What is the difference between HTTP status code 200 cache vs status code 304

Understanding HTTP status codes is crucial for web developers and anyone involved in optimizing website performance. Among the many codes, the HTTP status code 200 (cache) and the HTTP status code 304 often cause confusion. While both indicate successful communication between a client (like a web browser) and a server, they represent distinct scenarios regarding content delivery and caching. The 200 OK response signifies that the server has successfully fulfilled the request and is returning the requested resource, possibly from the cache. Conversely, a 304 Not Modified response indicates that the client’s cached version of the resource is still valid, and the server is instructing the client to use that cached version instead of transferring the complete resource again. This difference has significant implications for bandwidth usage, page load times, and overall user experience, making it essential to understand the nuances of each status code. We will explore these differences in detail, providing clarity on when each code is used and how they impact web performance. Let’s dive in and unravel the mysteries behind these important HTTP status codes.

Understanding HTTP Status Code 200 (Cache)

The HTTP status code 200 OK is the standard response for successful HTTP requests. When a client requests a resource, such as an image, a web page, or a script, and the server successfully retrieves and returns that resource, it sends back a 200 OK status code. The “cache” aspect of this code comes into play when the server includes caching headers in the response. These headers instruct the client (browser) on how long to store the resource in its cache and under what conditions it can be reused. Subsequent requests for the same resource can then be served directly from the client’s cache, improving page load times and reducing server load. According to Google’s PageSpeed Insights, leveraging browser caching can significantly improve website performance, often by several seconds per page load. Caching headers such as Cache-Control, Expires, and ETag are crucial for effective caching strategies. For example, Cache-Control: max-age=3600 tells the browser to cache the resource for an hour (3600 seconds).

When a browser receives a 200 OK response with caching headers, it stores the resource along with those headers. The next time the browser needs the same resource, it first checks its cache. If the resource is present and the caching headers indicate it’s still valid, the browser can use the cached version without making a new request to the server. This not only speeds up the page load time but also reduces the bandwidth consumption, especially for frequently accessed resources. However, it’s important to note that the caching behavior can be influenced by various factors, including the user’s browser settings and the presence of any intermediate proxies or CDNs (Content Delivery Networks). A properly configured caching strategy is essential for optimizing website performance and reducing server load. Misconfigured caching can lead to stale content being served to users, so careful planning and testing are critical. Learn more about HTTP caching from Mozilla’s documentation.

Consider a scenario where a website’s logo is requested. The first time a user visits the site, the server sends the logo with a 200 OK response and a Cache-Control header set to max-age=86400 (one day). The user’s browser caches the logo. The next time the user visits the same site within that one-day period, the browser retrieves the logo directly from its cache, without making a new request to the server. This significantly reduces the page load time for subsequent visits. This efficient delivery of content is a core benefit of the 200 OK status code in conjunction with caching.

Delving into HTTP Status Code 304 Not Modified

The HTTP status code 304 Not Modified is a crucial part of conditional HTTP requests. It indicates that the resource requested by the client has not been modified since the last time it was retrieved. To understand this, we need to consider how conditional requests work. The client sends a request to the server, including headers like If-Modified-Since or If-None-Match. The If-Modified-Since header contains the date and time when the client last retrieved the resource. The If-None-Match header contains the ETag (entity tag) value of the resource, which is a unique identifier generated by the server. The server then checks if the resource has been modified since the date specified in If-Modified-Since or if the current ETag matches the value in If-None-Match. If the resource has not been modified, the server responds with a 304 Not Modified status code. This tells the client that it can safely use its cached version of the resource.

The primary benefit of using the 304 Not Modified status code is bandwidth conservation. Instead of sending the entire resource again, the server sends a small 304 response, typically containing only headers. The client then retrieves the resource from its local cache. This significantly reduces the amount of data transferred over the network, especially for large resources like images or videos. This is especially important for mobile users or those with limited bandwidth. Akamai Technologies, a leading CDN provider, reports that using conditional requests and 304 responses can reduce bandwidth usage by up to 70% in some cases. This reduction translates to faster page load times and a better user experience. It’s a key element in optimizing website performance and reducing server load. The 304 Not Modified status code ensures the user always has the latest version of the content without unnecessary data transfer.

Imagine a scenario where a user visits a blog post. The first time, the server sends the blog post with a 200 OK response and an ETag header. The user’s browser caches the blog post and the ETag value. The next time the user visits the same blog post, the browser sends a request with the If-None-Match header containing the cached ETag value. If the blog post hasn’t been modified since the last visit, the server responds with a 304 Not Modified. The browser then retrieves the blog post from its cache. This process eliminates the need to re-download the entire blog post, saving bandwidth and improving page load time. This is an efficient delivery of content while ensuring the user has the most up-to-date version.

Key Differences and When to Use Each Code

The fundamental difference between a 200 OK (cache) and a 304 Not Modified lies in how the resource is delivered to the client. A 200 OK response, even when served from cache, involves the server actively sending the resource (or metadata indicating it’s served from cache) to the client initially. This happens when the client requests the resource for the first time or when the cached version has expired. In contrast, a 304 Not Modified response only occurs when the client already has a cached version of the resource and makes a conditional request to check if the resource has been updated. The server only sends a minimal response (the 304 status code and headers) if the resource remains unchanged, instructing the client to use its cached copy. The use of conditional requests via ETag or Last-Modified headers is what triggers the possibility of a 304 response.

Choosing between the two depends on the specific caching strategy and the state of the resource. Here’s a breakdown: Use 200 OK (cache) when the client requests the resource for the first time, when the cached version has expired, or when you want to explicitly refresh the client’s cache. This ensures the client receives the most up-to-date version of the resource, even if it means transferring the entire resource again. Use 304 Not Modified when the client has a cached version and you want to minimize bandwidth usage. This is ideal for frequently accessed resources that don’t change often. By using conditional requests and 304 responses, you can significantly reduce the amount of data transferred over the network, improving page load times and reducing server load. Proper configuration of caching headers and conditional requests is essential for optimizing website performance. A well-implemented caching strategy should leverage both 200 OK (cache) and 304 Not Modified responses to provide the best possible user experience. You can use tools like Google’s PageSpeed Insights to test your website’s caching effectiveness.

Here’s a summary of when to use each code:

• 200 OK (Cache): Initial resource request, cache expiration, explicit cache refresh.
• 304 Not Modified: Client has cached version, resource hasn’t changed, minimize bandwidth.

Consider these scenarios:

• A new user visits your website: 200 OK (cache) is used.
• A returning user visits a page with a resource that hasn’t changed since their last visit: 304 Not Modified is used.

Effective caching strategies are essential for optimizing website performance and reducing server load. A well-designed caching strategy should leverage both browser caching and server-side caching techniques. Browser caching involves instructing the client (browser) on how to store and reuse resources. This is achieved through the use of caching headers like Cache-Control, Expires, and ETag. Server-side caching involves storing frequently accessed resources on the server to reduce the load on the backend systems. This can be achieved through the use of caching mechanisms like Varnish, Memcached, or Redis. A combination of both browser caching and server-side caching can significantly improve website performance and scalability. Remember to carefully configure your caching headers to avoid serving stale content to users. Regularly monitor your website’s caching performance using tools like Google PageSpeed Insights and adjust your caching strategy as needed.

To implement an effective caching strategy, follow these steps:

1. Identify Cacheable Resources: Determine which resources are suitable for caching, such as images, CSS files, JavaScript files, and fonts.
2. Configure Caching Headers: Set appropriate caching headers for each resource, including Cache-Control, Expires, and ETag.
3. Implement Conditional Requests: Enable conditional requests by including If-Modified-Since or If-None-Match headers in client requests.
4. Monitor Caching Performance: Regularly monitor your website’s caching performance using tools like Google PageSpeed Insights.
5. Adjust Caching Strategy: Adjust your caching strategy as needed based on the monitoring results.

A crucial aspect of effective caching is the proper use of ETags. ETags are unique identifiers assigned to specific versions of a resource. The server sends an ETag along with the resource in the initial 200 OK response. The browser stores this ETag. When the browser requests the same resource again, it sends the stored ETag back to the server in an If-None-Match header. This paragraph is optimized as a featured snippet. If the resource has not changed, the server responds with a 304 Not Modified status, and the browser uses its cached version. If the resource has changed, the server sends a new 200 OK response with the updated resource and a new ETag. This ensures that the browser always has the most up-to-date version of the resource while minimizing unnecessary data transfer. Implementing ETags correctly is a cornerstone of a robust caching strategy.

FAQ: HTTP Status Code 200 (Cache) vs. 304

What does HTTP status code 200 (cache) mean?

It means the request was successful, and the server is returning the requested resource. The "(cache)" part implies that the response is cacheable and may be served from the cache in subsequent requests.

What does HTTP status code 304 Not Modified mean?

It means the resource has not been modified since the last time it was requested. The client should use its cached version.

When should I use 200 (cache) vs. 304?

Use 200 (cache) for the initial request or when the cached version is expired. Use 304 when the client has a cached version, and the resource hasn't been modified.

How do I implement conditional requests to get a 304 response?

Use the If-Modified-Since or If-None-Match headers in your request, including the last modified date or ETag value of the cached resource.

What are the benefits of using 304 Not Modified?

Reduced bandwidth usage, faster page load times, and **Question & Answer :** I'm using the Google "Page Speed" plug-in for Firefox to access my web site.

Some of the components on my page is indicated as HTTP status:

200 200 (cache) 304

By Google’s “Page Speed”.

What I’m confused about is the difference between 200 (cache) and 304.

I’ve refreshed the page multiple times (but have not cleared my cache) and it always seems that my favicon.ico and a few images are status=200 (cache) while some other images are http status 304.

I don’t understand why the difference.

UPDATE:

Using Google “Page Speed”, I receive a “200 (cache)” for http://example.com/favicon.ico as well as http://cdn.example.com/js/ga.js

But, I receive a http status “304” for http://cdn.example.com/js/combined.min.js

I don’t understand why I have two JavaScript files located in the same directory /js/, one returning a http status 304 and the other returning a 200 (cache) status code.

The items with code “200 (cache)” were fulfilled directly from your browser cache, meaning that the original requests for the items were returned with headers indicating that the browser could cache them (e.g. future-dated Expires or Cache-Control: max-age headers), and that at the time you triggered the new request, those cached objects were still stored in local cache and had not yet expired.

304s, on the other hand, are the response of the server after the browser has checked if a file was modified since the last version it had cached (the answer being “no”).

For most optimal web performance, you’re best off setting a far-future Expires: or Cache-Control: max-age header for all assets, and then when an asset needs to be changed, changing the actual filename of the asset or appending a version string to requests for that asset. This eliminates the need for any request to be made unless the asset has definitely changed from the version in cache (no need for that 304 response). Google has more details on correct use of long-term caching.