>So, what is it really? Is there one Write-Master that is asynchronously replicated to all other locations? Will writes be forwarded to this master and then replicated back?
Not a lot of detail, but that is mentioned:
"But we're going further. With D1, it will be possible to define a chunk of your Worker code that runs directly next to the database, giving you total control and maximum performance—each request first hits your Worker near your users, but depending on the operation, can hand off to another Worker deployed alongside a replica or your primary D1 instance to complete its work."
Not a lot of detail, but that is mentioned:
"But we're going further. With D1, it will be possible to define a chunk of your Worker code that runs directly next to the database, giving you total control and maximum performance—each request first hits your Worker near your users, but depending on the operation, can hand off to another Worker deployed alongside a replica or your primary D1 instance to complete its work."