https://go.dok.community/slack https://dok.community/ ABSTRACT OF THE TALK There is confusion amongst Kubernetes users as to when you should make a Deployment with a PVC and when you should use a StatefulSet with a PVC. There is also a general lack of understanding when it comes to disk access policies, what RWO/RWX mean and what they allow you to do. These concepts are complicated and require a deep level of understanding in order to avoid users making bad decisions that they come to regret later. In this talk we will explore when you should use each type and what things you need to think about before making a decision. We'll also explore in detail how to safely deploy a multi-replica application with persistence using Portainer. As part of the talk we will explore disk access policy, understand what RWO really is and how RWX disk access changes the equation in regards to persistence. TAKE-AWAYS From this talk delegates will learn the difference between persistent disk and StatefulSet. They will learn when to use each (and when not to) and what are the pitfalls they should be aware of when deploying multi replica apps. They will also learn what is disk access policy, what is RWO and how RWX disk access changes the equation in regards to persistence

1. STATEFUL APPLICATIONS
The Do’s and Dont’s
Neil Cresswell
CEO, Co-Founder Portainer.io

2. WHAT ARE
STATEFUL
APPLICATIONS
Generally…
• Applications that need to persist data whilst
running, and through container lifecycle events
(restarts, rescheduling, updates)
• Applications that need a persistent identity
• Applications that expect to be started/stopped
in a certain order

3. WHAT ARE THEY
NOT
Just a Persistent
Volume

4. HOW DOES
KUBERNETES
HANDLE STATE
• Persistent data is held in persistent
volumes, connected via a volume claim
• PersistentVolumes are (or should) be
addressable across all nodes in the
cluster
• Container hostnames can be made
consistent/predictable
• POD start/stop order can be made
predictable

5. HOW CAN I DEPLOY STATEFUL APPS
Two ways to deploy, depending on how much “state” your application
expects.
• 1) Deployment with a PVC
• Used if you ONLY need to persist data, and your application is OK with random
hostnames, and random start/stop order
• 2) StatefulSet with a PVC
• Used if you need to persist data AND your application needs consistent hostnames
and a predictable start/stop order

6. OK SO THIS ALL VERY
SIMPLE, WHAT GIVES..
Storage Access Policy can complicate
things, and confuse people.. RWO vs
RWX
• Just because your storage supports
RWX DOESN’T mean your application
does
• An Admin can manually set the access
policy of their storage to something of
their choosing; DON’T

7. BUT WHY?
To stop data corruption..
• Unless your application fully supports concurrent access to its underlying
files, you will either get “last changed wins” or “locking violations”
As an example..
• Hostpath does not support RWX, however you CAN set it
• You can then use a Deployment to deploy MYSQL with persistent
volume, and 2 replicas (this seems to be a common thing believed as
possible according to Kubernetes user groups, espc when NFS storage is
used)

8. BUT THAT ENDS UP LIKE..

9. OR, IF I TRY DO-BLOCK-STORAGE
• This DOESN’T work if manually setting to RWX, and so you get…

10. OK, SO THAT’S CLEAR.. WHAT ABOUT
STATE OTHER THAN STORAGE
• If you use Deployment type, the hostnames assigned are randomly generated,
and do not persist update/reschedule events
• If I put node 3iksk into “drain” mode, see what happens.. Pod xmd46 got
redeployed and its now named nh67k

11. NOW LET ME REPEAT THAT, USING
STATEFULSET
• See 5 instances..
• Let me put that node sk back into drain mode; we should see stateful-0 and
stateful-1 rescheduled, and retain their same name..

12. HOW ABOUT START-UP ORDER
• StatefulSets always start lowest to highest, and stop, highest to lowest..

13. SO IN
SUMMARY
• If you REALLY need persistent, use statefulsets
• Do not manually override your storage access policy
• Don’t assume your application supports concurrent
writes
• And finally, Use Portainer ☺
neil@portainer.io