Monitoring Varnish with Heartbeat and Pacemaker

Question

I have a pair of servers setup as high availability load balancers/reverse proxies. Each runs Ubuntu 12.04 x64 Server, Varnish, Heartbeat and Pacemaker, with Varnish load balancing traffic to back-end servers.

If either of the load balancers falls over, Heartbeat/Pacemaker transfers a group of virtual IPs over to the other server, and traffic flow resumes. This bit works fine.

What I haven't accounted for is if Varnish isn't running on either server. It's currently possible to stop Varnish without triggering any sort of action from Heartbeat/Pacemaker. I'd like the absense of an operational Varnish on the current server to trigger a move to the backup (rather than attempting to restart Varnish), but I'm struggling to find any sort of guidance online. Can anyone help?

Edit post-Daff's assistance:

I ended up with something a little different from my original request: Pacemaker attempts to restart Varnish once, and if that fails, it moves all resources to the passive node.

My setup is two servers, serverA (active) and serverB (passive). I'll assume that the messaging layer (Heartbeat or Corosync) is already setup and working. To allow Pacemaker to control Varnish, we need to fix Ubuntu's Varnish init script:

sudo vim /etc/init.d/varnish

Replace:

--start --quiet --pidfile ${PIDFILE} --exec ${DAEMON} -- \

in the start_varnish_d() function with:

--start --quiet --pidfile ${PIDFILE} --oknodo --exec ${DAEMON} -- \

so it works in accordance with the rules outlined here. Now setup a basic Pacemaker cluster on serverA with a two virtual IPs:

sudo crm configure property no-quorum-policy=ignore
sudo crm configure property stonith-enabled=false
sudo crm configure primitive virtual_ip_1 ocf:heartbeat:IPaddr params ip="192.168.1.134" nic="eth1" cidr_netmask="24" broadcast="192.168.1.255" op monitor interval="10s" timeout="20s"
sudo crm configure primitive virtual_ip_2 ocf:heartbeat:IPaddr params ip="192.168.1.135" nic="eth1" cidr_netmask="24" broadcast="192.168.1.255" op monitor interval="10s" timeout="20s"

Add a primitive for Varnish, providing a monitoring frequency, and generous timings for starting and stopping Varnish:

sudo crm configure primitive varnish lsb:varnish op monitor interval="10s" timeout="20s" op
start interval="0" timeout="15s" op stop interval="0" timeout="15s"

Group the varnish primitive with the virtual IPs, so Pacemaker migrates all resources to the passive node in the event of a failure:

sudo crm configure group cluster virtual_ip_1 virtual_ip_2 varnish

Set migration-threshold, so Pacemaker will tolerate two failures before moving all resources to the passive node. For Varnish, this means one initial failure, plus one failed restart attempt:

sudo crm_attribute --type rsc_defaults --attr-name migration-threshold --attr-value 2

Set a failure timeout. This seems to do two things:

1. Gives Pacemaker the time for one Varnish restart attempt before migrating to the passive node.

2. Prevents the failed node being marked as failed after 30s, allowing resources to be moved back to it without manually having to run crm resource cleanup varnish after a failure. This is good thing for my setup, as I don't have any weightings set on the nodes, but it could be a really bad idea in a different environment.

sudo crm_attribute --type rsc_defaults --attr-name failure-timeout --attr-value 30s

And that's it, but see Daff's answer below for comments about stickiness, which I didn't end up using. The only downside I can see is that if you manually put a node into standby, Pacemaker will shutdown Varnish on that node, thus clearing the in-memory cache. For me, that isn't a particularly big deal.

Answer 1

Your cluster architecture confuses me, as it seems you are running services that should be cluster-managed (like Varnish) standalone on two nodes at the same time and let the cluster resource manager (CRM) just juggle IP addresses around.

What is it you want to achieve with your cluster setup? Fault tolerance? Load balancing? Both? Mind you, I am talking about the cluster resources (Varnish, IP addresses, etc), not the backend servers to which Varnish distributes the load.

To me it sounds like you want an active-passive two-node cluster, which provides fault tolerance. One node is active and runs Varnish, the virtual IP addresses and possibly other resources, and the other node is passive and does nothing until the cluster resource manager moves resources over to the passive node, at which point it becomes active. This is a tried-and-true architecture that is as old as time itself. But for it to work you need to give the CRM full control over the resources. I recommend following Clusters from Scratch and modelling your cluster after that.

Edit after your updated question: your CIB looks good, and once you patched the Varnish init script so that repeated calls to "start" return 0 you should be able to add the following primitive (adjust the timeouts and intervals to your liking):

primitive p_varnish lsb:varnish \
    op monitor interval="10s" timeout="15s" \
    op start interval="0" timeout="10s" \
    op stop interval="0" timeout="10s" 

Don't forget to add it to the balancer group (the last element in the list):

group balancer eth0_gateway eth1_iceman_slider eth1_iceman_slider_ts \
    eth1_iceman_slider_pm eth1_iceman_slider_jy eth1_iceman eth1_slider \
    eth1_viper eth1_jester p_varnish

Edit 2: To decrease the migration threshold add a resource defaults section at the end of your CIB and set the migration-threshold property to a low number. Setting it to 1 means the resource will be migrated after a single failure. It is also a good idea to set resource stickiness so that a resource that has been migrated because of node failure (reboot or shutdown) does not automatically get migrated back later when the node is available again.

rsc_defaults $id="rsc-options" \
    resource-stickiness="100" \
    migration-threshold="1"