The Idempotency-Key HTTP Header Field

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.¶

2.1. Syntax

Idempotency-Key is an Item Structured Header [RFC8941]. Its value MUST be a String (Section 3.3.3 of [RFC8941]).¶

2.2. Uniqueness of Idempotency Key

The idempotency key 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. It is RECOMMENDED that a UUID [RFC4122] or a similar random identifier be used as an idempotency key.¶

The following example shows an idempotency key whose value is a UUID [RFC4122]:¶

Idempotency-Key: "8e03978e-40d5-43e8-bc93-6894a57f9324"

2.3. Idempotency Key Validity and Expiry

The resource MAY require time based idempotency keys to be able to purge or delete a key upon its expiry. The resource 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. 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

2.6. Idempotency Enforcement

• First time request (idempotency key and fingerprint has not been seen)¶

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

• Duplicate request (idempotency key and fingerprint has been seen)¶

  Retry¶

  The request was retried after the original request completed. The resource 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 SHOULD respond with a resource conflict error. See Error Scenarios for details.¶

2.7. Error Handling

If the Idempotency-Key request header is missing for a documented idempotent operation requiring this header, the resource 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": "https://developer.example.com/idempotency",
      "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: <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 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 15.5.21 of [RFC9110].¶

    HTTP/1.1 422 Unprocessable Content
    Content-Type: application/problem+json
    Content-Language: en
    {
      "type": "https://developer.example.com/idempotency",
      "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 Content
    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 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": "https://developer.example.com/idempotency",
      "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 documentation¶

    HTTP/1.1 409 Conflict
    Link: <https://developer.example.com/idempotency>;
    rel="describedby"; type="text/html"

Error scenarios above describe the status of failed idempotent requests after the resource prcocesses them. Clients MUST correct the requests (with the exception of 409 where no correction is required) before performing a retry operation, or the resource 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's documentation.¶

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¶

4.1. 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: https://pypi.org/project/django-idempotency-key¶

Organization: Chargebee¶

• Description: Chargebee uses custom HTTP header named chargebee-idempotency-key¶

• Reference: https://apidocs.chargebee.com/docs/api/idempotency¶

Organization: Twilio¶

• Description: Twilio uses custom HTTP header named I-Twilio-Idempotency-Token in webhooks¶

• Reference: https://www.twilio.com/docs/usage/webhooks/webhooks-connection-overrides¶

Organization: PayPal¶

• Description: PayPal uses custom HTTP header named PayPal-Request-Id¶

• Reference: https://developer.paypal.com/docs/business/develop/idempotency¶

Organization: RazorPay¶

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

• Reference: https://razorpay.com/docs/razorpayx/api/idempotency/¶

Organization: OpenBanking¶

• Description: OpenBanking uses custom HTTP header called x-idempotency-key¶

• Reference: https://openbankinguk.github.io/read-write-api-site3/v3.1.6/profiles/read-write-data-api-profile.html#request-headers¶

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: https://developer.squareup.com/docs/build-basics/using-rest-api¶

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: https://developers.google.com/standard-payments/payment-processor-service-api/rest/v1/TopLevel/capture¶

Organization: BBVA¶

• Description: BBVA Open Platform uses custom HTTP header called X-Unique-Transaction-ID¶

• Reference: https://bbvaopenplatform.com/apiReference/APIbasics/content/x-unique-transaction-id¶

Organization: WebEngage¶

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

• Reference: https://docs.webengage.com/docs/webhooks¶

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/rfc/rfc2119>.

[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/rfc/rfc4122>.

[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/rfc/rfc5234>.

[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/rfc/rfc7231>.

[RFC7807]

Nottingham, M. and E. Wilde, "Problem Details for HTTP APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, <https://www.rfc-editor.org/rfc/rfc7807>.

[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/rfc/rfc8174>.

[RFC8941]

Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, <https://www.rfc-editor.org/rfc/rfc8941>.

[RFC9110]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

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, <https://www.rfc-editor.org/rfc/rfc7942>.