*F**rom: *Patrick Hemmer <[email protected]>
*Sent: * 2014-06-16 11:25:40 EDT
*To: *Jan Friesse <[email protected]>, [email protected]
*Subject: *Re: [corosync] automatic membership discovery
On 2014/06/16 11:25, Patrick Hemmer wrote:
> Patrick,
>
>> I'm interested in having corosync automatically accept members into the
>> cluster without manual reconfiguration. Meaning that when I bring a new
>> node online, I want to configure it for the existing nodes, and those
>> nodes will automatically add the new node into their nodelist.
>> From a purely technical standpoint, this doesn't seem like it would be
>> hard to do. The only 2 things you have to do to add a node are add the
>> nodelist.node.X.nodeid and ring0_addr to cmap. When the new node comes
>> up, it starts sending out messages to the existing nodes. The ring0_addr
>> can be discovered from the source address, and the nodeid is in the message.
>>
> I need to think about this little deeper. It sounds like it may work,
> but I'm not entirely sure.
>
>> Going even further, when using the allow_downscale and last_man_standing
>> features, we can automatically remove nodes from the cluster when they
>> disappear. With last_man_standing, the quorum expected votes is
>> automatically adjusted when a node is lost, so it makes no difference
>> whether the node is offline, or removed. Then with the auto-join
>> functionality, it'll automatically be added back in when it
>> re-establishes communication.
>>
>> It might then even be possible to write the cmap data out to a file when
>> a node joins or leaves. This way if corosync restarts, and the
>> corosync.conf hasn't been updated, the nodelist can be read from this
>> save. If the save is out of date, and some nodes are unreachable, they
>> would simply be removed, and added when they join.
>> This wouldn't even have to be a part of corosync. Could have some
>> external utility watch the cmap values, and take care of setting them
>> when corosync is launched.
>>
>> Ultimately this allows us to have a large scale dynamically sized
>> cluster without having to edit the config of every node each time a node
>> joins or leaves.
>>
> Actually, this is exactly what pcs does.
Unfortunately pcs has lots of issues.
1. It assumes you will be using pacemaker as well.
In some of our uses, we are using corosync without pacemaker.
2. It still has *lots* of bugs. Even more once you start trying to use
non-fedora based distros.
Some bugs have been open on the project for a year and a half.
3. It doesn't know the real address of its own host.
What I mean is when a node is sitting behind NAT. We plan on running
corosync inside a docker container, and the container goes through
NAT if it needs to talk to another host. So pcs would need to know
the NAT address to advertise it to the other hosts. With the method
described here, that address is automatically discovered.
4. Doesn't handle automatic cleanup.
If you remove a node, something has to go and clean that node up.
Basically you would have to write a program to connect to the quorum
service and monitor for nodes going down, and then remove them. But
then what happens if that node was only temporarily down? Who is
responsible for adding it back into the cluster? If the node that
was down is responsible for adding itself back in, what if another
node joined the cluster while it was down? Its list will be
incomplete. You could do a few things to try and alleviate these
headaches, but automatic membership just feels more like the right
solution.
5. It doesn't allow you to adjust the config file.
>> This really doesn't sound like it would be hard to do. I might even be
>> willing to attempt implementing it myself if this sounds like something
>> that would be acceptable to merge into the code base.
>> Thoughts?
>>
> Yes, but question is if it is really worth of it. I mean:
> - With multicast you have FULLY dynamic membership
> - PCS is able to distribute config file so adding new node to UDPU
> cluster is easy
>
> Do you see any use case where pcs or multicast doesn't work? (to
> clarify. I'm not blaming your idea (actually I find it interesting) but
> I'm trying to find out real killer use case for this feature which
> implementation will take quite a lot time almost for sure).
Aside from the pcs issues mentioned above, having this in corosync just
feels like the right solution. No external processes involved, no
additional lines of communication, real-time on-demand updating. The end
goal might be able to be accomplished by modifying pcs to resolve the
issues, but is that the right way? If people want to use crmsh over pcs,
do they not get this functionality?
> Regards,
> Honza
>
>> -Patrick
>>
>>
>>
>> _______________________________________________
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>> [email protected]
>> http://lists.corosync.org/mailman/listinfo/discuss
>>
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