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While looking into this bug:

https://bugs.launchpad.net/heat/+bug/1290274

I was trying to find out why the original developers felt that tenant_ids
should be a 'varchar(256)'. In addition to moving from a regular varchar
into a text field, varchar(256) in MySQL will become a tinytext which
causes all sorts of performance issues, this just seems a _lot_ bigger
than anything I've ever seen shown as a tenant/project id. I'd expect at
worst varchar(64). Also it is probably safe to store as varbinary since
we don't ever sort on it or store case-insensitive equivalents to it.

So I was wondering where users can go to find out what to expect in
that field. I dug through the API documentation for Keystone and I see
nothing that really would constituate a format or even length. But maybe
I'm just not looking in the right place. Thanks!