NUT-19: Cached Responses

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To minimize the risk of loss of funds to due network errors during critical operations such as minting, swapping, and melting, we introduce a caching mechanism for successful responses. This allows wallet to replay requests on cached endpoints and allow it to recover the desired state after a network interruption.

Requests & Responses#

Any Mint implementation should elect a data structure D that maps request objects to their respective responses. D should be fit for fast insertion, look-up and deletion (eviction) operations. This could be an in-memory database or a dedicated caching service like Redis.

Derive & Repeat#

Upon receiving a request on a cached endpoint, the mint derives a unique key k for it which should depend on the method, path, and the payload of request.

The mint uses k to look up a response = D[k] and discriminates execution based on the following checks:

• If no cached response is found: request has no matching response. The mint processes request as per usual.
• If a cached response is found: request has a matching response. The mint returns the cached response.

Store#

For each successful response on a cached endpoint (status_code == 200), the mint stores the response in D under key k (D[k] = response).

Expiry#

The mint decides the ttl (Time To Live) of cached response, after which it can evict the entry from D.

Settings#

Support for NUT-19 is announced as an extension to the nuts field of the GetInfoResponse described in NUT-06.

The entry is structured as follows:

"nuts": {
    ...,
    "19": {
        "ttl": <int|null>,
        "cached_endpoints": [
            {
                "method": "POST",
                "path": "/v1/mint/bolt11",
            },
            {
                "method": "POST",
                "path": "/v1/swap",
            },
            ...
        ]
    }
}

Where:

• ttl is the number of seconds the responses are cached for
• cached_endpoints is a list of the methods and paths for which caching is enabled.
• path and method describe the cached route and its method respectively.

If ttl is null, the responses are expected to be cached indefinitely.