200--299 (Successful)
This class of status code indicates that the client's request was successfully received, understood, and accepted.
| Code | Description |
|---|---|
| 200 (Successful) | The server successfully processed the request. Generally, this means that the server provided the requested page. If you see this status for your robots.txt file, it means that Googlebot retrieved it successfully. The request has succeeded. The information returned with the response is dependent on the method used in the request, for example: GET an entity corresponding to the requested resource is sent in the response; HEAD the entity-header fields corresponding to the requested resource are sent in the response without any message-body; POST an entity describing or containing the result of the action; TRACE an entity containing the request message as received by the end server. |
| 201 (Created) | The request was successful and the server created a new resource. The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced by the URI(s) returned in the entity of the response, with the most specific URI for the resource given by a Location header field. The response SHOULD include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. The origin server MUST create the resource before returning the 201 status code. If the action cannot be carried out immediately, the server SHOULD respond with 202 (Accepted) response instead. A 201 response MAY contain an ETag response header field indicating the current value of the entity tag for the requested variant just created, see ETag section below. |
| 202 (Accepted) | The server has accepted the request, but hasn't yet processed it. The request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this. The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled. |
| 203 (Non-authoritative information) | The server successfully processed the request, but is returning information that may be from another source. The returned meta information in the entity-header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented MAY be a subset or super set of the original version. For example, including local annotation information about the resource might result in a super set of the meta information known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be 200 (OK). |
| 204 (No content) | The server successfully processed the request, but isn't returning any content. The server has fulfilled the request but does not need to return an entity-body, and might want to return updated meta information. The response MAY include new or updated meta information in the form of entity-headers, which if present SHOULD be associated with the requested variant. If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view, although any new or updated meta information SHOULD be applied to the document currently in the user agent's active view. The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields. |
| 205 (Reset content) | The server successfully processed the request, but isn't returning any content. Unlike a 204 response, this response requires that the requester reset the document view (for instance, clear a form for new input). The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response MUST NOT include an entity. |
| 206 (Partial content) | The server successfully processed a partial GET request. The server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field (see Range section below) indicating the desired range, and MAY have included an If-Range header field (see If-Range below) to make the request conditional. The response MUST include the following header fields: - Either a Content-Range header field (see Content Range section below) indicating the range included with this response, or a multi-part/byte ranges Content-Type including Content-Range fields for each part. If a Content-Length header field is present in the response, its value MUST match the actual number of OCTETs transmitted in the message-body. - Date - ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request - Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same variant If the 206 response is the result of an If-Range request that used a strong cache validator (see section 13.3.3 below), the response SHOULD NOT include other entity-headers. If the response is the result of an If-Range request that used a weak validator, the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers. Otherwise, the response MUST include all of the entity-headers that would have been returned with a 200 (OK) response to the same request. A cache MUST NOT combine a 206 response with other previously cached content if the ETag or Last-Modified headers do not match exactly, see 13.5.4 below. |
| 207 (Multi-Status) | (WebDAV) (RFC 4918) The message body that follows is an XML message and can contain a number of separate response codes, depending on how many sub-requests were made. |
| 226 (IM Used) | (RFC 3229) The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance |
3.11
Entity Tags
Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the ETag (section 14.19 below), If-Match (section 14.24 below), If-None-Match (section 14.26 below), and If-Range (section 14.27 below) header fields. The definition of how they are used and compared as cache validators is in section 13.3.3 below. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.entity-tag = [ weak ] opaque-tag
weak = "W/"
opaque-tag = quoted-string
A "strong entity tag" MAY be shared by two entities of a resource only if they are equivalent by octet equality. A "weak entity tag," indicated by the "W/" prefix, MAY be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.
An entity tag MUST be unique across all versions of all entities associated with a particular resource. A given entity tag value MAY be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities obtained by requests on different URIs does not imply the equivalence of those entities.
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.
13.5.4
Combining Byte Ranges
A response might transfer only a subrange of the bytes of an entity- body, either because the request included one or more Range specifications, or because a connection was broken prematurely. After several such transfers, a cache might have received several ranges of the same entity-body.If a cache has a stored non-empty set of subranges for an entity, and an incoming response transfers another subrange, the cache MAY combine the new subrange with the existing set if both the following conditions are met:
- Both the incoming response and the cache entry have a cache
validator.
- The two cache validators match using the strong comparison
function (see section 13.3.3 above).
If either requirement is not met, the cache MUST use only the most recent partial response (based on the Date values transmitted with every response, and using the incoming response if these values are equal or missing), and MUST discard the other partial information. 14.19
ETag
The ETag response-header field provides the current value of the entity tag for the requested variant. The headers used with entity tags are described in sections 14.24, 14.26 and 14.44 below. The entity tag MAY be used for comparison with other entities from the same resource (see section 13.3.3 above).ETag = "ETag" ":" entity-tagExamples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
14.24
If-Match
The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field. Entity tags are defined in section 3.11 above. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current entity of the resource.If-Match = "If-Match" ":" ( "*" | 1#entity-tag )If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-Match header) on that resource, or if "*" is given
and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.
A server MUST use the strong comparison function (see section 13.3.3 above) to compare the entity tags in If-Match.
If none of the entity tags match, or if "*" is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.
If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status, then the If-Match header MUST be ignored.
The meaning of "If-Match: *" is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.44 below) exists, and MUST NOT be performed if the representation does not exist.
A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.14.26
If-None-Match
The If-None-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used to prevent a method (e.g. PUT) from inadvertently modifying an existing resource when the client believes that the resource does not exist.As a special case, the value "*" matches any current entity of the resource.
If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MUST NOT perform the requested method, unless required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache- related header fields (particularly ETag) of one of the entities that matched. For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).
See section 13.3.3 above for rules on how to determine if two entities tags match. The weak comparison function can only be used with GET or HEAD requests.
If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.
If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status, then the If-None-Match header MUST be ignored. (See section 13.3.4 above for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.44 below) exists, and SHOULD be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.14.27
If-Range
If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body.The If-Range header allows a client to "short-circuit" the second request. Informally, its meaning is `if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity'.
If-Range = "If-Range" ":" ( entity-tag | HTTP-date )If the client has no entity tag for an entity, but does have a Last- Modified date, it MAY use that date in an If-Range header. (The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.) The If-Range header SHOULD only be used together with a Range header, and MUST be ignored if the request does not include a Range header, or if the server does not support the sub-range operation.
If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server SHOULD provide the specified sub-range of the entity using a 206 (Partial content) response. If the entity tag does not match, then the server SHOULD return the entire entity using a 200 (OK) response.
14.44
Vary
The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation. A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this response is the appropriate representation. See section 13.6 for use of the Vary header field by caches.Vary = "Vary" ":" ( "*" | 1#field-name )
An HTTP/1.1 server SHOULD include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server MAY include a Vary header field with a non-cacheable response that is subject to server-driven negotiation, since this might provide the user agent with useful information about the dimensions over which the response varies at the time of the response.
A Vary field value consisting of a list of field-names signals that the representation selected for the response is based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation. A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time for which the response is fresh.
The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names are case-insensitive.
A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address of the client), play a role in the selection of the response representation. The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server.
Resources:
W3.org
