How browser caching works when Cache-Control, ETag, Last-Modified headers are not sent by the server

If neither the Cache-Control header nor the ETag and Last-Modified headers are sent by the server, the browser's caching behavior will rely entirely on its default heuristics and configuration. Here's a detailed explanation of what happens in such a scenario:

No Cache-Control, ETag, or Last-Modified Headers

When a server response lacks caching headers like Cache-Control, ETag, and Last-Modified, the browser still tries to cache the resource, but its behavior is less predictable and relies on heuristic caching rules.

Heuristic Caching

Heuristic caching involves the browser using its own internal rules to determine how long a resource should be cached. Without explicit headers, the browser might consider factors like:

1. URL Heuristics:

   • URLs ending in common static file extensions (e.g., .css, .js, .png, .jpg) are more likely to be cached than dynamic URLs (e.g., URLs with query parameters).

2. Content-Type:

   • Resources with certain Content-Type headers (e.g., image/*, text/css, text/javascript) are more likely to be cached.

3. Protocol Defaults:

   • Resources served over HTTP/1.0, which lacks robust caching mechanisms compared to HTTP/1.1 or HTTP/2, may be cached for a shorter duration.

Example Scenario

Initial Request (No Cache Headers):

1. Initial Request:

   GET /example.js HTTP/1.1
   Host: example.com
   

2. Initial Response (No Cache Headers):

   HTTP/1.1 200 OK
   Content-Type: application/javascript
   Content-Length: 12345
   

Subsequent Request:

1. Heuristic Caching:

   The browser may cache the resource based on its own heuristics. The duration for which the resource is cached can vary between browsers and their specific implementations.

2. Revalidation:

   Without ETag or Last-Modified headers, the browser lacks a mechanism to validate the freshness of the cached resource. This can lead to:

   • Inconsistent Caching: Different browsers or even different versions of the same browser might handle caching differently.
   • Stale Data: The cached resource may become stale if the server content changes, as the browser has no way to check for updates.

Browser-Specific Heuristics

Browsers implement heuristic caching differently. Here are a few examples:

• Chrome: Chrome may use a heuristic of 10% of the age of the resource if no explicit caching headers are present.

• Firefox: Firefox may use its own algorithm, which could be similar to Chrome's but with variations based on user settings and version.

• Edge/Safari: Edge and Safari might use similar heuristics but can also be influenced by specific browser optimizations and user configurations.