Repair Schema Evolution & Compatibility Bugs

I have analyzed the stale writer scenario during your us-east-1 to us-west-2 failover. The issue stems from a lack of generation-based fencing, allowing workers with expired authority to commit writes to the global data store.

Distributed Fencing Implementation

To prevent stale writers from overwriting newer data, you must implement a fencing token (also known as a generation ID) that is validated at the storage layer.

// Proposed Fencing Logic for Background Workers
interface WriteRequest {
  regionId: string;
  generationId: number; // Increment this during failover
  payload: any;
}

async function performRegionalWrite(db: GlobalStore, request: WriteRequest) {
  // The storage layer must perform an atomic compare-and-swap or 
  // check a 'current_authority_generation' record.
  const currentGen = await db.getAuthorityGeneration();
  
  if (request.generationId < currentGen) {
    console.error(`Rejected stale write from generation ${request.generationId}. Current is ${currentGen}.`);
    throw new Error("STALE_AUTHORITY_ERROR");
  }

  return await db.put({
    ...request.payload,
    last_writer_gen: request.generationId,
    updated_at: new Date().toISOString()
  });
}

Invariant Analysis

Next steps

1. Update the regional promotion script to increment the authority_generation in your global metadata store.
2. Modify background worker loops to fetch the latest generationId before starting each batch.
3. Add a test case using a mock database that rejects writes with a decremented generation ID.