Re: automatic membership discovery

[Patrick Hemmer <corosync@xxxxxxxxxxxxxxx>]

From: Patrick Hemmer <corosync@xxxxxxxxxxxxxxx>

Sent: 2014-06-16 11:25:40 EDT

To: Jan Friesse <jfriesse@xxxxxxxxxx>, discuss@xxxxxxxxxxxx

Subject: Re: 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 If the save is out of date, and some nodes are unreachable, they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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