This extension is not distributed with Mercurial.
Author: Durham Goode
Current maintainer: Facebook
The hgsql extension allows multiple Mercurial servers to provide read and write access to a single repository at once. It does this by using a MySQL database to manage write locks and to propagate changes to the other servers.
This improves server scalability by allowing load to be distributed amongst multiple servers, and improves reliability by allowing individual servers to be taken down for repair without any downtime to the system as a whole.
This extension is still being developed and frequently changes. See the bitbucket page for the most recent documentation.