An Introduction to Caching

Ayush Srivastava's avatar

Ayush Srivastava

Understanding Caching: Client and Server Side

Caching is a critical aspect of modern web development. It allows us to store copies of data in locations closer to the user, reducing the amount of time it takes to serve this data. This article will cover both client-side and server-side caching, explaining what they are, why they are useful, and how to implement them.

What is Caching?

Caching is a technique used to store copies of data in temporary storage locations (cache) to speed up subsequent data retrieval. The main purpose of caching is to optimize the performance of your application by storing frequently accessed data in a place that can be accessed quickly.

Client-Side Caching

Client-side caching refers to the practice of storing data on the user's device, such as in their browser or local storage. This reduces the number of requests that need to be sent to the server, thereby improving the performance of the application and reducing server load.

One popular method for implementing client-side caching is using a Content Delivery Network (CDN). A CDN stores copies of your site's assets (like CSS, JavaScript, images) in various locations around the world. When a user visits your site, the CDN delivers these assets from the location closest to the user, reducing latency and improving loading time.

Another method for client-side caching is using the browser's local storage. Local storage allows you to save data in the user's browser, which can then be retrieved later, even if the user navigates away from your site and comes back later. This can be particularly useful for storing user preferences or other data that needs to persist across sessions.

Cookies are another form of client-side caching that play a crucial role in web development. Unlike browser cache and local storage, which store static resources, cookies are designed to store small pieces of data that represent stateful information for the user or the website. Here's more detailed information about cookies in the context of client-side caching:

  • Purpose of Cookies: Cookies are primarily used to remember stateful information between requests, such as user login status, shopping cart contents, or personalization settings. They can also be used for tracking user behavior across websites, although this practice is subject to privacy regulations.

  • Cookie Cache: Cookie cache refers to the mechanism by which a web browser stores cookies. These cookies are included in HTTP requests sent to the server, allowing the server to identify the client and maintain session state. Cookies are typically used for maintaining user sessions, tracking user activity, and implementing personalized experiences [0].

  • Expiration Time: Cookies have an expiration time, after which they are deleted. This ensures that sensitive data doesn't remain on the user's device indefinitely. The expiration time can be set by the server when creating the cookie [0].

  • Size Limitations: Cookies are limited in size; most browsers allow a maximum of 4KB per cookie. If more data needs to be stored, techniques like splitting the data across multiple cookies or using local storage are employed [0].

  • Security Considerations: Cookies can be made secure by setting the HttpOnly flag, which prevents them from being accessed via client-side scripts, mitigating the risk of cross-site scripting attacks. Additionally, cookies can be encrypted to protect against unauthorized access [1].

  • Performance Impact: Since cookies are sent with every HTTP request, including those for static resources, excessive use of cookies can negatively impact performance. Therefore, it's important to use cookies judiciously and avoid storing unnecessary data in them [3].

Here is a simple example of how to set a cookie in JavaScript:

document.cookie =
  'username=John Doe; expires=Thu,  18 Dec  2024  12:00:00 UTC; path=/';

And here's how to read a cookie:

function getCookie(name) {
  const nameEQ = name + '=';
  const ca = document.cookie.split(';');
  for (let i = 0; i < ca.length; i++) {
    let c = ca[i];
    while (c.charAt(0) == ' ') c = c.substring(1, c.length);
    if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length, c.length);
  return null;

Server-Side Caching

Server-side caching involves storing data on the server. This can significantly reduce the load on your database and improve the performance of your application, especially for read-heavy workloads. One common method for server-side caching is using a caching server like Redis. Redis is an in-memory data structure store that can be used as a cache, database, and message broker. It supports various types of data structures and offers a wide range of functionalities.

Here's an example of how to implement server-side caching with Redis in a node js application:

First, you need to install the redis pacakge via npm:

npm install redis
import express from 'express';
import redis from 'redis';
import { promisify } from 'util';
// Create a Redis client
const redisClient = redis.createClient({ host: 'localhost', port: 6379 });
// Promisify Redis commands for easier use
const redisGetAsync = promisify(redisClient.get).bind(redisClient);
const redisSetAsync = promisify(redisClient.set).bind(redisClient);
// Simulate an API call that returns data
function fetchData() {
  return new Promise((resolve) => {
    setTimeout(() => resolve('Hello, World!'), 2000); // Simulating a delay
const app = express();
app.get('/data', async (req, res) => {
  try {
    // Attempt to get data from Redis cache
    let cachedData = await redisGetAsync('myDataKey');
    if (cachedData) {
      // If data is found in cache, send it to the client
    } else {
      // If not in cache, fetch the data from the API (or another source)
      const data = await fetchData();
      // Store the data in Redis cache with a TTL (time-to-live) of  1 hour
      await redisSetAsync('myDataKey', data, 'EX', 3600);
      // Send the data to the client
  } catch (error) {
    res.status(500).send('An error occurred while processing your request.');
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);

In this example, the fetchData function simulates an API call that returns some data. The /data endpoint first tries to retrieve the data from the Redis cache. If it finds the data, it sends it to the client. If not, it fetches the data, stores it in the cache with a TTL of 1 hour, and then sends it to the client.

Remember to replace 'localhost' and 6379 with the actual host and port where your Redis server is running. Also, ensure that your Redis server is properly configured and running before executing this code.

Server-Side Caching with HTTP Cache Headers

Server-side caching can be further optimized by utilizing HTTP cache headers to instruct browsers and intermediaries how to cache the responses. These headers provide guidance on caching policies, such as how long to store a response, when to revalidate it with the server, and whether the response is cacheable at all.

Cache-Control Header

The Cache-Control header is essential for defining the caching behavior of server responses. It can specify directives such as:

  • public: Indicates that the response may be cached by any cache.
  • private: Specifies that the response is intended only for a single user and must not be stored by a shared cache.
  • max-age: Defines the maximum amount of time in seconds that a resource is considered fresh.
  • no-cache: Instructs caches to validate the response with the server before reusing it, ensuring it is up-to-date.
  • no-store: Completely prevents caching of the response.

For example, to set a response to be publicly cacheable for 1 hour, you could use:

Cache-Control: public, max-age=3600

ETag Header

The ETag header provides a mechanism for validating cached resources. When a browser requests a resource that is cached, it can include the ETag value in an If-None-Match header. The server then compares the current ETag with the one sent by the browser. If they match, the server responds with a 304 Not Modified, indicating that the cached version is still valid. If they differ, the server sends the updated resource along with the new ETag. Example of setting an ETag header in a response:

ETag: "686897696a7c876b7e"

Last-Modified Header

Similar to ETag, the Last-Modified header contains a timestamp indicating when the resource was last modified. Browsers can use this information in conjunction with If-Modified-Since headers to conditionally request resources based on their modification dates. Example of setting a Last-Modified header in a response:

Last-Modified: Wed,  11 Feb  2024  07:28:00 GMT

By combining these headers with server-side caching strategies, you can create a robust caching system that balances the benefits of reduced server load with the need for up-to-date content. It's crucial to configure these headers appropriately for each type of resource and to regularly review caching policies to ensure optimal performance.


Caching is a powerful tool for improving the performance of your web applications. Whether you choose to implement client-side or server-side caching depends on your specific needs and the nature of your application. By understanding the principles behind caching and knowing how to implement it effectively, you can significantly improve the speed and efficiency of your applications.

Read more about caching Here