Re: [Hawkular-dev] OpenShift Pet vs Cattle metaphor
----- Original Message -----
From: "Jiri Kremser" <jkremser(a)redhat.com>
To: "Discussions around Hawkular development"
<hawkular-dev(a)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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