Durable Object Lifecycle Event Emission for Local Dev Tooling

[MattieTK]

Summary

Add a mechanism for workerd to emit events when Durable Objects transition between lifecycle states (active, hibernated, inactive/evicted). This would enable Miniflare and local development tools to build resource inspectors that display DO state without the observer effect of waking hibernated objects.

Motivation

We're building local developer tooling that includes a resource inspector to help developers understand and debug their Durable Objects during development. A key piece of information developers need is the current lifecycle state of their DOs:

• Active (in-memory): Currently processing requests or has scheduled work
• Hibernated: In-memory but idle, with hibernatable WebSockets keeping the DO alive
• Inactive/Evicted: Removed from memory entirely

Heisenberg's problem of Durable Objects (the observer effect)

Inspecting a Durable Object to determine its state inherently changes that state. Any interaction with a DO will wake it up and transition it back to the active state. This creates an impossible situation for developer tooling:

Developer wants to see: "DO 'room-123' is currently hibernated"
What happens when we check: DO 'room-123' wakes up and becomes active
What developer sees: "DO 'room-123' is active" (incorrect/stale)

Currently this makes it impossible to build accurate observability tools that show the true runtime state of Durable Objects.

Use Cases

1. Local Resource Inspector: A dashboard in wrangler dev or Vite dev server showing:

   • List of instantiated DOs with their current lifecycle state
   • Visual indicators (e.g., green = active, yellow = hibernated, grey = evicted)
   • WebSocket connection counts per DO
   • Transition history/timeline

2. Debugging Hibernation Issues: Developers using the WebSocket Hibernation API encounter bugs where state isn't properly persisted before hibernation. Being able to see when hibernation actually occurs (without triggering a wake) might help them diagnose these issues.

3. Performance Profiling: Understanding how often DOs hibernate and wake can help developers optimise their applications for cost.

4. Educational/Learning: New developers learning about DO lifecycles would benefit from seeing the state machine in action.

Open to any possible solutions you think would work for us.

[Feature request partially Claude'd following discussions with the team]