ETag
An ETag (Entity Tag) is a web cache validation mechanism used in HTTP protocols to determine if a resource has changed since the last time it was fetched. ETags are part of the HTTP header and provide a more sophisticated way of controlling caching behavior than the Last-Modified header alone.
Functionality
ETags are unique identifiers assigned by a server to a specific version of a resource. When a server responds to an HTTP GET request, it can include an ETag in the response header. Here’s how the process works:
- Request: A client sends a request to fetch a resource.
- Response: The server responds with the resource and includes an ETag header.
- Subsequent Request: The next time the client requests the same resource, it includes an If-None-Match header with the ETag received previously.
- Server Check: If the ETag matches the current ETag for the resource, the server sends back a 304 Not Modified response, indicating the client should use the cached version. If the ETag does not match, the server sends the updated resource with a new ETag.
History
ETags were introduced as part of HTTP/1.1. They were designed to address some limitations of the Last-Modified date approach:
- Last-Modified can only have a one-second resolution, which might not be precise enough for frequently updated resources.
- Last-Modified might not reflect changes if the content changes without altering the file's modification time.
ETags provide a finer-grained control over cache validation, allowing servers to generate unique identifiers for each version of a resource, regardless of time-based changes.
Types of ETags
- Strong ETags: These ensure that the resource has not changed at all. A strong ETag must change if the resource changes in any way.
- Weak ETags: These are less strict and can be used when changes in the resource are not semantically significant (e.g., formatting changes). They start with "W/" in the ETag header.
Use Cases
- Caching: ETags are used extensively in web caching systems to reduce bandwidth usage and improve response times by validating cached copies of resources.
- Conditional Requests: They allow for conditional requests where the client can ask for a resource only if it has changed since the last request.
- Content Delivery Networks (CDNs): ETags help CDNs to efficiently manage and serve content by determining when content needs to be refreshed.
Limitations and Considerations
- ETags can increase the load on servers, as they need to generate and compare ETags for each request.
- They can also leak information about the server's internal state or implementation if not handled carefully.
- ETags can be less effective in environments where content is dynamically generated because the ETag might change even if the content does not (e.g., due to dynamic components like time stamps).
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