Best practices for RESTful web service design

Editor's Notes

• #7: ConstraintsThe REST architectural style describes the following six constraints applied to the architecture, while leaving the implementation of the individual components free to design:Client–serverA uniform interface separates clients from servers. This separation of concerns means that, for example, clients are not concerned with data storage, which remains internal to each server, so that the portability of client code is improved. Servers are not concerned with the user interface or user state, so that servers can be simpler and more scalable. Servers and clients may also be replaced and developed independently, as long as the interface between them is not altered.StatelessThe client–server communication is further constrained by no client context being stored on the server between requests. Each request from any client contains all of the information necessary to service the request, and any session state is held in the client.CacheableAs on the World Wide Web, clients can cache responses. Responses must therefore, implicitly or explicitly, define themselves as cacheable, or not, to prevent clients reusing stale or inappropriate data in response to further requests. Well-managed caching partially or completely eliminates some client–server interactions, further improving scalability and performance.Layered systemA client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers may improve system scalability by enabling load-balancing and by providing shared caches. They may also enforce security policies.Code on demand (optional)Servers can temporarily extend or customize the functionality of a client by the transfer of executable code. Examples of this may include compiled components such as Java applets and client-side scripts such as JavaScript.Uniform interfaceThe uniform interface between clients and servers, discussed below, simplifies and decouples the architecture, which enables each part to evolve independently. The four guiding principles of this interface are detailed below.The only optional constraint of REST architecture is "code on demand". One can characterise applications conforming to the REST constraints described in this section as "RESTful".[8] If a service violates any of the required constraints, it cannot be considered RESTful.Complying with these constraints, and thus conforming to the REST architectural-style, enables any kind of distributed hypermedia system to have desirable emergent properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability.
• #8: ConstraintsThe REST architectural style describes the following six constraints applied to the architecture, while leaving the implementation of the individual components free to design:Client–serverA uniform interface separates clients from servers. This separation of concerns means that, for example, clients are not concerned with data storage, which remains internal to each server, so that the portability of client code is improved. Servers are not concerned with the user interface or user state, so that servers can be simpler and more scalable. Servers and clients may also be replaced and developed independently, as long as the interface between them is not altered.StatelessThe client–server communication is further constrained by no client context being stored on the server between requests. Each request from any client contains all of the information necessary to service the request, and any session state is held in the client.CacheableAs on the World Wide Web, clients can cache responses. Responses must therefore, implicitly or explicitly, define themselves as cacheable, or not, to prevent clients reusing stale or inappropriate data in response to further requests. Well-managed caching partially or completely eliminates some client–server interactions, further improving scalability and performance.Layered systemA client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers may improve system scalability by enabling load-balancing and by providing shared caches. They may also enforce security policies.Code on demand (optional)Servers can temporarily extend or customize the functionality of a client by the transfer of executable code. Examples of this may include compiled components such as Java applets and client-side scripts such as JavaScript.Uniform interfaceThe uniform interface between clients and servers, discussed below, simplifies and decouples the architecture, which enables each part to evolve independently. The four guiding principles of this interface are detailed below.The only optional constraint of REST architecture is "code on demand". One can characterise applications conforming to the REST constraints described in this section as "RESTful".[8] If a service violates any of the required constraints, it cannot be considered RESTful.Complying with these constraints, and thus conforming to the REST architectural-style, enables any kind of distributed hypermedia system to have desirable emergent properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability.
• #15: Request received, continuing process.[2]This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers must not send a 1xx response to an HTTP/1.0 client except under experimental conditions.100 ContinueThis means that the server has received the request headers, and that the client should proceed to send the request body (in the case of a request for which a body needs to be sent; for example, a POST request). If the request body is large, sending it to a server when a request has already been rejected based upon inappropriate headers is inefficient. To have a server check if the request could be accepted based on the request's headers alone, a client must send Expect: 100-continue as a header in its initial request[2] and check if a 100 Continue status code is received in response before continuing (or receive 417 Expectation Failed and not continue).[2]101 Switching ProtocolsThis means the requester has asked the server to switch protocols and the server is acknowledging that it will do so.[2]102 Processing (WebDAV; RFC 2518)As a WebDAV request may contain many sub-requests involving file operations, it may take a long time to complete the request. This code indicates that the server has received and is processing the request, but no response is available yet.[3] This prevents the client from timing out and assuming the request was lost.2xx Success[edit]This class of status codes indicates the action requested by the client was received, understood, accepted and processed successfully.200 OKStandard response for successful HTTP requests. The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource. In a POST request the response will contain an entity describing or containing the result of the action.[2]201 CreatedThe request has been fulfilled and resulted in a new resource being created.[2]202 AcceptedThe 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.[2]203 Non-Authoritative Information (since HTTP/1.1)The server successfully processed the request, but is returning information that may be from another source.[2]204 No ContentThe server successfully processed the request, but is not returning any content.[2]205 Reset ContentThe server successfully processed the request, but is not returning any content. Unlike a 204 response, this response requires that the requester reset the document view.[2]206 Partial ContentThe server is delivering only part of the resource due to a range header sent by the client. The range header is used by tools like wget to enable resuming of interrupted downloads, or split a download into multiple simultaneous streams.[2]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.[4]208 Already Reported (WebDAV; RFC 5842)The members of a DAV binding have already been enumerated in a previous reply to this request, and are not being included again.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.[5]3xx Redirection[edit]The client must take additional action to complete the request.[2]This class of status code indicates that further action needs to be taken by the user agent to fulfil the request. The action required may be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent should not automatically redirect a request more than five times, since such redirections usually indicate an infinite loop.300 Multiple ChoicesIndicates multiple options for the resource that the client may follow. It, for instance, could be used to present different format options for video, list files with different extensions, or word sense disambiguation.[2]301 Moved PermanentlyThis and all future requests should be directed to the given URI.[2]302 FoundThis is an example of industry practice contradicting the standard.[2] The HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"),[6] but popular browsers implemented 302 with the functionality of a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to distinguish between the two behaviours.[7] However, some Web applications and frameworks use the 302 status code as if it were the 303.[8]303 See Other (since HTTP/1.1)The response to the request can be found under another URI using a GET method. When received in response to a POST (or PUT/DELETE), it should be assumed that the server has received the data and the redirect should be issued with a separate GET message.[2]304 Not ModifiedIndicates that the resource has not been modified since the version specified by the request headers If-Modified-Since or If-Match.[2] This means that there is no need to retransmit the resource, since the client still has a previously-downloaded copy.305 Use Proxy (since HTTP/1.1)The requested resource is only available through a proxy, whose address is provided in the response.[2] Many HTTP clients (such as Mozilla[9] and Internet Explorer) do not correctly handle responses with this status code, primarily for security reasons.[citation needed]306 Switch ProxyNo longer used.[2] Originally meant "Subsequent requests should use the specified proxy."[10]307 Temporary Redirect (since HTTP/1.1)In this case, the request should be repeated with another URI; however, future requests should still use the original URI.[2] In contrast to how 302 was historically implemented, the request method is not allowed to be changed when reissuing the original request. For instance, a POST request should be repeated using another POST request.[11]308 Permanent Redirect (approved as experimental RFC)[12]The request, and all future requests should be repeated using another URI. 307 and 308 (as proposed) parallel the behaviours of 302 and 301, but do not allow the HTTP method to change. So, for example, submitting a form to a permanently redirected resource may continue smoothly.4xx Client Error[edit]The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents should display any included entity to the user.400 Bad RequestThe request cannot be fulfilled due to bad syntax.[2]401 UnauthorizedSimilar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.[2] The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource. See Basic access authentication and Digest access authentication.402 Payment RequiredReserved for future use.[2] The original intention was that this code might be used as part of some form of digital cash or micropayment scheme, but that has not happened, and this code is not usually used. As an example of its use, however, Apple's defunct MobileMe service generated a 402 error if the MobileMe account was delinquent.[citation needed] In addition, YouTube uses this status if a particular IP address has made excessive requests, and requires the person to enter a CAPTCHA.403 ForbiddenThe request was a valid request, but the server is refusing to respond to it.[2] Unlike a 401 Unauthorized response, authenticating will make no difference.[2] On servers where authentication is required, this commonly means that the provided credentials were successfully authenticated but that the credentials still do not grant the client permission to access the resource (e.g. a recognized user attempting to access restricted content).404 Not FoundThe requested resource could not be found but may be available again in the future.[2] Subsequent requests by the client are permissible.405 Method Not AllowedA request was made of a resource using a request method not supported by that resource;[2] for example, using GET on a form which requires data to be presented via POST, or using PUT on a read-only resource.406 Not AcceptableThe requested resource is only capable of generating content not acceptable according to the Accept headers sent in the request.[2]407 Proxy Authentication RequiredThe client must first authenticate itself with the proxy.[2]408 Request TimeoutThe server timed out waiting for the request.[2] According to W3 HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."409 ConflictIndicates that the request could not be processed because of conflict in the request, such as an edit conflict.[2]410 GoneIndicates that the resource requested is no longer available and will not be available again.[2] This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource again in the future. Clients such as search engines should remove the resource from their indices. Most use cases do not require clients and search engines to purge the resource, and a "404 Not Found" may be used instead.411 Length RequiredThe request did not specify the length of its content, which is required by the requested resource.[2]412 Precondition FailedThe server does not meet one of the preconditions that the requester put on the request.[2]413 Request Entity Too LargeThe request is larger than the server is willing or able to process.[2]414 Request-URI Too LongThe URI provided was too long for the server to process.[2]415 Unsupported Media TypeThe request entity has a media type which the server or resource does not support.[2] For example, the client uploads an image as image/svg+xml, but the server requires that images use a different format.416 Requested Range Not SatisfiableThe client has asked for a portion of the file, but the server cannot supply that portion.[2] For example, if the client asked for a part of the file that lies beyond the end of the file.[2]417 Expectation FailedThe server cannot meet the requirements of the Expect request-header field.[2]418 I'm a teapot (RFC 2324)This code was defined in 1998 as one of the traditional IETF April Fools' jokes, in RFC 2324, Hyper Text Coffee Pot Control Protocol, and is not expected to be implemented by actual HTTP servers.419 Authentication TimeoutNot a part of the HTTP standard, 419 Authentication Timeout denotes that previously valid authentication has expired. It is used as an alternative to 401 Unauthorized in order to differentiate from otherwise authenticated clients being denied access to specific server resources.420 Enhance Your Calm (Twitter)Not part of the HTTP standard, but returned by the Twitter Search and Trends API when the client is being rate limited.[13] Other services may wish to implement the 429 Too Many Requests response code instead.422 Unprocessable Entity (WebDAV; RFC 4918)The request was well-formed but was unable to be followed due to semantic errors.[4]423 Locked (WebDAV; RFC 4918)The resource that is being accessed is locked.[4]424 Failed Dependency (WebDAV; RFC 4918)The request failed due to failure of a previous request (e.g. a PROPPATCH).[4]424 Method Failure (WebDAV)[14]Indicates the method was not executed on a particular resource within its scope because some part of the method's execution failed causing the entire method to be aborted.425 Unordered Collection (Internet draft)Defined in drafts of "WebDAV Advanced Collections Protocol",[15] but not present in "Web Distributed Authoring and Versioning (WebDAV) Ordered Collections Protocol".[16]426 Upgrade Required (RFC 2817)The client should switch to a different protocol such as TLS/1.0.[17]428 Precondition Required (RFC 6585)The origin server requires the request to be conditional. Intended to prevent "the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict."[18]429 Too Many Requests (RFC 6585)The user has sent too many requests in a given amount of time. Intended for use with rate limiting schemes.[18]431 Request Header Fields Too Large (RFC 6585)The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.[18]444 No Response (Nginx)Used in Nginx logs to indicate that the server has returned no information to the client and closed the connection (useful as a deterrent for malware).449 Retry With (Microsoft)A Microsoft extension. The request should be retried after performing the appropriate action.[19]Often search-engines or custom applications will ignore required parameters. Where no default action is appropriate, the Aviongoo website sends a "HTTP/1.1 449 Retry with valid parameters: param1, param2, . . ." response. The applications may choose to learn, or not.450 Blocked by Windows Parental Controls (Microsoft)A Microsoft extension. This error is given when Windows Parental Controls are turned on and are blocking access to the given webpage.[20]451 Unavailable For Legal Reasons (Internet draft)Defined in the internet draft "A New HTTP Status Code for Legally-restricted Resources".[21] Intended to be used when resource access is denied for legal reasons, e.g. censorship or government-mandated blocked access. A reference to the 1953 dystopian novel Fahrenheit 451, where books are outlawed.[22]451 Redirect (Microsoft)Used in Exchange ActiveSync if there either is a more efficient server to use or the server can't access the users' mailbox.[23]The client is supposed to re-run the HTTP Autodiscovery protocol to find a better suited server.[24]494 Request Header Too Large (Nginx)Nginx internal code similar to 413 but it was introduced earlier.[25][original research?]495 Cert Error (Nginx)Nginx internal code used when SSL client certificate error occurred to distinguish it from 4XX in a log and an error page redirection.496 No Cert (Nginx)Nginx internal code used when client didn't provide certificate to distinguish it from 4XX in a log and an error page redirection.497 HTTP to HTTPS (Nginx)Nginx internal code used for the plain HTTP requests that are sent to HTTPS port to distinguish it from 4XX in a log and an error page redirection.499 Client Closed Request (Nginx)Used in Nginx logs to indicate when the connection has been closed by client while the server is still processing its request, making server unable to send a status code back.[26]5xx Server Error[edit]The server failed to fulfill an apparently valid request.[2]Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.500 Internal Server ErrorA generic error message, given when no more specific message is suitable.[2]501 Not ImplementedThe server either does not recognize the request method, or it lacks the ability to fulfill the request.[2]502 Bad GatewayThe server was acting as a gateway or proxy and received an invalid response from the upstream server.[2]503 Service UnavailableThe server is currently unavailable (because it is overloaded or down for maintenance).[2] Generally, this is a temporary state.504 Gateway TimeoutThe server was acting as a gateway or proxy and did not receive a timely response from the upstream server.[2]505 HTTP Version Not SupportedThe server does not support the HTTP protocol version used in the request.[2]506 Variant Also Negotiates (RFC 2295)Transparent content negotiation for the request results in a circular reference.[27]507 Insufficient Storage (WebDAV; RFC 4918)The server is unable to store the representation needed to complete the request.[4]508 Loop Detected (WebDAV; RFC 5842)The server detected an infinite loop while processing the request (sent in lieu of 208).509 Bandwidth Limit Exceeded (Apache bw/limited extension)This status code, while used by many servers, is not specified in any RFCs.510 Not Extended (RFC 2774)Further extensions to the request are required for the server to fulfil it.[28]511 Network Authentication Required (RFC 6585)The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g. "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).[18]598 Network read timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network read timeout behind the proxy to a client in front of the proxy.[citation needed]599 Network connect timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network connect timeout behind the proxy to a client in front of the proxy.[citation needed]
• #16: Request received, continuing process.[2]This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers must not send a 1xx response to an HTTP/1.0 client except under experimental conditions.100 ContinueThis means that the server has received the request headers, and that the client should proceed to send the request body (in the case of a request for which a body needs to be sent; for example, a POST request). If the request body is large, sending it to a server when a request has already been rejected based upon inappropriate headers is inefficient. To have a server check if the request could be accepted based on the request's headers alone, a client must send Expect: 100-continue as a header in its initial request[2] and check if a 100 Continue status code is received in response before continuing (or receive 417 Expectation Failed and not continue).[2]101 Switching ProtocolsThis means the requester has asked the server to switch protocols and the server is acknowledging that it will do so.[2]102 Processing (WebDAV; RFC 2518)As a WebDAV request may contain many sub-requests involving file operations, it may take a long time to complete the request. This code indicates that the server has received and is processing the request, but no response is available yet.[3] This prevents the client from timing out and assuming the request was lost.2xx Success[edit]This class of status codes indicates the action requested by the client was received, understood, accepted and processed successfully.200 OKStandard response for successful HTTP requests. The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource. In a POST request the response will contain an entity describing or containing the result of the action.[2]201 CreatedThe request has been fulfilled and resulted in a new resource being created.[2]202 AcceptedThe 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.[2]203 Non-Authoritative Information (since HTTP/1.1)The server successfully processed the request, but is returning information that may be from another source.[2]204 No ContentThe server successfully processed the request, but is not returning any content.[2]205 Reset ContentThe server successfully processed the request, but is not returning any content. Unlike a 204 response, this response requires that the requester reset the document view.[2]206 Partial ContentThe server is delivering only part of the resource due to a range header sent by the client. The range header is used by tools like wget to enable resuming of interrupted downloads, or split a download into multiple simultaneous streams.[2]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.[4]208 Already Reported (WebDAV; RFC 5842)The members of a DAV binding have already been enumerated in a previous reply to this request, and are not being included again.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.[5]3xx Redirection[edit]The client must take additional action to complete the request.[2]This class of status code indicates that further action needs to be taken by the user agent to fulfil the request. The action required may be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent should not automatically redirect a request more than five times, since such redirections usually indicate an infinite loop.300 Multiple ChoicesIndicates multiple options for the resource that the client may follow. It, for instance, could be used to present different format options for video, list files with different extensions, or word sense disambiguation.[2]301 Moved PermanentlyThis and all future requests should be directed to the given URI.[2]302 FoundThis is an example of industry practice contradicting the standard.[2] The HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"),[6] but popular browsers implemented 302 with the functionality of a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to distinguish between the two behaviours.[7] However, some Web applications and frameworks use the 302 status code as if it were the 303.[8]303 See Other (since HTTP/1.1)The response to the request can be found under another URI using a GET method. When received in response to a POST (or PUT/DELETE), it should be assumed that the server has received the data and the redirect should be issued with a separate GET message.[2]304 Not ModifiedIndicates that the resource has not been modified since the version specified by the request headers If-Modified-Since or If-Match.[2] This means that there is no need to retransmit the resource, since the client still has a previously-downloaded copy.305 Use Proxy (since HTTP/1.1)The requested resource is only available through a proxy, whose address is provided in the response.[2] Many HTTP clients (such as Mozilla[9] and Internet Explorer) do not correctly handle responses with this status code, primarily for security reasons.[citation needed]306 Switch ProxyNo longer used.[2] Originally meant "Subsequent requests should use the specified proxy."[10]307 Temporary Redirect (since HTTP/1.1)In this case, the request should be repeated with another URI; however, future requests should still use the original URI.[2] In contrast to how 302 was historically implemented, the request method is not allowed to be changed when reissuing the original request. For instance, a POST request should be repeated using another POST request.[11]308 Permanent Redirect (approved as experimental RFC)[12]The request, and all future requests should be repeated using another URI. 307 and 308 (as proposed) parallel the behaviours of 302 and 301, but do not allow the HTTP method to change. So, for example, submitting a form to a permanently redirected resource may continue smoothly.4xx Client Error[edit]The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents should display any included entity to the user.400 Bad RequestThe request cannot be fulfilled due to bad syntax.[2]401 UnauthorizedSimilar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.[2] The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource. See Basic access authentication and Digest access authentication.402 Payment RequiredReserved for future use.[2] The original intention was that this code might be used as part of some form of digital cash or micropayment scheme, but that has not happened, and this code is not usually used. As an example of its use, however, Apple's defunct MobileMe service generated a 402 error if the MobileMe account was delinquent.[citation needed] In addition, YouTube uses this status if a particular IP address has made excessive requests, and requires the person to enter a CAPTCHA.403 ForbiddenThe request was a valid request, but the server is refusing to respond to it.[2] Unlike a 401 Unauthorized response, authenticating will make no difference.[2] On servers where authentication is required, this commonly means that the provided credentials were successfully authenticated but that the credentials still do not grant the client permission to access the resource (e.g. a recognized user attempting to access restricted content).404 Not FoundThe requested resource could not be found but may be available again in the future.[2] Subsequent requests by the client are permissible.405 Method Not AllowedA request was made of a resource using a request method not supported by that resource;[2] for example, using GET on a form which requires data to be presented via POST, or using PUT on a read-only resource.406 Not AcceptableThe requested resource is only capable of generating content not acceptable according to the Accept headers sent in the request.[2]407 Proxy Authentication RequiredThe client must first authenticate itself with the proxy.[2]408 Request TimeoutThe server timed out waiting for the request.[2] According to W3 HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."409 ConflictIndicates that the request could not be processed because of conflict in the request, such as an edit conflict.[2]410 GoneIndicates that the resource requested is no longer available and will not be available again.[2] This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource again in the future. Clients such as search engines should remove the resource from their indices. Most use cases do not require clients and search engines to purge the resource, and a "404 Not Found" may be used instead.411 Length RequiredThe request did not specify the length of its content, which is required by the requested resource.[2]412 Precondition FailedThe server does not meet one of the preconditions that the requester put on the request.[2]413 Request Entity Too LargeThe request is larger than the server is willing or able to process.[2]414 Request-URI Too LongThe URI provided was too long for the server to process.[2]415 Unsupported Media TypeThe request entity has a media type which the server or resource does not support.[2] For example, the client uploads an image as image/svg+xml, but the server requires that images use a different format.416 Requested Range Not SatisfiableThe client has asked for a portion of the file, but the server cannot supply that portion.[2] For example, if the client asked for a part of the file that lies beyond the end of the file.[2]417 Expectation FailedThe server cannot meet the requirements of the Expect request-header field.[2]418 I'm a teapot (RFC 2324)This code was defined in 1998 as one of the traditional IETF April Fools' jokes, in RFC 2324, Hyper Text Coffee Pot Control Protocol, and is not expected to be implemented by actual HTTP servers.419 Authentication TimeoutNot a part of the HTTP standard, 419 Authentication Timeout denotes that previously valid authentication has expired. It is used as an alternative to 401 Unauthorized in order to differentiate from otherwise authenticated clients being denied access to specific server resources.420 Enhance Your Calm (Twitter)Not part of the HTTP standard, but returned by the Twitter Search and Trends API when the client is being rate limited.[13] Other services may wish to implement the 429 Too Many Requests response code instead.422 Unprocessable Entity (WebDAV; RFC 4918)The request was well-formed but was unable to be followed due to semantic errors.[4]423 Locked (WebDAV; RFC 4918)The resource that is being accessed is locked.[4]424 Failed Dependency (WebDAV; RFC 4918)The request failed due to failure of a previous request (e.g. a PROPPATCH).[4]424 Method Failure (WebDAV)[14]Indicates the method was not executed on a particular resource within its scope because some part of the method's execution failed causing the entire method to be aborted.425 Unordered Collection (Internet draft)Defined in drafts of "WebDAV Advanced Collections Protocol",[15] but not present in "Web Distributed Authoring and Versioning (WebDAV) Ordered Collections Protocol".[16]426 Upgrade Required (RFC 2817)The client should switch to a different protocol such as TLS/1.0.[17]428 Precondition Required (RFC 6585)The origin server requires the request to be conditional. Intended to prevent "the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict."[18]429 Too Many Requests (RFC 6585)The user has sent too many requests in a given amount of time. Intended for use with rate limiting schemes.[18]431 Request Header Fields Too Large (RFC 6585)The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.[18]444 No Response (Nginx)Used in Nginx logs to indicate that the server has returned no information to the client and closed the connection (useful as a deterrent for malware).449 Retry With (Microsoft)A Microsoft extension. The request should be retried after performing the appropriate action.[19]Often search-engines or custom applications will ignore required parameters. Where no default action is appropriate, the Aviongoo website sends a "HTTP/1.1 449 Retry with valid parameters: param1, param2, . . ." response. The applications may choose to learn, or not.450 Blocked by Windows Parental Controls (Microsoft)A Microsoft extension. This error is given when Windows Parental Controls are turned on and are blocking access to the given webpage.[20]451 Unavailable For Legal Reasons (Internet draft)Defined in the internet draft "A New HTTP Status Code for Legally-restricted Resources".[21] Intended to be used when resource access is denied for legal reasons, e.g. censorship or government-mandated blocked access. A reference to the 1953 dystopian novel Fahrenheit 451, where books are outlawed.[22]451 Redirect (Microsoft)Used in Exchange ActiveSync if there either is a more efficient server to use or the server can't access the users' mailbox.[23]The client is supposed to re-run the HTTP Autodiscovery protocol to find a better suited server.[24]494 Request Header Too Large (Nginx)Nginx internal code similar to 413 but it was introduced earlier.[25][original research?]495 Cert Error (Nginx)Nginx internal code used when SSL client certificate error occurred to distinguish it from 4XX in a log and an error page redirection.496 No Cert (Nginx)Nginx internal code used when client didn't provide certificate to distinguish it from 4XX in a log and an error page redirection.497 HTTP to HTTPS (Nginx)Nginx internal code used for the plain HTTP requests that are sent to HTTPS port to distinguish it from 4XX in a log and an error page redirection.499 Client Closed Request (Nginx)Used in Nginx logs to indicate when the connection has been closed by client while the server is still processing its request, making server unable to send a status code back.[26]5xx Server Error[edit]The server failed to fulfill an apparently valid request.[2]Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.500 Internal Server ErrorA generic error message, given when no more specific message is suitable.[2]501 Not ImplementedThe server either does not recognize the request method, or it lacks the ability to fulfill the request.[2]502 Bad GatewayThe server was acting as a gateway or proxy and received an invalid response from the upstream server.[2]503 Service UnavailableThe server is currently unavailable (because it is overloaded or down for maintenance).[2] Generally, this is a temporary state.504 Gateway TimeoutThe server was acting as a gateway or proxy and did not receive a timely response from the upstream server.[2]505 HTTP Version Not SupportedThe server does not support the HTTP protocol version used in the request.[2]506 Variant Also Negotiates (RFC 2295)Transparent content negotiation for the request results in a circular reference.[27]507 Insufficient Storage (WebDAV; RFC 4918)The server is unable to store the representation needed to complete the request.[4]508 Loop Detected (WebDAV; RFC 5842)The server detected an infinite loop while processing the request (sent in lieu of 208).509 Bandwidth Limit Exceeded (Apache bw/limited extension)This status code, while used by many servers, is not specified in any RFCs.510 Not Extended (RFC 2774)Further extensions to the request are required for the server to fulfil it.[28]511 Network Authentication Required (RFC 6585)The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g. "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).[18]598 Network read timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network read timeout behind the proxy to a client in front of the proxy.[citation needed]599 Network connect timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network connect timeout behind the proxy to a client in front of the proxy.[citation needed]
• #17: Request received, continuing process.[2]This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers must not send a 1xx response to an HTTP/1.0 client except under experimental conditions.100 ContinueThis means that the server has received the request headers, and that the client should proceed to send the request body (in the case of a request for which a body needs to be sent; for example, a POST request). If the request body is large, sending it to a server when a request has already been rejected based upon inappropriate headers is inefficient. To have a server check if the request could be accepted based on the request's headers alone, a client must send Expect: 100-continue as a header in its initial request[2] and check if a 100 Continue status code is received in response before continuing (or receive 417 Expectation Failed and not continue).[2]101 Switching ProtocolsThis means the requester has asked the server to switch protocols and the server is acknowledging that it will do so.[2]102 Processing (WebDAV; RFC 2518)As a WebDAV request may contain many sub-requests involving file operations, it may take a long time to complete the request. This code indicates that the server has received and is processing the request, but no response is available yet.[3] This prevents the client from timing out and assuming the request was lost.2xx Success[edit]This class of status codes indicates the action requested by the client was received, understood, accepted and processed successfully.200 OKStandard response for successful HTTP requests. The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource. In a POST request the response will contain an entity describing or containing the result of the action.[2]201 CreatedThe request has been fulfilled and resulted in a new resource being created.[2]202 AcceptedThe 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.[2]203 Non-Authoritative Information (since HTTP/1.1)The server successfully processed the request, but is returning information that may be from another source.[2]204 No ContentThe server successfully processed the request, but is not returning any content.[2]205 Reset ContentThe server successfully processed the request, but is not returning any content. Unlike a 204 response, this response requires that the requester reset the document view.[2]206 Partial ContentThe server is delivering only part of the resource due to a range header sent by the client. The range header is used by tools like wget to enable resuming of interrupted downloads, or split a download into multiple simultaneous streams.[2]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.[4]208 Already Reported (WebDAV; RFC 5842)The members of a DAV binding have already been enumerated in a previous reply to this request, and are not being included again.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.[5]3xx Redirection[edit]The client must take additional action to complete the request.[2]This class of status code indicates that further action needs to be taken by the user agent to fulfil the request. The action required may be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent should not automatically redirect a request more than five times, since such redirections usually indicate an infinite loop.300 Multiple ChoicesIndicates multiple options for the resource that the client may follow. It, for instance, could be used to present different format options for video, list files with different extensions, or word sense disambiguation.[2]301 Moved PermanentlyThis and all future requests should be directed to the given URI.[2]302 FoundThis is an example of industry practice contradicting the standard.[2] The HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"),[6] but popular browsers implemented 302 with the functionality of a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to distinguish between the two behaviours.[7] However, some Web applications and frameworks use the 302 status code as if it were the 303.[8]303 See Other (since HTTP/1.1)The response to the request can be found under another URI using a GET method. When received in response to a POST (or PUT/DELETE), it should be assumed that the server has received the data and the redirect should be issued with a separate GET message.[2]304 Not ModifiedIndicates that the resource has not been modified since the version specified by the request headers If-Modified-Since or If-Match.[2] This means that there is no need to retransmit the resource, since the client still has a previously-downloaded copy.305 Use Proxy (since HTTP/1.1)The requested resource is only available through a proxy, whose address is provided in the response.[2] Many HTTP clients (such as Mozilla[9] and Internet Explorer) do not correctly handle responses with this status code, primarily for security reasons.[citation needed]306 Switch ProxyNo longer used.[2] Originally meant "Subsequent requests should use the specified proxy."[10]307 Temporary Redirect (since HTTP/1.1)In this case, the request should be repeated with another URI; however, future requests should still use the original URI.[2] In contrast to how 302 was historically implemented, the request method is not allowed to be changed when reissuing the original request. For instance, a POST request should be repeated using another POST request.[11]308 Permanent Redirect (approved as experimental RFC)[12]The request, and all future requests should be repeated using another URI. 307 and 308 (as proposed) parallel the behaviours of 302 and 301, but do not allow the HTTP method to change. So, for example, submitting a form to a permanently redirected resource may continue smoothly.4xx Client Error[edit]The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents should display any included entity to the user.400 Bad RequestThe request cannot be fulfilled due to bad syntax.[2]401 UnauthorizedSimilar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.[2] The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource. See Basic access authentication and Digest access authentication.402 Payment RequiredReserved for future use.[2] The original intention was that this code might be used as part of some form of digital cash or micropayment scheme, but that has not happened, and this code is not usually used. As an example of its use, however, Apple's defunct MobileMe service generated a 402 error if the MobileMe account was delinquent.[citation needed] In addition, YouTube uses this status if a particular IP address has made excessive requests, and requires the person to enter a CAPTCHA.403 ForbiddenThe request was a valid request, but the server is refusing to respond to it.[2] Unlike a 401 Unauthorized response, authenticating will make no difference.[2] On servers where authentication is required, this commonly means that the provided credentials were successfully authenticated but that the credentials still do not grant the client permission to access the resource (e.g. a recognized user attempting to access restricted content).404 Not FoundThe requested resource could not be found but may be available again in the future.[2] Subsequent requests by the client are permissible.405 Method Not AllowedA request was made of a resource using a request method not supported by that resource;[2] for example, using GET on a form which requires data to be presented via POST, or using PUT on a read-only resource.406 Not AcceptableThe requested resource is only capable of generating content not acceptable according to the Accept headers sent in the request.[2]407 Proxy Authentication RequiredThe client must first authenticate itself with the proxy.[2]408 Request TimeoutThe server timed out waiting for the request.[2] According to W3 HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."409 ConflictIndicates that the request could not be processed because of conflict in the request, such as an edit conflict.[2]410 GoneIndicates that the resource requested is no longer available and will not be available again.[2] This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource again in the future. Clients such as search engines should remove the resource from their indices. Most use cases do not require clients and search engines to purge the resource, and a "404 Not Found" may be used instead.411 Length RequiredThe request did not specify the length of its content, which is required by the requested resource.[2]412 Precondition FailedThe server does not meet one of the preconditions that the requester put on the request.[2]413 Request Entity Too LargeThe request is larger than the server is willing or able to process.[2]414 Request-URI Too LongThe URI provided was too long for the server to process.[2]415 Unsupported Media TypeThe request entity has a media type which the server or resource does not support.[2] For example, the client uploads an image as image/svg+xml, but the server requires that images use a different format.416 Requested Range Not SatisfiableThe client has asked for a portion of the file, but the server cannot supply that portion.[2] For example, if the client asked for a part of the file that lies beyond the end of the file.[2]417 Expectation FailedThe server cannot meet the requirements of the Expect request-header field.[2]418 I'm a teapot (RFC 2324)This code was defined in 1998 as one of the traditional IETF April Fools' jokes, in RFC 2324, Hyper Text Coffee Pot Control Protocol, and is not expected to be implemented by actual HTTP servers.419 Authentication TimeoutNot a part of the HTTP standard, 419 Authentication Timeout denotes that previously valid authentication has expired. It is used as an alternative to 401 Unauthorized in order to differentiate from otherwise authenticated clients being denied access to specific server resources.420 Enhance Your Calm (Twitter)Not part of the HTTP standard, but returned by the Twitter Search and Trends API when the client is being rate limited.[13] Other services may wish to implement the 429 Too Many Requests response code instead.422 Unprocessable Entity (WebDAV; RFC 4918)The request was well-formed but was unable to be followed due to semantic errors.[4]423 Locked (WebDAV; RFC 4918)The resource that is being accessed is locked.[4]424 Failed Dependency (WebDAV; RFC 4918)The request failed due to failure of a previous request (e.g. a PROPPATCH).[4]424 Method Failure (WebDAV)[14]Indicates the method was not executed on a particular resource within its scope because some part of the method's execution failed causing the entire method to be aborted.425 Unordered Collection (Internet draft)Defined in drafts of "WebDAV Advanced Collections Protocol",[15] but not present in "Web Distributed Authoring and Versioning (WebDAV) Ordered Collections Protocol".[16]426 Upgrade Required (RFC 2817)The client should switch to a different protocol such as TLS/1.0.[17]428 Precondition Required (RFC 6585)The origin server requires the request to be conditional. Intended to prevent "the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict."[18]429 Too Many Requests (RFC 6585)The user has sent too many requests in a given amount of time. Intended for use with rate limiting schemes.[18]431 Request Header Fields Too Large (RFC 6585)The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.[18]444 No Response (Nginx)Used in Nginx logs to indicate that the server has returned no information to the client and closed the connection (useful as a deterrent for malware).449 Retry With (Microsoft)A Microsoft extension. The request should be retried after performing the appropriate action.[19]Often search-engines or custom applications will ignore required parameters. Where no default action is appropriate, the Aviongoo website sends a "HTTP/1.1 449 Retry with valid parameters: param1, param2, . . ." response. The applications may choose to learn, or not.450 Blocked by Windows Parental Controls (Microsoft)A Microsoft extension. This error is given when Windows Parental Controls are turned on and are blocking access to the given webpage.[20]451 Unavailable For Legal Reasons (Internet draft)Defined in the internet draft "A New HTTP Status Code for Legally-restricted Resources".[21] Intended to be used when resource access is denied for legal reasons, e.g. censorship or government-mandated blocked access. A reference to the 1953 dystopian novel Fahrenheit 451, where books are outlawed.[22]451 Redirect (Microsoft)Used in Exchange ActiveSync if there either is a more efficient server to use or the server can't access the users' mailbox.[23]The client is supposed to re-run the HTTP Autodiscovery protocol to find a better suited server.[24]494 Request Header Too Large (Nginx)Nginx internal code similar to 413 but it was introduced earlier.[25][original research?]495 Cert Error (Nginx)Nginx internal code used when SSL client certificate error occurred to distinguish it from 4XX in a log and an error page redirection.496 No Cert (Nginx)Nginx internal code used when client didn't provide certificate to distinguish it from 4XX in a log and an error page redirection.497 HTTP to HTTPS (Nginx)Nginx internal code used for the plain HTTP requests that are sent to HTTPS port to distinguish it from 4XX in a log and an error page redirection.499 Client Closed Request (Nginx)Used in Nginx logs to indicate when the connection has been closed by client while the server is still processing its request, making server unable to send a status code back.[26]5xx Server Error[edit]The server failed to fulfill an apparently valid request.[2]Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.500 Internal Server ErrorA generic error message, given when no more specific message is suitable.[2]501 Not ImplementedThe server either does not recognize the request method, or it lacks the ability to fulfill the request.[2]502 Bad GatewayThe server was acting as a gateway or proxy and received an invalid response from the upstream server.[2]503 Service UnavailableThe server is currently unavailable (because it is overloaded or down for maintenance).[2] Generally, this is a temporary state.504 Gateway TimeoutThe server was acting as a gateway or proxy and did not receive a timely response from the upstream server.[2]505 HTTP Version Not SupportedThe server does not support the HTTP protocol version used in the request.[2]506 Variant Also Negotiates (RFC 2295)Transparent content negotiation for the request results in a circular reference.[27]507 Insufficient Storage (WebDAV; RFC 4918)The server is unable to store the representation needed to complete the request.[4]508 Loop Detected (WebDAV; RFC 5842)The server detected an infinite loop while processing the request (sent in lieu of 208).509 Bandwidth Limit Exceeded (Apache bw/limited extension)This status code, while used by many servers, is not specified in any RFCs.510 Not Extended (RFC 2774)Further extensions to the request are required for the server to fulfil it.[28]511 Network Authentication Required (RFC 6585)The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g. "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).[18]598 Network read timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network read timeout behind the proxy to a client in front of the proxy.[citation needed]599 Network connect timeout error (Unknown)This status code is not specified in any RFCs, but is used by Microsoft HTTP proxies to signal a network connect timeout behind the proxy to a client in front of the proxy.[citation needed]