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Re: Re: path based authz and write-through proxy

From: Nico Kadel-Garcia <nkadel_at_gmail.com>
Date: Fri, 25 Sep 2015 08:28:22 -0400

On Fri, Sep 25, 2015 at 7:51 AM, Andreas Stieger <Andreas.Stieger_at_gmx.de> wrote:
> Hi,
> Nico Kadel-Garcia wrote:
>> On Fri, Sep 25, 2015 at 4:37 AM, Andreas Stieger <Andreas.Stieger_at_gmx.de> wrote:
>> > Nico Kadel-Garcia wrote:
>> >> On Thu, Sep 24, 2015 at 3:34 PM, Aaron Friesen <AFriesen_at_spirae.com> wrote:
>> >> > I have been tasked with setting up a mirror of several repositories with write-through back to the master.
>> >>
>> >> What is your engineering time worth? Wandisco publishes a very nice
>> >> multi-master setup that does precisely this, at
>> >
>> > Not "precisely", it's synchronous replication with a variant of Paxos.
>> It's a more complete multi-master solution. The result is even better:
>> full high availability access with multiple Subversion servers,
>> synchronized write access to the core repository, and you don't have
>> to write the potentially fragile and split-brain prone hooks yourself.
> Yes. Synchronous replication. Paxos. Why repeat the definition?

Because "Paxos" doesn't mean anything to most system admins who are
just starting out, and I wanted it clear for new mailing list members.

> Split-brain is an irrelevant consideration for a write-through asynchronous configuration.

Until your primary master goes toes up, for whatever hardware or
software reason, or needs downtime for maintenance, and you wind up
having to manually switch around the services to link to a new
designated master, or lose all write access while the favored master
is offline. Unless ou're ready and able to take the old master
completely offline and never replace it, you than have to pull from
the new "master" to the old "master" to remain consistent.

That can be.... difficult to avoid when you don't have complete
network control and can't trivially access the disabled previous
master to ensure that it stays read-only. It's also one of the very
classic problem of any master/slave database relationship. Swapping
the master and slave to provide full failover can be quite tricky and
error prone, and it's very easy to accidentally leave two masters live
with divergent histories. If you're able to do that manually, great!
Enjoy the benefits of your expertise and competence.

For a new admin, the results of a mis-step or failure to fully
implement genuine high availability can be very expensive. Been there,
done that, got paid to clean up the problems, for various source
control systems and databases.
Received on 2015-09-25 14:28:37 CEST

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