ALPs have many client-resolvable query parameters which greatly reduce cache-hit by multiplying cache keys. E.g. https://host/alp?adpos=X will multiply on the possible space of adpos query parameter, even though it's never used by the page itself.

The proposal is to use https://host/alp as the cache URL/key and replace it when AMP runtime starts up to https://host/alp?adpos=X. The replacing URL will be specified via viewer param as #replaceUrl=....

Some important notes:

1. Replacement should be done, ideally, very early on and definitely before installDocumentStateService is called.
2. The replacement URL must match the original URLs origin and source origin