Internet-Draft The Idempotency HTTP Header Field November 2020
Jena & Dalal Expires 27 May 2021 [Page]
Network Working Group
Intended Status:
Standards Track
J. Jena
PayPal, Inc.
S. Dalal

The Idempotency HTTP Header Field


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

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 27 May 2021.

Table of Contents

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 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 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

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.

  • 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 key.

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 or fingerprint has not been seen)

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

  • Duplicate request (idempotency key or fingerprint has been seen)


    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.

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: <>;
  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: <>;
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: <>;
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].

Header Field Name: Idempotency-Key

Applicable Protocol: Hypertext Transfer Protocol (HTTP)

Status: Standard

        Jayadeba Jena

        Sanjay Dalal

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

Organization: Adyen

Organization: Dwolla

Organization: Interledger

Organization: WorldPay

Organization: Yandex

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 HTTP_IDEMPOTENCY_KEY
  • 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-Id
  • Reference:

Organization: RazorPay

  • Description: RazorPay uses custom HTTP header named X-Payout-Idempotency
  • Reference:

Organization: OpenBanking

  • Description: OpenBanking uses custom HTTP header called x-idempotency-key
  • 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-Unique-Transaction-ID
  • Reference:

Organization: WebEngage

  • Description: WebEngage uses custom HTTP header called x-request-id to identify webhook POST requests uniquely to achieve events idempotency.
  • Reference:

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"

7. References

7.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, , <>.
Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, , <>.
Dusseault, L., Ed., "HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)", RFC 4918, DOI 10.17487/RFC4918, , <>.
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

7.2. Informative References

Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <>.

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 and Stripe as well as Internet Draft on POST Once Exactly 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.
Sanjay Dalal