Inter-process schema validation cache

[dvisockas]

#363 introduced a in-process schema validation cache. The problem with this approach is that when the MCP server boots - the cache is stale and needs to be warmed, which might not always be trivial to do.

Could we extract the default cache implementation and allow users to configure a different cache store, say for an inter-process long lived cache?

MCP.configuration do |config|
  config.schema_validation_cache = FileBasedSchemaCache.new
end

Happy to put up a PR just want to align on the approach first

[dvisockas]

Example of how this could look like: https://github.com/modelcontextprotocol/ruby-sdk/compare/main...dvisockas:mcp-ruby-sdk:dvisockas/schema-cache-configuration?expand=1

[koic]

Thanks for the follow-up.

I'd rather hold off on pluggable cache backends for now. The cost we're recovering is relatively narrow (~32-100ms per distinct schema, paid once per process), which long-lived servers only pay once after boot.

Exposing a SchemaCache interface would also be a long-term API commitment, and persistent caches introduce correctness concerns: entries would need invalidation whenever the validator or metaschema dialect changes, otherwise stale results could silently bypass validation.

If the practical concern is simply that 1000 entries is not enough, a small MCP.configuration.schema_validation_cache_size knob would address that with much less API surface. That said, while such an option could be worth adding given a concrete use case, there does not seem to be a clear need for it at the moment.