Stale-while-revalidate for DNS caching

[notpushkin]

Some background: stale-while-revalidate is a caching mechanism in HTTP that allows reusing old query result while simultaneiusly refetching a fresh response to use for the next request. For example, with the following header:

Cache-Control: max-age=1, stale-while-revalidate=59

• If a request is repeated within the next 1 second, the previously cached value will still be fresh, and used as-is, without any revalidation.

• If a request is repeated between 1 and 60 seconds later, then the cached value will be stale, but will be used to fulfill the API request. At the same time, "in the background," a revalidation request will be made to populate the cache with a fresh value for future use.

This could work with dnscrypt-proxy as well. E. g. with the following settings:

cache = true
cache_min_ttl = 2400  # 40 minutes
cache_stale_while_revalidate = 19200  # 6 hours - 40 minutes

• If a request is repeated within the next 40 minutes, the previously cached value will still be fresh, and used as-is, without any revalidation.

• If a request is repeated between 40 minutes and 6 hours, then the cached value will be stale, but will be returned to the client (e. g. browser) with TTL of 1 (so that it doesn't cache it any further). At the same time, "in the background," a revalidation request will be made to populate the cache with a fresh value for future use.