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Re: Help with Scaled Cartridges Racing (mariadb galera)






On Tue, Feb 25, 2014 at 3:29 PM, John Hawley <jhawley redhat com> wrote:
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.

​You could use Configure-Order to set dependency ​
​between different cartridges.​

​This is used by phpmyadmin like this:​

Configure-Order:
- mysql
- phpmyadmin

​However, there is a drawback. User is able to remove the base cartridge without the need to remove the dependent one, i.e., they can remove the master cart while slaves are still attached to your app.​ (is this a real issue or just some kind of "feature"?)


 

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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