One document matched: draft-nottingham-http-problem-03.xml
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<!ENTITY problem-schema SYSTEM 'http-problem-03.rnc'>
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<rfc ipr="trust200902" docName="draft-nottingham-http-problem-03" category="info">
<front>
<title abbrev="Problem Details">Problem Details for HTTP APIs</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Akamai</organization>
<address>
<email>mnot@mnot.net</email>
<uri>http://www.mnot.net/</uri>
</address>
</author>
<author initials="E." surname="Wilde" fullname="Erik Wilde">
<organization>EMC</organization>
<address>
<email>erik.wilde@emc.com</email>
</address>
</author>
<date year="2013"/>
<keyword>status</keyword>
<keyword>HTTP</keyword>
<keyword>error</keyword>
<keyword>problem</keyword>
<keyword>API</keyword>
<keyword>JSON</keyword>
<keyword>XML</keyword>
<abstract>
<t>This document defines a "problem detail" as a way to carry
machine-readable details of errors in a HTTP response, to avoid the need
to invent new error response formats for HTTP APIs.</t>
</abstract>
<note title="Note to Readers">
<t>This draft should be discussed on the <eref
target="https://www.ietf.org/mailman/listinfo/apps-discuss">apps-discuss
mailing list</eref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>HTTP <xref target="RFC2616"/> status codes are sometimes not
sufficient to convey helpful information about an error. While humans
behind Web browsers can be informed about the nature of the problem with
an HTML response body, non-human consumers of so-called "HTTP APIs" are
usually not.</t>
<t>This specification defines simple JSON <xref target="RFC4627"/> and
XML <xref target="W3C.REC-xml-20081126"/> document formats to suit this
purpose. They are designed to be reused by HTTP APIs, which can identify
distinct "problem types" specific to their needs.</t>
<t>Thus, API clients can be informed of both the high-level error
class (using the status code) and the finer-grained details of the
problem (using one of these formats).</t>
<t>Consider a response that indicates that the client's
account doesn't have enough credit. The 403 Forbidden status code
might be deemed most appropriate to use, as it will inform HTTP-generic
software (such as client libraries, caches and proxies) of the general
semantics of the response.</t>
<t>However, that doesn't give the API client enough information about
why the request was forbidden, the applicable account balance, or how to
correct the problem. If these details are included in the response body
in a machine-readable format, the client can treat it appropriately.</t>
<t>This specification does this by identifying a specific type of
problem (e.g., "out of credit") with a URI <xref target="RFC3986"/>;
APIs can do this by nominating new URIs under their control, or by
reusing existing ones.</t>
<t>Additionally, problems can contain other information, such as a URI
that identifies the specific occurrence of the problem (effectively
giving an identifier to the concept "The time Joe didn't have enough
credit last Thursday"), which may be useful for support or forensic
purposes. See below for a full list.</t>
<t>The data model for problem details is a JSON <xref target="RFC4627"/>
object; when formatted as a JSON document, it uses the
"application/api-problem+json" media type. <xref target="xml-syntax"/>
defines how to express them in an equivalent XML format, which uses
the "application/api-problem+xml" media type.</t>
<t>Note that problem details are (naturally) not the only way to convey
the details of a problem in HTTP; if the response is still a
representation of a resource, for example, it's often preferable to
accommodate describing the relevant details in that application's
format.</t>
<t>Instead, the aim of this specification is to define common error
formats for those applications that need one, so that they aren't
required to define their own, or worse, tempted to re-define the
semantics of existing HTTP status codes.</t>
</section>
<section title="Requirements">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"/>.</t>
</section>
<section title="The Problem Details JSON Object">
<t>The canonical model for problem details is a JSON <xref
target="RFC4627"/> object.</t>
<t>When serialised as a JSON document, that format is identified with
the "application/api-problem+json" media type.</t>
<t>For example, a HTTP response carrying JSON problem details:</t>
<figure><artwork><![CDATA[
HTTP/1.1 403 Forbidden
Content-Type: application/api-problem+json
Content-Language: en
{
"problemType": "http://example.com/probs/out-of-credit",
"title": "You do not have enough credit.",
"detail": "Your current balance is 30, but that costs 50.",
"problemInstance": "http://example.net/account/12345/msgs/abc",
"balance": 30,
"accounts": ["http://example.net/account/12345",
"http://example.net/account/67890"]
}]]></artwork></figure>
<t>Here, the out-of-credit problem (identified by its problemType URI)
indicates the reason for the 403 in "title", gives a reference for the
specific problem occurrence with "problemInstance", gives
occurrence-specific details in "detail", and adds two extensions;
"balance" conveys the account's balance, and "account" gives a link
where the account can be topped up.</t>
<t>Note that "problemType" is case-sensitive in the JSON object, as are
all other member names.</t>
<section title="Required Members">
<t>A problem details object MUST have the following members:</t>
<t><list style='symbols'>
<t>"problemType" (string) - An absolute URI <xref
target="RFC3986"/> that identifies the problem type. When
dereferenced, it SHOULD provide human-readable documentation for
the problem type (e.g., using HTML).</t>
<t>"title" (string) - A short, human-readable summary of the
problem type. It SHOULD NOT change from occurrence to occurrence of
the problem, except for purposes of localisation.</t>
</list></t>
<t>Consumers MUST use the problemType string as the primary
identifier for the problem type; the title string is advisory, and
included only for users who are not aware of the semantics of the
URI, and don't have the ability to discover them (e.g., offline log
analysis). Consumers SHOULD NOT automatically dereference the
problemType URI.</t>
</section>
<section title="Optional Members">
<t>Furthermore, a problem details object MAY have the following
members:</t>
<t><list style='symbols'>
<t>"httpStatus" (number) - The HTTP status code set by the origin
server for this occurrence of the problem.</t>
<t>"detail" (string) - An human readable explanation specific to
this occurrence of the problem.</t>
<t>"problemInstance" (string) - An absolute URI that identifies the
specific occurrence of the problem. It may or may not yield
further information if dereferenced.</t>
</list></t>
<t>The httpStatus member, if present, is only advisory; it conveys the
HTTP status code used for the convenience of the consumer. Generators
MUST use the same status code in the actual HTTP response, to assure
that generic HTTP software that does not understand this format still
behaves correctly. See <xref target="security-considerations"/> for
further caveats regarding its use.</t>
<t>The detail member, if present, SHOULD focus on helping the
client correct the problem, rather than giving debugging
information.</t>
<t>Consumers SHOULD NOT be parse the detail member for information;
extensions are more suitable and less error-prone ways to obtain
such information.</t>
</section>
<section title="Extension Members">
<t>Finally, problem type definitions MAY extend the problem details
object with additional members.</t>
<t>Clients consuming problem details MUST ignore any such extensions
that they don't recognise; this allows problem types to evolve and
include additional information in the future.</t>
</section>
</section>
<section anchor="defining" title="Defining New Problem Types">
<t>When an HTTP API needs to define a response that indicates an error
condition, it might be appropriate to do so by defining a new problem
type.</t>
<t>Before doing so, it's important to understand what they are good for,
and what's better left to other mechanisms.</t>
<t>Problem details are not a debugging tool for the underlying
implementation; rather, they are a way to expose greater detail about
the HTTP interface itself. New problem types need to carefully consider
the Security Considerations (<xref target="security-considerations"/>);
in particular the risk of exposing attack vectors by exposing
implementation internals through error messages.</t>
<t>Likewise, truly generic problems – i.e., conditions that could
potentially apply to any resource on the Web – are usually better
expressed as plain status codes. For example, a "write access
disallowed" problem is probably unnecessary, since a 403 Forbidden
status code on a PUT request is self-explanatory.</t>
<t>Finally, an application may have a more appropriate way to carry
an error in a format that it already defines. Problem details
are intended to avoid the necessity of establishing new "fault" or
"error" document formats, not to replace existing domain-specific
formats.</t>
<t>That said, it is possible to add support for problem details to
existing HTTP APIs using HTTP content negotiation (e.g., using the
Accept request header to indicate a preference for this format).</t>
<t>New problem type definitions MUST document:
<list style="numbers">
<t>A problemType URI (typically, with the "http" scheme),</t>
<t>A title that appropriately describes it (think short), and</t>
<t>The HTTP status code for it to be used with.</t>
</list></t>
<t>Problem types MAY specify the use of the Retry-After response
header in appropriate circumstances.</t>
<t>A problem's problemType URI SHOULD resolve to HTML documentation
that explains how to resolve the problem.</t>
<t>A problem type definition MAY specify additional members on the
Problem Details object. For example, an extension might use
typed links <xref target="RFC5988"/> to another resource that can be
used by machines to resolve the problem.</t>
<t>If such additional members are defined, their names SHOULD start with
a letter (ALPHA, as per <xref target="RFC5234"/> and SHOULD consist of
characters from ALPHA, DIGIT and "_" (so that it can be serialised in
formats other than JSON), and SHOULD be three characters or longer.</t>
<section title="Example">
<t>For example, if your are publishing an HTTP API to your online
shopping cart, you might need to indicate that the user is out of
credit (our example from above), and therefore cannot make the
purchase.</t>
<t>If you already have an application-specific format that can
accommodate this information, it's probably best to do that. However,
if you don't, you might consider using one of the problem details
formats; JSON if your API is JSON-based, or XML if it uses that
convention.</t>
<t>To do so, you might look for an already-defined problemType URI
that suits your purposes. If one is available, you can reuse that URI
by documenting use of both this specification and that specific
URI value in your API.</t>
<t>If one isn't available, you'd reference this specification, mint
and document a new problemType URI (which ought to be under your
control and stable over time), an appropriate title and the HTTP
status code that it will be used with, along with what it means and
how it should be handled. This information could appear within the
rest of your API's documentation.</t>
<t>In summary: a problemInstance URI will always identify a specific
occurrence of a problem. On the other hand, problemType URIs can be
reused if an appropriate description of a problem type is already
available someplace else, or they can be created for new problem
types.</t>
</section>
</section>
<section anchor="security-considerations" title="Security Considerations">
<t>When defining a new problem type, the information included must be
carefully vetted. Likewise, when actually generating a problem –
however it is serialised – the details given must also be
scrutinised.</t>
<t>Risks include leaking information that can be exploited to
compromise the system, access to the system, or the privacy of users
of the system.</t>
<t>Generators providing links to occurrence information are encouraged
to avoid making implementation details such as a stack dump available
through the HTTP interface, since this can expose sensitive details of
the server implementation, its data, and so on.</t>
<t>The "httpStatus" member duplicates the information available in the
HTTP status code itself, thereby bringing the possibility of
disagreement between the two. Their relative precedence is not clear,
since a disagreement might indicate that (for example) an intermediary
has modified the HTTP status code in transit. As such, those defining
problem types as well as generators and consumers of problems need to
be aware that generic software (such as proxies, load balancers,
firewalls, virus scanners) are unlikely to know of or respect the
status code conveyed in this member.</t>
</section>
<section title="IANA Considerations">
<t>This specification defines two new Internet media types
<xref target="RFC6838"/>:</t>
<figure><artwork><![CDATA[
Type name: application
Subtype name: api-problem+json
Required parameters: None
Optional parameters: None; unrecognised parameters
should be ignored
Encoding considerations: Same as [RFC4627]
Security considerations: see [this document]
Interoperability considerations: None.
Published specification: [this document]
Applications that use this media type: HTTP
Additional information:
Magic number(s): n/a
File extension(s): n/a
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Mark Nottingham <mnot@mnot.net>
Intended usage: COMMON
Restrictions on usage: None.
Author: Mark Nottingham <mnot@mnot.net>
Change controller: IESG
]]></artwork></figure>
<figure><artwork><![CDATA[
Type name: application
Subtype name: api-problem+xml
Required parameters: None
Optional parameters: None; unrecognised parameters
should be ignored
Encoding considerations: Same as [RFC3023]
Security considerations: see [this document]
Interoperability considerations: None.
Published specification: [this document]
Applications that use this media type: HTTP
Additional information:
Magic number(s): n/a
File extension(s): n/a
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Mark Nottingham <mnot@mnot.net>
Intended usage: COMMON
Restrictions on usage: None.
Author: Mark Nottingham <mnot@mnot.net>
Change controller: IESG
]]></artwork></figure>
</section>
<section title="Acknowledgements">
<t>The authors would like to thank
Jan Algermissen,
Mike Amundsen,
Subbu Allamaraju,
Roy Fielding,
Sam Johnston,
Mike McCall,
Julian Reschke, and
James Snell
for early review of this specification (even if some disagree
with parts of it).
</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc2616;
&rfc3986;
&rfc4627;
&rfc5234;
</references>
<references title="Informative References">
&xml;
<reference anchor="ISO-19757-2">
<front>
<title>Information Technology --- Document Schema Definition Languages (DSDL) --- Part 2: Grammar-based Validation --- RELAX NG</title>
<author>
<organization>International Organization for Standardization</organization>
</author>
<date year="2003"/>
</front>
<seriesInfo name="ISO/IEC" value="19757-2"/>
</reference>
&rfc3023;
&rfc5988;
&rfc6838;
</references>
<section anchor="xml-syntax" title="HTTP Problems and XML">
<t>Some HTTP-based APIs use XML <xref
target="W3C.REC-xml-20081126"/> as their primary format convention. Such
APIs MAY express problem details using the format defined in this
appendix.</t>
<t>The OPTIONAL RELAX NG schema <xref target="ISO-19757-2"/>
for the XML format is:</t>
<figure><artwork>&problem-schema;</artwork></figure>
<t>The media type for this format is "application/api-problem+xml".</t>
<t>Extension arrays and objects can be serialised into the XML format by
considering an element containing a child or children to represent an
object, except for elements that contain only child element(s) named 'i',
which are considered arrays. For example, an alternate version of the
example above would appear in XML as:</t>
<figure><artwork><![CDATA[
HTTP/1.1 403 Forbidden
Content-Type: application/api-problem+xml
Content-Language: en
<?xml version="1.0" encoding="UTF-8"?>
<problem xmlns="urn:ietf:rfc:XXXX">
<problemType>http://example.com/probs/out-of-credit</problemType>
<title>You do not have enough credit.</title>
<detail>Your current balance is 30, but that costs 50.</detail>
<problemInstance>
http://example.net/account/12345/msgs/abc
</problemInstance>
<balance>30</balance>
<accounts>
<i>http://example.net/account/12345</i>
<i>http://example.net/account/67890</i>
</accounts>
</problem>]]></artwork></figure>
<t>Note that this format uses an XML Namespace. This is primarily to allow
embedding it into other formats; it does not imply that it can be extended
with content from other namespaces. The RELAX NG schema explicitly only
allows elements from the one namespace used in the XML format. Any
extension arrays and objects MUST be serialised using that namespace.</t>
</section>
</back>
</rfc>
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