Network Working Group                                            J. Jena
Intended status: Standards Track                                S. Dalal
Expires: 13 May 2023                                     9 November 2022

                 The Idempotency-Key HTTP Header Field


   The HTTP Idempotency-Key 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.

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   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Building Blocks for
   HTTP APIs Working Group mailing list (, which is
   archived at

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Table of Contents

   1.  Introduction
     1.1.  Notational Conventions
   2.  The Idempotency-Key HTTP Request Header Field
     2.1.  Syntax
     2.2.  Uniqueness of Idempotency Key
     2.3.  Idempotency Key Validity and Expiry
     2.4.  Idempotency Fingerprint
     2.5.  Responsibilities
     2.6.  Idempotency Enforcement Scenarios
     2.7.  Error Scenarios
   3.  IANA Considerations
     3.1.  The Idempotency-Key HTTP Request Header Field
   4.  Implementation Status
     4.1.  Implementing the Concept
   5.  Security Considerations
   6.  Examples
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Imported ABNF
   Authors' Addresses

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?
   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

   For many use cases of HTTP APIs, duplicated resources are a severe
   problem from a 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

   {::boilerplate bcp14-tagged}

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] and includes, by reference, the IMF-fixdate
   rule as defined in Section 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

2.  The Idempotency-Key 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

   "Idempotency-Key" is an Item Structured Header [RFC8941].  Its value
   MUST be a String.  Refer to Section 3.3.3 of [RFC8941] for ABNF of

   Idempotency-Key = sf-string

   Clients MUST NOT include more than one "Idempotency-Key" header field
   in the same request.

   The following example shows an idempotency key using "UUID"

   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 it in the 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.

   *  Checksum of the entire request payload.

   *  Checksum of selected element(s) in the request payload.

   *  Field value match for each field in the request payload.

   *  Field value match for selected element(s) in the request payload.

   *  Request digest/signature.

2.5.  Responsibilities


   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

   *  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

   For each request, server SHOULD

   *  Identify idempotency key from the HTTP "Idempotency-Key" request
      header field.

   *  Generate idempotency fingerprint if required.

   *  Check for idempotency considering various scenarios including the
      ones described in section below.

2.6.  Idempotency Enforcement Scenarios

   *  First time request (idempotency key and fingerprint has not been

      The resource server SHOULD process the request normally and
      respond with an appropriate response and status code.

   *  Duplicate request (idempotency key and fingerprint has been seen)


      The request was retried after the original request completed.  The
      resource server SHOULD respond with the result of the previously
      completed operation, success or an error.  See Error Scenarios for
      details on errors.

      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.

2.7.  Error Scenarios

   If the "Idempotency-Key" request header is missing for a documented
   idempotent operation requiring this header, the resource server
   SHOULD reply with an HTTP "400" status code with body containing a
   link pointing to relevant documentation.  Following examples shows an
   error response describing the problem using [RFC7807].

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json
Content-Language: en
  "type": "",
  "title": "Idempotency-Key is missing",
  "detail": "This operation is idempotent and it requires correct usage of Idempotency Key.",

   Alternately, using the HTTP header "Link", the client can be informed
   about the error as shown below.

   HTTP/1.1 400 Bad Request
   Link: <>;
     rel="describedby"; type="text/html"

   If there is an attempt to reuse an idempotency key with a different
   request payload, the resource server SHOULD 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

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/problem+json
Content-Language: en
  "type": "",
  "title": "Idempotency-Key is already used",
  "detail": "This operation is idempotent and it requires correct usage of Idempotency Key. Idempotency Key MUST not be reused across different payloads of this operation.",

   The server can also inform the client by using the HTTP header "Link"
   as shown below.

   HTTP/1.1 422 Unprocessable Entity
   Link: <>;
   rel="describedby"; type="text/html"

   If the request is retried, while the original request is still being
   processed, the resource server SHOULD reply with an HTTP "409" status
   code with body containing problem description.

HTTP/1.1 409 Conflict
Content-Type: application/problem+json
Content-Language: en
  "type": "",
  "title": "A request is outstanding for this Idempotency-Key",
  "detail": "A request with the same Idempotency-Key for the same operation is being processed or is outstanding.",

   Or, alternately using the HTTP header "Link" pointing to the relevant

   HTTP/1.1 409 Conflict
   Link: <>;
   rel="describedby"; type="text/html"

   Error scenarios above describe the status of failed idempotent
   requests after the resource server prcocesses them.  Clients MUST
   correct the requests (with the execption of 409 where no correction
   is required) before performing a retry operation, or the the resource
   server MUST fail the request and return one of the above errors.

   For other 4xx/5xx errors, such as 401, 403, 500, 502, 503, 504, 429,
   or any other HTTP error code that is not listed here, the client
   SHOULD act appropriately by following the resource server's

3.  IANA Considerations

3.1.  The Idempotency-Key HTTP Request Header Field

   The Idempotency-Key field name should be added to the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry".

   Field Name:  Idempotency-Key

   Status:  permanent

   Specification document:  This specification, Section 2

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

   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

   *  Description: Stripe uses custom HTTP header named "Idempotency-

   *  Reference:

   Organization: Adyen

   *  Description: Adyen uses custom HTTP header named "Idempotency-Key"

   *  Reference:

   Organization: Dwolla

   *  Description: Dwolla uses custom HTTP header named "Idempotency-

   *  Reference:

   Organization: Interledger

   *  Description: Interledger uses custom HTTP header named

   *  Reference:

   Organization: WorldPay

   *  Description: WorldPay uses custom HTTP header named "Idempotency-

   *  Reference:

   Organization: Yandex

   *  Description: Yandex uses custom HTTP header named "Idempotency-

   *  Reference:


   *  Description: Http4s is a minimal, idiomatic Scala interface for
      HTTP services.

   *  Reference:

   Organization: Finastra

   *  Description: Finastra uses custom HTTP header named "Idempotency-

   *  Reference:

   Organization: Datatrans

   *  Description: Datatrans focuses on the technical processing of
      payments, including hosting smart payment forms and correctly
      routing payment information.

   *  Reference:

4.1.  Implementing the Concept

   This is a list of implementations that implement the general concept,
   but do so using different mechanisms:

   Organization: Django

   *  Description: Django uses custom HTTP header named

   *  Reference:

   Organization: Twilio

   *  Description: Twilio uses custom HTTP header named "I-Twilio-
      Idempotency-Token" in webhooks

   *  Reference:

   Organization: PayPal

   *  Description: PayPal uses custom HTTP header named "PayPal-Request-

   *  Reference:

   Organization: RazorPay

   *  Description: RazorPay uses custom HTTP header named "X-Payout-

   *  Reference:

   Organization: OpenBanking

   *  Description: OpenBanking uses custom HTTP header called "x-

   *  Reference:

   Organization: Square

   *  Description: To make an idempotent API call, Square recommends
      adding a property named "idempotency_key" with a unique value in
      the request body.

   *  Reference:

   Organization: Google Standard Payments

   *  Description: Google Standard Payments API uses a property named
      "requestId" in request body in order to provider idempotency in
      various use cases.

   *  Reference:

   Organization: BBVA

   *  Description: BBVA Open Platform uses custom HTTP header called "X-

   *  Reference:

   Organization: WebEngage

   *  Description: WebEngage uses custom HTTP header called "x-request-
      id" to identify webhook POST requests uniquely to achieve events

   *  Reference:

5.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns specific to the idempotency

   Resource servers that do not implement strong idempotency keys, such
   as UUIDs, or have appropriate controls to validate the idempotency
   keys, could be victim to various forms of security attacks from
   malicious clients:

   *  Injection attacks-When the resource server does not validate the
      idempotency key in the client request and performs a idempotent
      cache lookup, there can be security attacks (primarily in the form
      of injection), compromising the server.

   *  Data leaks-When an idempotency implementation allows low entropy
      keys, attackers MAY determine other keys and use them to fetch
      existing idempotent cache entries, belonging to other clients.

   To prevent such situations, the specification recommends the
   following best practices for idempotency key implementation in the
   resource server.

   *  Establish a fixed format for the idempotency key and publish the
      key's specification.

   *  Always validate the key as per its published specification before
      processing any request.

   *  On the resource server, implement a unique composite key as the
      idempotent cache lookup key.  For example, a composite key MAY be
      implemented by combining the idempotency key sent by the client
      with other client specific attributes known only to the resource

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"

7.  References

7.1.  Normative References

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [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,

   [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,

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,

   [RFC8941]  Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

7.2.  Informative References

   [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,

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

   The rules below are defined in [RFC7230]:

    obs-text      = <obs-text, see [RFC7230], Section 3.2.6>


   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 (
   standards/blob/master/ and Stripe
   ( as well as Internet Draft on
   POST Once Exactly (
   poe-00) authored by Mark Nottingham.

   The authors take all responsibility for errors and omissions.

Authors' Addresses

   Jayadeba Jena


   Sanjay Dalal