ETag (Entity Tag)

An ETag (entity tag) is an opaque identifier the server attaches to a specific version of a resource, usually a hash of its content or a version number. When the resource changes, the ETag changes.

Why it matters

Once a cached copy goes stale, the browser can ask the server whether it is still valid instead of downloading it again. It sends the stored tag in an If-None-Match header. If the tag still matches, the server replies 304 Not Modified with no body, saving the transfer but not the round trip.

ETags can be strong ("abc123", byte-identical) or weak (W/"abc123", semantically equivalent). They also work for conditional writes with If-Match, preventing lost updates in APIs.

Example

GET /styles.css
200 OK
ETag: "5d8c72a5"

GET /styles.css
If-None-Match: "5d8c72a5"
304 Not Modified

Common mistakes

• Multiple servers generating ETags from file inode and timestamps, so the same file gets different tags behind a load balancer and revalidation always fails.
• Assuming an ETag replaces freshness headers. Without max-age, the browser may revalidate on every load.
• Compression layers rewriting content but leaving the tag identical, or changing it inconsistently.

Related

• HTTP caching
• Cache-Control
• Browser cache
• Load balancer

यह पेज सामान्य जानकारी है, जिसे हमारी संपादकीय टीम ने लिखा है और दस्तावेज़ीकरण बदलने पर अपडेट किया जाता है। सर्वर और प्रदाता अलग-अलग होते हैं, इसलिए बदलाव पहले सुरक्षित तरीके से जाँचें। देखें: संपादकीय नीति और अस्वीकरण.