13.3 Validation Model


When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if the cached entry is good, and we do not want to pay the overhead of an extra round trip if the cached entry is invalid, the HTTP/1.1 protocol supports the use of conditional methods.

The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated validator in the request.

The server then checks that validator against the current validator for the entity, and, if they match (see section 13.3.3), it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round trip if it does not match.

In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.

The protocol includes both positive and negative senses of cache- validating conditions. That is, it is possible to request either that a method be performed if and only if a validator matches or if and only if no validators match.
      Note: a response that lacks a validator may still be cached, and
      served from cache until it expires, unless this is explicitly
      prohibited by a cache-control directive. However, a cache cannot
      do a conditional retrieval if it does not have a validator for the
      entity, which means it will not be refreshable after it expires.

13.3.1 Last-Modified Dates


The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered to be valid if the entity has not been modified since the Last-Modified value.

13.3.2 Entity Tag Cache Validators


The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.

Entity Tags are described in section 3.11. The headers used with entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.

13.3.3 Weak and Strong Validators


Since both origin servers and caches will compare two validators to decide if they represent the same or different entities, one normally would expect that if the entity (the entity-body or any entity- headers) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong validator."

However, there might be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the entity change. A validator that does not always change when the resource changes is a "weak validator."

Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities.
      Note: One example of a strong validator is an integer that is
      incremented in stable storage every time an entity is changed.
      An entity's modification time, if represented with one-second
      resolution, could be a weak validator, since it is possible that
      the resource might be modified twice during a single second.
      Support for weak validators is optional. However, weak validators
      allow for more efficient caching of equivalent objects; for
      example, a hit counter on a site is probably good enough if it is
      updated every few days or weeks, and any value during that period
      is likely "good enough" to be equivalent.


A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.

Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent entity.

Clients MAY issue simple (non-subrange) GET requests with either weak validators or strong validators. Clients MUST NOT use weak validators in other forms of request.

The only function that the HTTP/1.1 protocol defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not:
      - The strong comparison function: in order to be considered equal,
        both validators MUST be identical in every way, and both MUST
        NOT be weak.
      - The weak comparison function: in order to be considered equal,
        both validators MUST be identical in every way, but either or
        both of them MAY be tagged as "weak" without affecting the
        result.


An entity tag is strong unless it is explicitly tagged as weak. Section 3.11 gives the syntax for entity tags.

A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules:
      - The validator is being compared by an origin server to the
        actual current validator for the entity and,
      - That origin server reliably knows that the associated entity did
        not change twice during the second covered by the presented
        validator.


or
      - The validator is about to be used by a client in an If-
        Modified-Since or If-Unmodified-Since header, because the client
        has a cache entry for the associated entity, and
      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and
      - The presented Last-Modified time is at least 60 seconds before
        the Date value.


or
      - The validator is being compared by an intermediate cache to the
        validator stored in its cache entry for the entity, and
      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and
      - The presented Last-Modified time is at least 60 seconds before
        the Date value.


This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60- second limit guards against the possibility that the Date and Last- Modified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation MAY use a value larger than 60 seconds, if it is believed that 60 seconds is too short.

If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it MAY do this only if the Last-Modified time is strong in the sense described here.

A cache or origin server receiving a conditional request, other than a full-body GET request, MUST use the strong comparison function to evaluate the condition.

These rules allow HTTP/1.1 caches and clients to safely perform sub- range retrievals on values that have been obtained from HTTP/1.0

servers.

13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates


We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types ought to be used, and for what purposes.

HTTP/1.1 origin servers:
      - SHOULD send an entity tag validator unless it is not feasible to
        generate one.
      - MAY send a weak entity tag instead of a strong entity tag, if
        performance considerations support the use of weak entity tags,
        or if it is unfeasible to send a strong entity tag.
      - SHOULD send a Last-Modified value if it is feasible to send one,
        unless the risk of a breakdown in semantic transparency that
        could result from using this date in an If-Modified-Since header
        would lead to serious problems.


In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value.

In order to be legal, a strong entity tag MUST change whenever the associated entity value changes in any way. A weak entity tag SHOULD change whenever the associated entity changes in a semantically significant way.
      Note: in order to provide semantically transparent caching, an
      origin server must avoid reusing a specific strong entity tag
      value for two different entities, or reusing a specific weak
      entity tag value for two semantically different entities. Cache
      entries might persist for arbitrarily long periods, regardless of
      expiration times, so it might be inappropriate to expect that a
      cache will never again attempt to validate an entry using a
      validator that it obtained at some point in the past.


HTTP/1.1 clients:
      - If an entity tag has been provided by the origin server, MUST
        use that entity tag in any cache-conditional request (using If-
        Match or If-None-Match).
      - If only a Last-Modified value has been provided by the origin
        server, SHOULD use that value in non-subrange cache-conditional
        requests (using If-Modified-Since).
      - If only a Last-Modified value has been provided by an HTTP/1.0
        origin server, MAY use that value in subrange cache-conditional
        requests (using If-Unmodified-Since:). The user agent SHOULD
        provide a way to disable this, in case of difficulty.
      - If both an entity tag and a Last-Modified value have been
        provided by the origin server, SHOULD use both validators in
        cache-conditional requests. This allows both HTTP/1.0 and
        HTTP/1.1 caches to respond appropriately.


An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since or If-Unmodified-Since header field) and one or more entity tags (e.g., in an If-Match, If-None-Match, or If-Range header field) as cache validators, MUST NOT return a response status of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in the request.

An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity tags as cache validators, MUST NOT return a locally cached response to the client unless that cached response is consistent with all of the conditional header fields in the request.
      Note: The general principle behind these rules is that HTTP/1.1
      servers and clients should transmit as much non-redundant
      information as is available in their responses and requests.
      HTTP/1.1 systems receiving this information will make the most
      conservative assumptions about the validators they receive.
      HTTP/1.0 clients and caches will ignore entity tags. Generally,
      last-modified values received or used by these systems will
      support transparent and efficient caching, and so HTTP/1.1 origin
      servers should provide Last-Modified values. In those rare cases
      where the use of a Last-Modified value as a validator by an
      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
      origin servers should not provide one.

13.3.5 Non-validating Conditionals


The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry.