There is no such thing as a 100% available system, and any goal of
trying to achieve that is not a real goal. Serious High Availabiltiy
people talk about 99.9999% uptime, etc. But without knowing more my
best guess is that you don't need anything like that at all, because
very few people do.
Just how many users are you planning to serve with that dotLRN
instance? How many of them using the site each day? How much does
downtime hurt your users? And how important, critical, or
irreplacable is the content each user contributes each day? You have
to start with those questions first.
Last I heard, PostgreSQL has several asynchronous replication tools
that meet some people's needs, and a better asynchronous replication
tool (Slony) on the way. It currently does not have anything
equivalent to Oracle's Archivelog Mode at all, but the Point in Time
Recovery work currently underway should allow building something like
it, and there seems to be interest in doing so.
Note that with Oracle properly set up in Archivelog mode, you can, at
least in theory, guarantee that you will never lose a committed
transaction. With PostgreSQL that is currently not possible. But in
reality, how many PostgreSQL or Oracle installations does
that really matter to, in practice? Probably not very many.
(But the Archivelog feature certainly is nice to have.)
Ask yourself how much data you can afford to lose, and how much time
you can afford to lose.
On the data, ask yourself: "Worst case, can I afford to lose all data
in my database since my last PostgreSQL backup (typically 1 day's
worth)?" If yes, great, just backup nightly like most all OpenACS
users do.
On the other hand, if your answer is no, but your answer to the
question, "Worst case, can I affored to lose some
transactions, maybe a few minutes or a few hours worth?" is
yes, then investigate the various current PostgreSQL
asynchronous replication tool.
But in the unlikely event that your answer really is, "No, I
can't afford to ever lose a transaction, no matter what.",
then you should not be using PostgreSQL. You need to use Oracle (or
something equivalent), and you need to put a whole lot more time,
money, and thought into the problem.
In all cases, make sure you have good backups, and a good procedure
for bringing up a new website using them, preferably on a spare
machine. If you can't afford to be down for very long, there are
various Linux High Availability tools that can help you fail over to a
backup server, etc. Googling should find them. So far I havn't heard
of anyone at all doing that sort of stuff with OpenACS, but it should
be feasible.
