ETag

An ETag (entity tag) is an identifier a server assigns to a specific version of a resource, often a hash of its contents. Clients send it back via the If-None-Match header so the server can reply 304 Not Modified when nothing has changed, saving bandwidth. It also helps detect mid-update conflicts.

The everyday use of ETags is conditional requests. The first response includes an ETag header; the browser caches it. On the next request it sends If-None-Match: "<etag>". If the resource is unchanged, the server returns an empty 304 Not Modified, and the browser reuses its cached copy — no body transferred. This is a major bandwidth saver for CDNs and APIs alike.

ETags also enable optimistic concurrency. A client can send If-Match: "<etag>" on an update so the write only succeeds if the resource hasn't changed since it was read. If someone else modified it first, the server returns 412 Precondition Failed instead of silently clobbering their change.

In object storage, the ETag of an uploaded object is frequently a content hash, which lets you verify integrity after upload — a useful check at the end of a multipart transfer.

Related terms

Cache-Control

Cache-Control is an HTTP header that tells browsers and CDNs how to cache a response — for how long, and whether they may store it at all. Directives like max-age, public, private, no-cache, and immutable set freshness, control revalidation, and decide how aggressively an asset is reused.

CDN

A CDN (content delivery network) is a globally distributed set of servers that cache copies of your files close to users. When someone requests an asset, the nearest edge location serves it instead of your origin, cutting latency and offloading traffic. CDNs speed up images, video, downloads, and static sites worldwide.

MIME type

A MIME type (media type) is a standardized label like image/png or application/json that tells clients what kind of data a file contains. Sent in the Content-Type header, it tells browsers whether to render, download, or parse a response, and lets servers validate uploads by format.

Multipart upload

Multipart upload splits a large file into smaller parts that are uploaded independently — often in parallel — then reassembled into one object on the server. It improves throughput, survives flaky networks by retrying only failed parts, and is the standard way to move large files into object storage reliably.

How files.link fits in

Validators like ETag are part of why CDN-served files on files.link revalidate cheaply instead of re-downloading unchanged assets. affordable CDN delivery.

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