Add ConnectionManager for robust React lifecycle handling

[douglance]

Summary

• Add ConnectionManager singleton that uses reference counting and deferred cleanup to handle React StrictMode's double-mount behavior
• Implement TanStack Query-style pattern: retain() / release() instead of direct connect/disconnect
• Use useSyncExternalStore for tear-free state reads in React

Changes

• New: src/sdk/connection_manager.ts - Core ConnectionManager implementation
• Modified: src/react/SpacetimeDBProvider.ts - Now uses ConnectionManager
• Modified: src/react/connection_state.ts - Simplified type, imports from ConnectionManager
• Modified: src/sdk/db_connection_builder.ts - Added getUri() and getModuleName() methods
• Modified: src/sdk/db_connection_impl.ts - Minor type updates for ConnectionManager integration
• New: tests/connection_manager.test.ts - 33 unit tests
• Docs: Added React Integration section to TypeScript reference
• Docs: Added StrictMode compatibility note to README

Test plan

• pnpm test passes (104 tests)
• pnpm build passes
• Manual browser test with React StrictMode - single WebSocket connection

---

Motivation

The Problem

React StrictMode double-mounts components in development to help catch bugs:

1. Mount → creates WebSocket connection
2. Unmount → destroys WebSocket connection
3. Remount → creates another WebSocket connection

Without ConnectionManager, you get either:

• Two connections (if you don't clean up on unmount)
• Broken connection (if you clean up on unmount, the remount races with cleanup)

Why ConnectionManager? (vs alternatives)

Alternative	Problem
Don't clean up on unmount	Leaks connections, memory leaks, stale subscriptions
Disable StrictMode	Hides real bugs, production still has edge cases
Connection in module scope	React can't react to state changes, no useSyncExternalStore

ConnectionManager approach (TanStack Query pattern):

• Reference counting: multiple components share one connection
• Deferred cleanup: setTimeout(0) lets StrictMode remount cancel cleanup
• External store: connection state lives outside React but syncs properly

---

Why Two useSyncExternalStore Usages?

The SDK now has two places using useSyncExternalStore. They solve different problems:

	useTable	ConnectionManager
Subscribes to	Table row changes	Connection state changes
External store	DbConnection.db.tableName callbacks	ConnectionManager singleton
Re-renders when	Row inserted/deleted/updated	isActive, identity, error change
Manages lifecycle?	❌ No - assumes connection exists	✅ Yes - owns connect/disconnect

useTable assumes a connection exists

// useTable.ts - subscribes to TABLE DATA
useEffect(() => {
  const connection = connectionState.getConnection()!;  // assumes this exists
  if (connectionState.isActive && connection) {
    const cancel = connection.subscriptionBuilder().subscribe(query);
    return () => cancel.unsubscribe();
  }
}, [...]);

ConnectionManager manages WHEN the connection exists

// OLD SpacetimeDBProvider - broken with StrictMode
useEffect(() => {
  const conn = builder.build();    // Creates WebSocket
  return () => conn.disconnect();  // Destroys WebSocket
}, [builder]);
// StrictMode: build → disconnect → build = TWO CONNECTIONS RACING

// NEW SpacetimeDBProvider - works with StrictMode  
useEffect(() => {
  ConnectionManager.retain(key, builder);   // refCount++, maybe create
  return () => ConnectionManager.release(key); // refCount--, deferred cleanup
}, [key, builder]);
// StrictMode: retain(1) → release(0, schedule) → retain(1, cancel) = ONE CONNECTION

The Layered Architecture

ConnectionManager (lifecycle: retain/release, deferred cleanup)
    ↓
SpacetimeDBProvider (React context + useSyncExternalStore for connection state)
    ↓
useTable (useSyncExternalStore for table data)

• useTable syncs data from an existing connection
• ConnectionManager syncs connection state AND manages when the connection exists

[CLAassistant]

All committers have signed the CLA.

[bfops]

Thank you for opening this and the detailed description! @cloutiertyler also tried to fix the StrictMode problem but did so by briefly delaying disconnection (#4017).

What do you think of that approach?

[douglance]

@bfops I was watching him work on that PR on stream. That's why I wanted to open this one. There are scenarios where that solution doesn't work. The timing is not guaranteed.

This solution (basically just using a singleton outside of React), guarantees the system timing works as expected in all scenarios. It's a robust pattern used by the biggest libraries solving this problem.

I would consider it the best solution for this problem.