[Hawkular-dev] OpenShift Pet vs Cattle metaphor

[Lukas Krejci]

I have hardly any hands-on experience with Openshift or Kubernetes, but I was 
under the impression the pods can be configured using environment variables.

If that is true, couldn't the agent just read its feed id from a well-known 
env var? Or if that is not possible then surely we can have some wrapper 
script that would take the env var and pass it on to the agent as a -D system 
property (which I believe the agent accepts already).

On Wednesday, October 19, 2016 4:00:21 PM CEST Matt Wringe wrote:
> ----- Original Message -----
> 
> > From: "Jiri Kremser" <jkremser at redhat.com>
> > To: "Discussions around Hawkular development"
> > <hawkular-dev at lists.jboss.org> Sent: Wednesday, 19 October, 2016 11:49:26
> > AM
> > Subject: Re: [Hawkular-dev] OpenShift Pet vs Cattle metaphor
> > 
> > Hi Matt,
> > thanks for response.
> > 
> > PetSets are meant for clustering of pods which have persistent storage. If
> > this is not your use case, what exactly are you trying to do? There may be
> > better ways to handle it.
> > 
> > I am trying to figure out how to monitor wildfly in the openshift. If I am
> > not mistaken all the metric ids contain the feed id, feed id (at least for
> > wildfly is autogenerated if it's not provided in the xml config). If
> > container/pod is killed and re-created its history is lost with the feed
> > id. That's why I thought the Pet Sets with persistent ids can help.
> 
> I don't know if this sounds like the right approach.
> 
> The pods you are monitoring are still "cattle". They may be started,
> stopped, removed, increase in the number of instances running, decreased in
> the number of instances running or some other change may occur.
> 
> I might have a replica set of 3 pods right now, 10 minutes from now due to
> increased load my replica set may autoscale to 5 pods, and 2 hours from now
> it may decrease down to 2 pods.
> 
> If you want to track metrics from individual pods, then you should be
> setting the feed id to something unique like the namespace plus the pod
> name. The feed will change when the pod is recreated, but that is what you
> want, its no longer the same thing being monitored.
> 
> If you want to keep track of something like a particular replica set, then
> you need to use inventory or tags and query based on that.
> > jk
> > 
> > On Wed, Oct 19, 2016 at 3:49 PM, Matt Wringe < mwringe at redhat.com > wrote:
> > 
> > 
> > ----- Original Message -----
> > 
> > > From: "Jiri Kremser" < jkremser at redhat.com >
> > > To: "Discussions around Hawkular development" <
> > > hawkular-dev at lists.jboss.org >
> > > Sent: Thursday, 13 October, 2016 8:21:10 AM
> > > Subject: [Hawkular-dev] OpenShift Pet vs Cattle metaphor
> > > 
> > > Hello,
> > > today, I was on L&L about storage in OpenShift and I learn interesting
> > > thing.
> > > I always thought, that everything needs to be immutable and stateless
> > > and
> > > all the state needs to be handled by means of NFS persistent volumes.
> > > Luckily, there is a feature in Kubernetess (since 1.3) that allows the
> > > PODs
> > > to be treated as pets. It's called PetSet [1] and it assigns a unique ID
> > > (and persistent DNS record) to a POD that runs in this "mode".
> > 
> > For OpenShift, we would have moved to using PetSets for our Cassandra pod,
> > but its not a fully supported feature yet. In the next version we will be
> > moving over to using it.
> > 
> > It will make changing the cluster size for Cassandra nodes a lot easier
> > once we can use this.
> > 
> > > Common use-case for PetSet is a set of pods with a relational DBs that
> > > uses
> > > some kind of master-slave replication and slaves needs to know the
> > > master's
> > > address. But it can be used for anything. We can use the hostname as the
> > > feed id for instance.
> > > 
> > > I don't know how much this will be popular because it kind of defeats
> > > the
> > > purpose of immutable infrastructure but it can save us some work with
> > > the
> > > feed identity. And of course we need to support also the "normal" POD
> > > scenario.
> > 
> > PetSets are meant for clustering of pods which have persistent storage. If
> > this is not your use case, what exactly are you trying to do? There may be
> > better ways to handle it.
> > 
> > > [1]: http://kubernetes.io/docs/user-guide/petset/
> > > 
> > > jk
> > > 
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-- 
Lukas Krejci