Just released IdempotencyPlug, a plug based library that makes POST and PATCH requests idempotent using an Idempotency-Key HTTP header. It follows the IETF Idempotency-Key HTTP Header Field specification draft.

The details

I wrote a blog post on the process and motivations: https://danschultzer.com/posts/idempotencyplug-idempotent-post-requests

Some TLDR excerpts:

An idempotent request ensures that a request only affects the resource at most once. In REST all methods are idempotent except POST and PATCH.

Take the endpoint POST /api/payments that initiate a new payment charge at a payment processor. If the client experiences a network interruption during the request, how can the client safely retry the request without creating a new charge?

These are the requirements for idempotent request handling:

• Require a single Idempotency-Key HTTP header for all POST and PATCH requests
• Idempotency-Key value MUST be unique for a URI
• Idempotency-Key value MUST NOT be reused with a different request payload
• First-time requests MUST be processed normally, and the response cached
• Duplicate requests MUST return the cached response
• Concurrent requests MUST return an error
• We MUST handle unexpected process termination
• The cached responses SHOULD expire after 24 hours
• The cache SHOULD be distributed and persisted

Links

Github: https://github.com/danschultzer/idempotency_plug
Hex: https://hex.pm/packages/idempotency_plug
Docs: https://hexdocs.pm/idempotency_plug/README.html

I hope you find it useful, and as always, any contributions are much appreciated!