[rabbitmq-discuss] Disk node vs ram node

Thu Aug 5 20:43:09 BST 2010

> 1.  Assuming I have 3 nodes, node A, B, and C. Does node A become a disk
> node by default when I set up the vhosts/users/permissions?

The disk nodes are the ones explicitly listed in the "rabbitmqctl
cluster" command.

So,
% rabbitmqctl cluster rabbit at A rabbit at B
results in a cluster where A and B are disk nodes.  If that command is
run on either A or B,  the cluster has just the two nodes.
If, on the other hand, that command is run on C, you get a cluster with
A and B disc nodes (same as above) and C as a ram node.

> 2. With the same node arrangement listed above.... I set up A(created vhost,
> users, set permissions etc) and then clustered B against A,B (rabbitmqctl
> cluster nodeA nodeB). I took down A and my messages did not persist even
> though B should be a disk node. Is there something wrong with my steps?

Queues exist physically only on the node on which they were declared.
Clustering just makes them (and exchanges) accesible transparently on
all of the nodes.

Thus, if the queue was declared on A, and A was taken down, the messages
stored on the queue would become inaccesible.

If you want to never lose messages, have a look at our high-availability
clustering guide in the documentation section of the website.

> 3. What is the conventional/non-conventional way to determine whether or not
> individual nodes in a cluster are a disk or ram nodes?

To find out whether nodes are disc or ram, run
% rabbitmqctl status
on any node in the cluster.

Unfortunately, due to a bug, that command will not report the correct
node type, so it's not of much help to you now.  This problem has been
fixed in our default branch and will be in the release.

Cheers,
Alex