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An entity tag is an opaque validator associated with a selected HTTP representation and commonly supplied in an ETag HTTP header field. It allows requests to refer to a previously observed version or equivalence class without interpreting the representation's contents. The tag's spelling need not reveal how the server generated it.^[ref]

A strong entity tag has no weak marker; a weak entity tag has the prefix W/. Strong comparison succeeds only when both tags are non-weak and their opaque values match character by character. Weak comparison checks the opaque values while ignoring whether either tag is marked weak.^[ref]

For example, "v1" and W/"v1" match weakly but not strongly. Two occurrences of "v1" match under either comparison. Tags "AbC" and "abc" match under neither: field-name case insensitivity does not make the opaque value case-insensitive.

A useful HTTP conditional request illustrates the purpose. Suppose a client retains representation data tagged "v1", then sends GET with If-None-Match: "v1". If the selected current tag matches, this precondition fails and the server returns 304 without transferring that response content. The retained data can then be used according to the applicable caching rules. If-None-Match uses weak comparison, while If-Match uses strong comparison.^[ref]

An entity tag is therefore a protocol validator, not an instruction to lowercase or decode its opaque string. Different representations of one resource can require different validators. Compare the tag for the selected representation under the comparison rule required by the field, rather than assuming that equal resource URLs alone establish equal data.

HTTP cache validation checks whether a stored HTTP response remains suitable for reuse, commonly by sending an HTTP conditional request containing a validator such as an entity tag. It supports reusing stored representation data while checking with a server whether the relevant state has changed.^[ref]

Validation and HTTP freshness answer different questions. A stored response may still be fresh according to its lifetime and age, allowing reuse under the applicable rules without this exchange. A response that is no longer fresh is not thereby known to have different content at the origin: validation can establish that reuse remains appropriate.^[ref]

Suppose a cache stores a successful /manual response with a $12000$-octet body and entity tag "manual-4". Assume that it is otherwise eligible for reuse after successful validation. A later GET sends If-None-Match: "manual-4". If the selected HTTP representation still matches, the origin returns 304 without a representation body. The cache identifies the matching stored response, updates its fields according to the validation rules, and reuses its stored data.^[ref][ref]

In that unchanged branch, the origin avoids retransmitting the $12000$ representation-body octets. The validation request and response still have a cost; the body count alone is not a measurement of total network savings. Nor is 304 an empty replacement document: the useful data are the body already held by the cache.

If the selected representation changed, an otherwise successful validation request can instead receive a full response with its new data. The cache uses that response to satisfy the request and may store it subject to the ordinary storage constraints.^[ref] A validator alone does not make every stored response reusable: selection inputs, applicable cache controls and the result of validation still matter.

The comparison rule also matters. If-None-Match uses weak entity tag comparison, so HTTP cache validation can accept the server's declared equivalence through a weak entity tag. An update requiring an exact precondition through If-Match instead uses strong entity tag comparison.^[ref]

An HTTP conditional request carries one or more preconditions to be evaluated before applying its HTTP request method. The condition can ask whether the selected HTTP representation still has a previously observed entity tag. This makes the request depend on the state the client knows, rather than applying an operation unconditionally.^[ref]

For HTTP cache validation, suppose an earlier retrieval of /color stored the text blue with entity tag "v7". A later GET with If-None-Match: "v7" asks for the representation only if its tag no longer matches. If the selected representation still matches, the condition is false and the server returns status 304 instead of transferring the representation data again. If it has changed, an otherwise successful retrieval can return the new data.^[ref]

A second use prevents overwriting another client's work. Suppose two clients read version "v7". The first makes an update and the server assigns version "v8". The second submits a different PUT with If-Match: "v7". The stale tag does not match, so the server must not apply that update; in this example it returns 412. Checking and applying the condition as part of the operation prevents the second client from silently replacing the first client's changes.^[ref]

The two field names deliberately use different comparisons: If-Match requires strong entity tag comparison, while If-None-Match uses weak comparison and can match a weak entity tag. Failed If-Match need not always yield 412: the specification permits a success response when the requested change appears already applied. Authentication, redirects and other normal request checks also precede precondition evaluation. An HTTP conditional request therefore combines method semantics, a particular condition and the specified evaluation order.

HTTP content negotiation provides mechanisms for choosing among available HTTP representations, for example different formats, languages or content encodings. One resource can have several representations without requiring that its identifier change for every preference.^[ref]

For a concrete example, stipulate that /color offers both an HTML representation and plain text. A client sends Accept: text/html;q=0.8, text/plain;q=0.5. The weights express a preference for HTML. If this server honors that ranking and both choices are available, it selects HTML and declares Content-Type: text/html. The Accept request field describes preferences; the Content-Type response field identifies the selected media type.^[ref]

The quality weight q lies between zero and one, defaults to one when omitted, and uses zero for an unacceptable choice. These weights are preference values, not probabilities. The example stipulates a server policy; it is not a claim that all servers must select by a universal maximum-weight algorithm.^[ref]

In HTTP proactive negotiation, the server makes its choice using information from the request. A response field Vary: Accept indicates that the request's Accept value might influence selection. This matters when reusing a stored response: the resource identifier alone need not describe every relevant selection input.^[ref]

In HTTP reactive negotiation, the client selects after receiving an initial response containing alternatives, potentially making a further request. This can expose the choices directly but can require an additional exchange. Neither mechanism guarantees that every requested representation exists, nor that the server knows the user's eventual purpose.^[ref]

HTTP freshness describes whether a stored HTTP response is young enough for reuse according to its applicable lifetime. The comparison isThe strict inequality matters: a response exactly at its lifetime boundary is no longer fresh.^[ref]

The HTTP freshness lifetime can come from explicit cache controls or, where permitted, a heuristic. The HTTP current age accounts for age accumulated before the response entered the present cache as well as its residence there. Counting only the seconds since this cache received the response can therefore underestimate its age.^[ref]

For an own example, suppose the applicable lifetime is $60$ seconds and the correctly calculated current age at a chosen instant is $20$ seconds. Keep the lifetime fixed. After another $30$ seconds, the age is $50$, so $60>50$ and the response remains fresh. After another $40$ seconds from the original instant, the age is $60$ and the strict comparison fails. After $45$ seconds it is $65$, again stale. These calculations start from a given current age; they do not derive it from local residence time alone.

Freshness and HTTP cache validation answer different questions. An old response might still match the origin's present HTTP representation. Validation can establish that reuse remains appropriate without retransmitting the representation body. Conversely, a fresh response has not thereby been checked against the origin at every use.^[ref]

Other rules still govern reuse. For example, the unqualified Cache-Control: no-cache response directive requires successful validation before subsequent use, even though the response can be stored. Stale-serving rules also have explicit conditions. A freshness calculation is therefore one part of deciding whether a stored HTTP response may satisfy a particular request, rather than a complete permission test.^[ref]

An HTTP representation consists of data and metadata describing a selected state or form of a resource. It is the information transferred or referenced to express that resource, rather than the resource identifier itself. A resource can have more than one available representation.^[ref]

For an invented /color resource, one representation is the plain-text bytes blue with type text/plain. Another is an HTML document displaying the same color information, with type text/html. The underlying application value can agree while the bytes and media types differ. HTTP content negotiation can help select among available representations according to request information.

An HTTP response also contains control information about the exchange, such as its HTTP status code. It is not simply another name for representation data. A HEAD response, for example, can describe a selected representation without transferring its content, while a GET can transfer that content.^[ref]

Representation metadata tells a recipient how to interpret the data. A media type concerns format, character-set information concerns text encoding, and an entity tag supplies a validator for the selected representation. These properties can differ between two forms of the same resource.

A URL therefore does not promise one immutable byte sequence. A resource can change with time or offer different forms; conversely, matching application meaning does not require byte-identical representations. When comparing responses, record which representation was selected and compare the relevant data and metadata rather than only the identifier.

An HTTP response is a message returned in response to an HTTP request, carrying an HTTP status code, associated HTTP header fields and any permitted content. An exchange can include informational responses before its final response.^[ref]

For an invented plain-text resource, a successful GET can return status 200, Content-Type: text/plain; charset=utf-8 and the content blue. These four letters occupy four bytes in UTF-8, so a corresponding content length is four. The HTTP representation's data, its media type and the status describing this exchange have different roles.

A response need not contain a representation body. A successful PUT can return 204 without response content, and a HEAD response carries no content even when the corresponding GET would retrieve data. Interpreting a response therefore requires the request method as well as the status code.^[ref]

In HTTP/1.1, HTTP/1.1 200 OK is a possible status line, followed by field lines, an empty line and any message body allowed by the framing rules. This spelling is one version's concrete syntax for the broader response abstraction.^[ref]

The received content can change between requests while the resource identifier stays the same. Conversely, the same content can accompany different exchange metadata. Inspect status, fields and content together instead of treating any one of them as the entire response or as a guarantee of an application-specific result.

An HTTP shared cache stores HTTP responses for reuse by more than one user. It is often part of an intermediary, though that deployment is not required by the definition. An HTTP private cache is instead dedicated to a single user.^[ref]

For an own example, suppose four users request the same public manual through one shared cache. The first request obtains a complete, cacheable response with a $1200$-octet body. Assume the following three requests match the required HTTP cache key inputs, arrive while the stored response remains reusable, and have no directives requiring forwarding. They can be satisfied from that stored response. The origin then sends the body once instead of four times, avoiding $3(1200)=3600$ origin-body octets in this stipulated sequence. This does not count headers, validation traffic or delivery from the cache to the users.

Matching the target identifier alone need not be sufficient. If the response uses Vary: Accept, the relevant request-field matching rules also matter; one user's negotiated HTTP representation is not automatically suitable for another request's preferences.^[ref] HTTP content negotiation and cache selection therefore interact.

Storage and reuse are separate decisions. The unqualified Cache-Control: private response directive forbids storage by a shared cache. Other responses must still satisfy the ordinary storage constraints, and the cache must consider HTTP freshness or successful HTTP cache validation when constructing a later response.^[ref]

A shared cache also interprets s-maxage when present. For Cache-Control: max-age=600, s-maxage=60, its explicit freshness lifetime is sixty seconds rather than six hundred. This gives the origin a way to specify a shared reuse interval independently of the corresponding private-cache interval.^[ref]

The benefit comes from reusing an eligible response across users, not from assuming all users receive identical content. State selection inputs and cache controls before extrapolating the simple manual example to personalized or negotiated resources.