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Re: Help with Scaled Cartridges Racing (mariadb galera)

On 02/25/2014 01:47 AM, Andrew Lau wrote:
> Hi guys,
> I've been working on bringing MariaDB Galera [1] to OpenShift and so far
> I've quite successful with the help from many different people. I'm
> stuck on a few things though which I'm hoping someone could help me with.

Just a general word of warning, I found Galera clustering to have some
rather nasty rough edges, particularly with the possibility of some
transactions never actually making it out to the other nodes.  I
confirmed that pathology with Monty, and I don't know if there's been a
resolution to it yet.  I'm not trying to deter, just trying to make it
known that Galera is not a solve-all solution.

> *- Determining a master gear for the database initialization*
> With the galera cluster, we need to have one gear create the database,
> drop test user. etc. all the normal stuff, however only one needs to do
> this. The problem is, starting the cartridge with min 3 gears, they will
> all race to start, this makes using a hook method not possible because
> there'll be too many inconsistencies. I'd ideally not want to be
> creating two separate cartridges just for this.

You are probably going to have to solve this similarly to the way I
solved it with the original master/slave stuff and have an explicit
"init" cartridge that has that logic in it, and then instantiate a your
"slaves".  The init node obviously shouldn't be scalable, but you can
make the "slaves" scale at will at that point.  If you make a dependency
for the slaves to require the init that should prevent anything too
weird, and prevent a race for the "slaves" to come up before the init
has completed and settled.

Not ideal, obviously, but I don't know of anyway to prevent that race
condition you are describing, and it's not a dissimilar one to what I
was seeing with the master/slave stuff.

- John 'Warthog9' Hawley

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