Support multiple LDAP servers for automatic failover

[oyvindhagberg]

A recent outage highlighted the need for mreg to support multiple LDAP servers for failover purposes.
Here's a dump from a discussion we had internally (names have been removed for privacy):

https://django-auth-ldap.readthedocs.io/en/latest/multiconfig.html

is this something that can be implemented via just settings?

static classes defining how many ldap backends can be set up seems like a very poor way to go about this

does django ldap really not support defining a server list?

with the python ldap3 library it's possible to set up a server pool and round robin between them for connections

apparently the "solution" is to make AUTH_LDAP_SERVER_URI a callable and write your own code to check for availability

way to go shoving the problem onto users

If your server location is even more dynamic than this, you may provide a function (or any callable object) that returns the URI. The callable is passed a single positional argument: request. You should assume that this will be called on every request, so if it’s an expensive operation, some caching is in order.

does it bind on every request? sounds expensive

for the actual check you could probably get away fairly cheap by using socket to check if a server is available, and implement some basic strategies like round robin and random selection

to avoid doing this on every single connection i would use cachetools.TTLCache