Network Working Group M. Nottingham
Internet-Draft Akamai
Intended status: Informational E. Wilde
Expires: December 5, 2013 EMC
June 3, 2013
Problem Details for HTTP APIs
draft-nottingham-http-problem-04
Abstract
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.
Note to Readers
This draft should be discussed on the apps-discuss mailing list [1].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 5, 2013.
Copyright Notice
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document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The Problem Details JSON Object . . . . . . . . . . . . . . . 4
3.1. Required Members . . . . . . . . . . . . . . . . . . . . . 5
3.2. Optional Members . . . . . . . . . . . . . . . . . . . . . 5
3.3. Extension Members . . . . . . . . . . . . . . . . . . . . 6
4. Defining New Problem Types . . . . . . . . . . . . . . . . . . 6
4.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. HTTP Problems and XML . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
HTTP [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.
This specification defines simple JSON [RFC4627] and XML
[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.
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).
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.
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; for example, triggering a transfer of more credit into
the acccount.
This specification does this by identifying a specific type of
problem (e.g., "out of credit") with a URI [RFC3986]; APIs can do
this by nominating new URIs under their control, or by reusing
existing ones.
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.
The data model for problem details is a JSON [RFC4627] object; when
formatted as a JSON document, it uses the "application/
api-problem+json" media type. Appendix A defines how to express them
in an equivalent XML format, which uses the "application/
api-problem+xml" media type.
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
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representation of a resource, for example, it's often preferable to
accommodate describing the relevant details in that application's
format. Likewise, in many situations, there is an appropriate HTTP
status code that does not require extra detail to be conveyed.
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.
2. Requirements
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 [RFC2119].
3. The Problem Details JSON Object
The canonical model for problem details is a JSON [RFC4627] object.
When serialised as a JSON document, that format is identified with
the "application/api-problem+json" media type.
For example, a HTTP response carrying JSON problem details:
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"]
}
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.
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Note that "problemType" is case-sensitive in the JSON object, as are
all other member names.
3.1. Required Members
A problem details object MUST have the following members:
o "problemType" (string) - An absolute URI [RFC3986] that identifies
the problem type. When dereferenced, it SHOULD provide human-
readable documentation for the problem type (e.g., using HTML).
o "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.
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.
3.2. Optional Members
Furthermore, a problem details object MAY have the following members:
o "httpStatus" (number) - The HTTP status code ([RFC2616], Section
6) generated by the origin server for this occurrence of the
problem.
o "detail" (string) - An human readable explanation specific to this
occurrence of the problem.
o "problemInstance" (string) - An absolute URI that identifies the
specific occurrence of the problem. It may or may not yield
further information if dereferenced.
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 Section 5 for further caveats
regarding its use.
The detail member, if present, SHOULD focus on helping the client
correct the problem, rather than giving debugging information.
Consumers SHOULD NOT parse the detail member for information;
extensions are more suitable and less error-prone ways to obtain such
information.
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3.3. Extension Members
Finally, problem type definitions MAY extend the problem details
object with additional members.
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.
4. Defining New Problem Types
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.
Before doing so, it's important to understand what they are good for,
and what's better left to other mechanisms.
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 (Section 5); in particular the
risk of exposing attack vectors by exposing implementation internals
through error messages.
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.
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.
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).
New problem type definitions MUST document:
1. A problemType URI (typically, with the "http" scheme),
2. A title that appropriately describes it (think short), and
3. The HTTP status code for it to be used with.
Problem types MAY specify the use of the Retry-After response header
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in appropriate circumstances.
A problem's problemType URI SHOULD resolve to HTML documentation that
explains how to resolve the problem.
A problem type definition MAY specify additional members on the
Problem Details object. For example, an extension might use typed
links [RFC5988] to another resource that can be used by machines to
resolve the problem.
If such additional members are defined, their names SHOULD start with
a letter (ALPHA, as per [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.
4.1. Example
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.
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.
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.
If one isn't available, you could 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.
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.
5. Security Considerations
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
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scrutinised.
Risks include leaking information that can be exploited to compromise
the system, access to the system, or the privacy of users of the
system.
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.
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.
6. IANA Considerations
This specification defines two new Internet media types [RFC6838]:
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
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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
7. Acknowledgements
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).
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References
[ISO-19757-2]
International Organization for Standardization,
"Information Technology --- Document Schema Definition
Languages (DSDL) --- Part 2: Grammar-based Validation ---
RELAX NG", ISO/IEC 19757-2, 2003.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013.
[W3C.REC-xml-20081126]
Sperberg-McQueen, C., Yergeau, F., Bray, T., Maler, E.,
and J. Paoli, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
URIs
[1] <https://www.ietf.org/mailman/listinfo/apps-discuss>
Appendix A. HTTP Problems and XML
Some HTTP-based APIs use XML [W3C.REC-xml-20081126] as their primary
format convention. Such APIs MAY express problem details using the
format defined in this appendix.
The OPTIONAL RELAX NG schema [ISO-19757-2] for the XML format is:
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default namespace ns = "urn:ietf:rfc:XXXX"
start |= problem
problem =
element problem {
(element problemType { xsd:anyURI }
& element title { xsd:string }
& element detail { xsd:string }?
& element httpStatus { xsd:positiveInteger }?
& element problemInstance { xsd:anyURI }?),
anyNsElement
}
anyNsElement =
(element ns:* { anyNsElement | text }
| attribute * { text })*
The media type for this format is "application/api-problem+xml".
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:
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>
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
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using that namespace.
Authors' Addresses
Mark Nottingham
Akamai
Email: mnot@mnot.net
URI: http://www.mnot.net/
Erik Wilde
EMC
6801 Koll Center Parkway
Pleasanton, CA 94566
U.S.A.
Phone: +1-925-6006244
Email: erik.wilde@emc.com
URI: http://dret.net/netdret/
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