caching options about mbtileserver

[brendan-ward]

@xinyuluo thanks for the question.

The cache control headers control caching on the client (browser) side, those do not correspond to caching on the server.

At present we have no caching on the server. In the earliest versions of this tool, we had in-memory caching for the most frequently used tiles, but that added unnecessary complexity and we removed it early on. At present, the mbtiles files are effectively one of the best cases for on-disk caches anyway, so there isn't (in my opinion, not validated by testing) much benefit to having a secondary on-disk cache.

Depending on your use case and your server capacity, there may be some benefit to having an in-memory cache for most frequently requested tiles, but you should implement that yourself along with a reverse proxy; it is out of scope for mbtileserver. Our focus is to make this run on servers with tiny amounts of memory, so I don't want to build in an in-memory cache by default.