[onlydnaq]

> At the moment there are few enough that they could fetch a list of all of them and cache it (the public registry lacks a documented API for doing that right now, but we can certainly provide one)

Might I suggest having a bloom filter containing all the existing type declaration (which would be quite small) and only querying the registry if the bloom filter reports the type declaration as a positive.

Since the filter can be really small it will probably scale a lot better than a complete list of all type-declarations, and a new filter could be downloaded by the clients every now and then.

[mirhagk]

Is there an efficient diff algorithm for bloom filters?

[Yugnik]

Depends on the bloom filter, but for the fixed size, fixed hash functions, and other implementations in the same general vein, it would just be XOR of both of the bloom filters. Anything that comes out true is missing from what would be the combined filter. Slightly different for counting filters, where something along the lines of subtraction would get you what you need (anything non-zero is different).