when it works at least //in the foreseeable future//. (Yes, changing is a possibility but it’s not currently foreseeable whether or when it will happen.)
Not being able to create user tables on the replicas wasn't
foreseeable. It was possible, and then it was decided that that couldn't
be supported anymore in combination with other reliability features
that were desired.
Not being able to select any wiki's
database from any shard is foreseeable, as in our DBAs have //already
foreseen// that it's a likely enough possibility to tell us not to count
on it remaining available.
Brad Jorsch (Anomie) Senior Software Engineer Wikimedia Foundation