when it becomes outdated. There are a few common strategies:

Time-based expiration: Set a cache TTL (Time-to-Live) to invalidate cached data after a certain period.
Event-based invalidation: Invalidate the cache when certain events occur (e.g., when a user updates their profile or posts new content).
Manual invalidation: Allow developers to manually trigger cache invalidation when needed.

In Next.js, this can be achieved using Incremental Static Regeneration (ISR) or getStaticProps revalidation.

8. Best Practices for Caching in SSR Apps

Use Cache-Control Headers: Leverage HTTP headers like Cache-Control, ETag, and Last-Modified for proper cache management.
Use Content Delivery Networks (CDNs): CDNs can cache static content and SSR pages at the edge for faster delivery.
Cache Data: Cache API responses and database queries on the server-side to avoid redundant operations.
Incremental Static Regeneration (ISR): Use ISR in Next.js to regenerate static pages without rebuilding the entire site.
Use Edge Caching: Cache content at the edge to reduce latency for users in different geographic locations.
Ensure Cache Invalidation: Use effective strategies to invalidate and refresh cached data as needed to prevent serving outdated content.