add-unit relation question

[David Britton]

Hi All --

I've hit a problem about which I need some advice.  I can think of two ways
to solve it, but it seems to me a common problem that people must have hit
before.

My charm (landscape charm, unpublished for now) relates with postgresql
over the "db-admin" relation.  This relation allows direct access to the
database as a super user.  The two things the connecting charm needs from
postgresql are the connection details (host, super-user name, password) and
postgresql to *allow* the connecting unit access to the database through
the pg_hba.conf file, adding a db user, setting a password.

This all works fine for the case of the first client unit connecting.  In
my case -- "landscape".  The second landscape unit I add however hits a
problem.  It keys off the presence of *host* (part of the connection
details) to know if it is *ok to connect**.  *This of course is already set
in the relation context by the postgresql unit from the previous unit that
connected  What is missing is the second requirement of allowing the
connecting landscape unit access to the database through the pg_hba.conf
file.

I can think of three ways around this situation:

1) Avoid -- Don't restrict on ip addresses in the pg_hba.conf file.

2) Workaround -- In landscape, make the connecting process resilient.  It
would retry for a timeout period before considering it a failure.  This
would work fine, but would need to be part of the README for any connecting
service, as it's very non-obvious why you need to do this.

3) Formally Fix -- Use a setting like "clients-allowed" that postgresql
will relation-set with the list of ips that are allowed to connect.  Make
*that* the official way to key on when a connection should be attempted.
 Document in the README.  The old way would still work, but not for the N+1
case (which afaict, it never did).

My question is, what approach should I choose?  Is there something that I
am missing?  This seems like a pretty typical problem.  As *soon* as a new
unit is added, the relation variables are in the same state from the
previous unit, which in some cases, make sense, but only if nothing on the
server side needs to be done to allow that new client to connect.

How have other people solved this?

Thanks for your help! :)
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