Flush is a multi access device

The writes to the flush fd are atomic, and can b locking, thus serializing write access to a resource, like a data base. The atomic write limit is 4096, plenty of space for an sql ite update.  One can give sql a simple put str method, then chain a flush. Simple setup, a few lines of linux command.

Threading should not be an issue, it makes no sense to have multiple readers on the same fd. Flush can maintain an fd  index, associated with callback and a device ID.  One flush modules running multiple queues.