Network Working Group J. Jena
Internet-Draft PayPal, Inc.
Intended status: Standards Track S. Dalal
Expires: May 27, 2021 November 23, 2020
The Idempotency HTTP Header Field
draft-idempotency-header-01
Abstract
The "HTTP" Idempotency request header field can be used to carry
idempotency key in order to make non-idempotent "HTTP" methods such
as "POST" or "PATCH" fault-tolerant.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Idempotency HTTP Request Header Field . . . . . . . . . . 3
2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Uniqueness of Idempotency Key . . . . . . . . . . . . . . 4
2.3. Idempotency Key Validity and Expiry . . . . . . . . . . . 4
2.4. Idempotency Fingerprint . . . . . . . . . . . . . . . . . 4
2.5. Responsibilities . . . . . . . . . . . . . . . . . . . . 4
2.6. Idempotency Enforcement Scenarios . . . . . . . . . . . . 5
2.7. Error Scenarios . . . . . . . . . . . . . . . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
3.1. The Idempotency-Key HTTP Request Header Field . . . . . . 6
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 7
4.1. Implementing the Concept . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 12
Appendix B. Appendix . . . . . . . . . . . . . . . . . . . . . . 12
B.1. Appendix A. Imported ABNF . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Idempotence is the property of certain operations in mathematics and
computer science whereby they can be applied multiple times without
changing the result beyond the initial application. It does not
matter if the operation is called only once, or 10s of times over.
The result "SHOULD" be the same.
Idempotency is important in building a fault-tolerant "HTTP API". An
"HTTP" request method is considered "idempotent" if the intended
effect on the server of multiple identical requests with that method
is the same as the effect for a single such request. According to
[RFC7231], "HTTP" methods "OPTIONS", "HEAD", "GET", "PUT" and
"DELETE" are idempotent while methods "POST" and "PATCH" are not.
Let's say a client of an "HTTP API" wants to create (or update) a
resource using a "POST" method. Since "POST" is "NOT" an idempotent
method, calling it multiple times can result in duplication or wrong
updates. Consider a scenario where the client sent a "POST" request
to the server, but it got a timeout. Following questions arise : Is
the resource actually created (or updated)? Did the timeout occur
during sending of the request, or when receiving of the response?
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Can the client safely retry the request, or does it need to figure
out what happened in the first place? If "POST" had been an
idempotent method, such questions may not arise. Client would safely
retry a request until it actually gets a valid response from the
server.
For many use cases of "HTTP API", duplicate resource is a severe
problem from business perspective. For example, duplicate records
for requests involving any kind of money transfer "MUST NOT" be
allowed. In other cases, processing of duplicate webhook delivery is
not expected.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] and includes, by reference, the IMF-fixdate
rule as defined in Section 7.1.1.1 of [RFC7231].
The term "resource" is to be interpreted as defined in Section 2 of
[RFC7231], that is identified by an URI. The term "resource server"
is to be interpreted as "origin server" as defined in Section 3 of
[RFC7231].
2. The Idempotency HTTP Request Header Field
An idempotency key is a unique value generated by the client which
the resource server uses to recognize subsequent retries of the same
request. The "Idempotency-Key" HTTP request header field carries
this key.
2.1. Syntax
The "Idempotency-Key" request header field describes
Idempotency-Key = idempotency-key-value
idempotency-key-value = opaque-value
opaque-value = DQUOTE *idempotencyvalue DQUOTE
idempotencyvalue = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
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Clients MUST NOT include more than one "Idempotency-Key" header field
in the same request.
The following example shows an idempotency key using "UUID"
[RFC4122]:
Idempotency-Key: "8e03978e-40d5-43e8-bc93-6894a57f9324"
2.2. Uniqueness of Idempotency Key
The idempotency key that is supplied as part of every "POST" request
MUST be unique and "MUST" not be reused with another request with a
different request payload.
Uniqueness of the key "MUST" be defined by the resource owner and
"MUST" be implemented by the clients of the resource server. It is
"RECOMMENDED" that "UUID" [RFC4122] or a similar random identifier be
used as an idempotency key.
2.3. Idempotency Key Validity and Expiry
The resource MAY enforce time based idempotency keys, thus, be able
to purge or delete a key upon its expiry. The resource server SHOULD
define such expiration policy and publish in related documentation.
2.4. Idempotency Fingerprint
An idempotency fingerprint MAY be used in conjunction with an
idempotency key to determine the uniqueness of a request. Such a
fingerprint is generated from request payload data by the resource
server. An idempotency fingerprint generation algorithm MAY use one
of the following or similar approaches to create a fingerprint.
o Checksum of the entire request payload.
o Checksum of selected element(s) in the request payload.
o Field value match for each field in the request payload.
o Field value match for selected element(s) in the request payload.
o Request digest/signature.
2.5. Responsibilities
Client
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Clients of "HTTP API" requiring idempotency, SHOULD understand the
idempotency related requirements as published by the server and use
appropriate algorithm to generate idempotency keys.
For each request, client SHOULD
o Send a unique idempotency key in the HTTP "Idempotency-Key"
request header field.
Resource Server
Resource server MUST publish idempotency related specification. This
specification MUST include expiration related policy if applicable.
Server is responsible for managing the lifecycle of the idempotency
key.
For each request, server SHOULD
o Identify idempotency key from the HTTP "Idempotency-Key" request
header field.
o Generate idempotency fingerprint if required.
o Check for idempotency considering various scenarios including the
ones described in section below.
2.6. Idempotency Enforcement Scenarios
o First time request (idempotency key or fingerprint has not been
seen)
The resource server SHOULD process the request normally and
respond with an appropriate response and status code.
o Duplicate request (idempotency key or fingerprint has been seen)
Retry
The request was retried after the original request completed. The
resource server MUST respond with the result of the previously
completed operation, success or an error.
Concurrent Request
The request was retried before the original request completed.
The resource server MUST respond with a resource conflict error.
See Error Scenarios for details.
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2.7. Error Scenarios
If the "Idempotency-Key" request header is missing for a documented
idempotent operation requiring this header, the resource server MUST
reply with an "HTTP" "400" status code with body containing a link
pointing to relevant documentation. Alternately, using the "HTTP"
header "Link", the client can be informed about the error as shown
below.
HTTP/1.1 400 Bad Request
Link: <https://developer.example.com/idempotency>;
rel="describedby"; type="text/html"
If there is an attempt to reuse an idempotency key with a different
request payload, the resource server MUST reply with a "HTTP" "422"
status code with body containing a link pointing to relevant
documentation. The status code "422" is defined in Section 11.2 of
[RFC4918]. The server can also inform the client by using the "HTTP"
header "Link" as shown below.
HTTP/1.1 422 Unprocessable Entity
Link: <https://developer.example.com/idempotency>;
rel="describedby"; type="text/html"
If the request is retried, while the original request is still being
processed, the resource server MUST reply with an "HTTP" "409" status
code with body containing a link or the "HTTP" header "Link" pointing
to the relevant documentation.
HTTP/1.1 409 Conflict
Link: <https://developer.example.com/idempotency>;
rel="describedby"; type="text/html"
For other errors, the resource MUST return the appropriate status
code and error message.
3. IANA Considerations
3.1. The Idempotency-Key HTTP Request Header Field
The "Idempotency-Key" request header should be added to the permanent
registry of message header fields (see [RFC3864]), taking into
account the guidelines given by HTTP/1.1 [RFC7231].
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Header Field Name: Idempotency-Key
Applicable Protocol: Hypertext Transfer Protocol (HTTP)
Status: Standard
Authors:
Jayadeba Jena
Email: jjena@paypal.com
Sanjay Dalal
Email: sanjay.dalal@cal.berkeley.edu
Change controller: IETF
Specification document: this specification,
Section 2 "The Idempotency HTTP Request Header Field"
4. Implementation Status
Note to RFC Editor: Please remove this section before publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
Organization: Stripe
o Description: Stripe uses custom HTTP header named "Idempotency-
Key"
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o Reference: https://stripe.com/docs/idempotency
Organization: Adyen
o Description: Adyen uses custom HTTP header named "Idempotency-Key"
o Reference: https://docs.adyen.com/development-resources/api-
idempotency/
Organization: Dwolla
o Description: Dwolla uses custom HTTP header named "Idempotency-
Key"
o Reference: https://docs.dwolla.com/
Organization: Interledger
o Description: Interledger uses custom HTTP header named
"Idempotency-Key"
o Reference: https://github.com/interledger/
Organization: WorldPay
o Description: WorldPay uses custom HTTP header named "Idempotency-
Key"
o Reference: https://developer.worldpay.com/docs/wpg/idempotency
Organization: Yandex
o Description: Yandex uses custom HTTP header named "Idempotency-
Key"
o Reference: https://cloud.yandex.com/docs/api-design-
guide/concepts/idempotency
4.1. Implementing the Concept
This is a list of implementations that implement the general concept,
but do so using different mechanisms:
Organization: Django
o Description: Django uses custom HTTP header named
"HTTP_IDEMPOTENCY_KEY"
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o Reference: https://pypi.org/project/django-idempotency-key
Organization: Twilio
o Description: Twilio uses custom HTTP header named "I-Twilio-
Idempotency-Token" in webhooks
o Reference: https://www.twilio.com/docs/usage/webhooks/webhooks-
connection-overrides
Organization: PayPal
o Description: PayPal uses custom HTTP header named "PayPal-Request-
Id"
o Reference: https://developer.paypal.com/docs/business/develop/
idempotency
Organization: RazorPay
o Description: RazorPay uses custom HTTP header named "X-Payout-
Idempotency"
o Reference: https://razorpay.com/docs/razorpayx/api/idempotency/
Organization: OpenBanking
o Description: OpenBanking uses custom HTTP header called "x-
idempotency-key"
o Reference: https://openbankinguk.github.io/read-write-api-
site3/v3.1.6/profiles/read-write-data-api-profile.html#request-
headers
Organization: Square
o Description: To make an idempotent API call, Square recommends
adding a property named "idempotency_key" with a unique value in
the request body.
o Reference: https://developer.squareup.com/docs/build-basics/using-
rest-api
Organization: Google Standard Payments
o Description: Google Standard Payments API uses a property named
"requestId" in request body in order to provider idempotency in
various use cases.
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o Reference: https://developers.google.com/standard-payments/
payment-processor-service-api/rest/v1/TopLevel/capture
Organization: BBVA
o Description: BBVA Open Platform uses custom HTTP header called "X-
Unique-Transaction-ID"
o Reference:
https://bbvaopenplatform.com/apiReference/APIbasics/content/x-
unique-transaction-id
Organization: WebEngage
o Description: WebEngage uses custom HTTP header called "x-request-
id" to identify webhook POST requests uniquely to achieve events
idempotency.
o Reference: https://docs.webengage.com/docs/webhooks
5. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to the idempotency
keys.
For idempotent request handling, the resources MAY make use of the
value in the idempotency key to look up a cache or a persistent store
for duplicate requests matching the key. If the resource does not
validate the value of the idempotency key prior to performing such a
lookup, it MAY lead to various forms of security attacks and
compromise. To avoid such situations, the resource SHOULD publish
the expected format of the idempotency key, algorithm used to
generate it and always validate the key value as per the published
specification before processing any request.
6. Examples
The first example shows an idempotency-key header field with key
value using UUID version 4 scheme:
Idempotency-Key: "8e03978e-40d5-43e8-bc93-6894a57f9324"
Second example shows an idempotency-key header field with key value
using some random string generator:
Idempotency-Key: "clkyoesmbgybucifusbbtdsbohtyuuwz"
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
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[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
7.3. URIs
[1] https://github.com/paypal/api-standards/blob/master/
patterns.md#idempotency
[2] https://stripe.com/docs/idempotency
[3] https://tools.ietf.org/html/draft-nottingham-http-poe-00
Appendix A. Acknowledgments
The authors would like to thank Mark Nottingham for his support for
this Internet Draft. We would like to acknowledge that this draft is
inspired by Idempotency related patterns described in API
documentation of PayPal [1] and Stripe [2] as well as Internet Draft
on POST Once Exactly [3] authored by Mark Nottingham.
The authors take all responsibility for errors and omissions.
Appendix B. Appendix
B.1. Appendix A. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [RFC7230]:
obs-text = <obs-text, see [RFC7230], Section 3.2.6>
Authors' Addresses
Jayadeba Jena
PayPal, Inc.
Email: jjena@paypal.com
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Sanjay Dalal
Email: sanjay.dalal@cal.berkeley.edu
URI: https://github.com/sdatspun2
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