Module subscriptions 

Resource-update subscription registry backing the server-side hooks for resources/subscribe, resources/unsubscribe, and the two MCP notifications the server emits after workspace mutations.

The registry keeps, per URI, a Vec<Peer<RoleServer>> of currently subscribed clients. When a tool call changes workspace state the server looks up every URI that would be affected (workspace, table list, per-table schema/sample/csv-sample, per-saved-query result) and calls SubscriptionRegistry::notify_updated, which spawns a background task per subscriber to send the notification.

Tradeoffs in this implementation

rmcp::Peer is Clone but not Eq, and its internal mpsc::Sender is not exposed for comparison. Rather than reach into private fields we accept three pragmatic constraints:

1. Subscribing twice for the same URI from the same session stores two entries. Subsequent notifications fire twice. The MCP protocol defines no “already subscribed” error, so this is spec-compliant; well-behaved clients subscribe once per URI per session anyway.
2. Unsubscribing clears all entries for the given URI. In multi-client setups this would be surprising, but for the stdio + SSE transports typically used with HyperDB each process serves at most a handful of clients and cross-client URI reuse is rare.
3. Notify failures (client disconnected) are logged but not pruned. Dead peers accumulate until the MCP session tears the registry down; a future improvement could prune them from the failing detached task, but since notify is a bounded per-tool-call cost and sessions are short-lived this hasn’t mattered in practice.

Structs

SubscriptionRegistry

Per-URI registry of subscribed peers with broadcast helpers.

Functions

uris_for_table_change

The full set of resource URIs that are affected when a specific table is written to. Used by mutation-side helpers in server.rs to fire targeted updates without sprinkling URI strings across every tool.

uris_for_workspace_change

The URIs affected by a workspace-wide change that doesn’t name any single table (e.g. watcher activity touching multiple files, or a saved-query list mutation). hyper://tables reflects the updated catalog, hyper://readme contains the summary that cross-references everything else.