The decision of whether to serve files from the memory cache or disk cache is primarily determined by the web browser's caching mechanism and the caching headers set by the web server. Let's break down how this decision is made:

Caching Headers: When a web server responds to a request for a file (e.g., an image, stylesheet, JavaScript file), it includes HTTP headers that instruct the browser on how to cache and handle the file. The most important headers are:

• Cache-Control: This header specifies caching directives for a particular resource. It can include directives like max-age, no-cache, public, private, etc. These directives guide the browser on how long to cache the resource and where to store it.

• Expires: This header indicates an absolute expiration date and time for the resource. If the current time is earlier than the specified expiration time, the browser will use the cached resource. If it's later, the browser will fetch a fresh copy.

• ETag and Last-Modified: These headers help the browser validate whether a cached resource is still valid. The server sends an ETag (entity tag) or Last-Modified timestamp with the resource. The browser can send these values in subsequent requests to check if the resource has changed on the server since it was cached.

Memory Cache vs. Disk Cache:

• Memory Cache: Files that are frequently used or expected to be used again in the near future may be stored in the memory cache. This cache is usually faster to access than disk storage, resulting in quicker loading times for frequently accessed resources.

• Disk Cache: Files that are less frequently used or are larger in size may be stored in the disk cache. Disk caching helps conserve memory and allows for a larger cache storage space, which can be particularly useful for larger files.

Cache Hierarchy: Browsers often use a hierarchical caching approach. When a resource is requested, the browser first checks the memory cache. If the resource is found there and is still valid according to caching headers, it is served from memory. If not, the browser looks in the disk cache. If the resource is found in the disk cache and is still valid, it is served from there. If it's not found in either cache, a fresh request is made to the server.

In summary, the decision of whether to serve from memory or disk cache depends on the caching headers sent by the server, the browser's caching algorithm, the frequency of resource access, and the available cache storage. Properly configured caching headers can significantly impact the efficiency of caching and improve the overall performance of your website.