[rabbitmq-discuss] nodename variable and mnesia directory

[Tim Watson]

Hi

On 10/19/2012 03:41 PM, Elizabeth Liao wrote:
> Hi,
>
> We are currently running rabbitmq on a system where there is a need to change the hostname occasionally.  Right now, when the hostname is changed, we also update the NODENAME variable in the rabbitmq-env.conf file.

The first thing I'm going to say is that this isn't really a desirable 
state of affairs. Why do you need to change NODENAME at all, just 
because the host name has changed? This seems like an odd step to me - 
can you explain the rationale behind it?

> The problem we're having right now is that due to a bug when setting up the rabbitmq-env.conf file, sometimes the NODENAME variable exists but is not set to anything ( NODENAME=) so rabbit never successfully starts up.  Even after setting the NODENAME variable to the hostname, rabbit still doesn't start up properly.

Can you explain a bit more about this 'bug' please? What exactly is 
going wrong?

> The only way that I could find to recover from this situation without killing the erlang process, was to also set a MNESIA_DIR variable. After I did that, once the NODENAME was set to the correct hostname, rabbit started up properly.

Now this makes no sense at all to me. If you are trying to recover 
'without killing the erlang process' then how are you planning on 
changing the NODENAME? Once an erlang node has become part of a 
distributed system, you cannot arbitrarily change the node's name 
without restarting the net_kernel AFAIK so I can't see how this is even 
possible. Also, because you're stating that 'rabbit started up properly' 
I'm assuming that you have in fact taken the node offline (using 
`rabbitmqctl stop` or one of the various service scripts that invokes 
it) and that you are now starting the node again. The rabbit startup 
scripts (and associated infrastructure) is not set up to handle changing 
the name of a node post-installation in this manner, so I would not 
expect this to work smoothly.

> A couple of questions:
> Is this the proper way to fix the situation?

No. As I said a moment ago, rabbit is not set up for this kind of 
scenario so I'd be very cautious about doing this and would really 
recommend against it.

> Will this have unintended consequences?

I have no idea, but it is entirely possible that various things could 
break in obscure and alarming ways. I really would not do this if I were 
you.

> We are running rabbitmq-server-2.8.6 and erlang-R15B.
>
> Thanks!

Liz. I would like to help you find a safe work-around so that you can 
use rabbit in your environment. I would really like to understand why 
the changing of a machine's host-name is causing problems for you. I 
fully realise that after changing the hostname, it is possible that a 
restart may be required (although I'd have to do some research to verify 
whether or not this is a hard requirement). But I cannot understand why 
a change to the machine's host name requires changing the rabbit 
NODENAME at all, unless you've altered the rabbit start scripts to use 
'longnames' configuration (i.e., including the complete host name in the 
node name) in which case, a simple solution to your problem might be to 
switch back to using 'shortnames'. If erlang is started using -sname 
rabbit@`hostname -s` and your /etc/hosts configuration supports 
resolving the machine's IP to the short version of the name, then 
changing the hostname shouldn't cause all of these headaches.

For example, if I launch rabbit with something like this:

env RABBITMQ_NODENAME=rabbit RABBITMQ_NODE_PORT=5672 
RABBITMQ_LOG_BASE="/tmp" 
RABBITMQ_MNESIA_DIR="/tmp/rabbitmq-rabbit-mnesia" 
RABBITMQ_PID_FILE="/tmp/rabbitmq-rabbit.pid" 
RABBITMQ_PLUGINS_EXPAND_DIR="/tmp/rabbitmq-rabbit-plugins-scratch" 
RABBITMQ_ENABLED_PLUGINS_FILE="/tmp/enabled-plugins" 
RABBITMQ_CONFIG_FILE="/tmp/etc/rabbit" ./scripts/rabbitmq-server

Then change the hostname, then stop rabbit and start it again, 
everything is fine.

Cheers,
Tim

> Liz
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