The Idempotency-Key HTTP Header Field

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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.¶

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 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.¶

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.¶

Idempotency-Key = sf-string

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

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.",
}

HTTP/1.1 400 Bad Request
Link: <https://developer.example.com/idempotency>;
  rel="describedby"; type="text/html"

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.",
}

HTTP/1.1 422 Unprocessable Content
Link: <https://developer.example.com/idempotency>;
rel="describedby"; type="text/html"

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.",
}

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

3. IANA Considerations

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¶

• Description: Stripe uses custom HTTP header named Idempotency-Key¶

• Reference: https://stripe.com/docs/idempotency¶

Organization: Adyen¶

• Description: Adyen uses custom HTTP header named Idempotency-Key¶

• Reference: https://docs.adyen.com/development-resources/api-idempotency/¶

Organization: Dwolla¶

• Description: Dwolla uses custom HTTP header named Idempotency-Key¶

• Reference: https://docs.dwolla.com/¶

Organization: Interledger¶

• Description: Interledger uses custom HTTP header named Idempotency-Key¶

• Reference: https://github.com/interledger/¶

Organization: WorldPay¶

• Description: WorldPay uses custom HTTP header named Idempotency-Key¶

• Reference: https://developer.worldpay.com/docs/wpg/idempotency¶

Organization: Yandex¶

• Description: Yandex uses custom HTTP header named Idempotency-Key¶

• Reference: https://cloud.yandex.com/docs/api-design-guide/concepts/idempotency¶

Organization: http4s.org¶

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

• Reference: https://github.com/http4s/http4s¶

Organization: Finastra¶

• Description: Finastra uses custom HTTP header named Idempotency-Key¶

• Reference: https://developer.fusionfabric.cloud/¶

Organization: Datatrans¶

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

• Reference: https://docs.datatrans.ch/docs/api-endpoints¶

5. Security Considerations

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

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 server.¶

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"

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>

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.¶

Authors' Addresses