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5th chunk of `content/en/blog/_posts/2018-03-00-How-To-Integrate-Rollingupdate-Strategy.md`
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Because TPR (currently CRD) does not support the rolling upgrade strategy, we needed to integrate the RollingUpdate strategy into WQ-RDS. Fortunately, the [Kubernetes repo][5] is a treasure for learning. In the process of implementation, there are some points to share:  

* **MySQL Sharding Cluster has ****changed**: Each StatefulSet has its corresponding ControllerRevision, which records all the revision data and order (like git). Whenever StatefulSet is syncing, StatefulSet Controller will firstly compare it's spec to the latest corresponding ControllerRevision data (similar to git diff). If changed, a new ControllerrRevision will be generated, and the revision number will be incremented by 1. WQ-RDS borrows the process, MySQL Sharding Cluster object will record all the revision and order in ControllerRevision.
* **How to perform a Rolling ****Upgrade**: Statefulset recreates pods in strictly decreasing order. The difference is that WQ-RDS updates shards instead of recreating them, as shown below:
![](https://lh6.googleusercontent.com/B4ig8krCsXwvMeBy8NamQi1DrihUEzBcRTHCqhn9kUvlcpPrFoYUNAxn61qh8S2HXcdg31QpOhWSsYHP0jI4QxPkKpZ5oY-k9gFp1eK63qt6rwTMMWMiBs45DObY6rw2R7c0lNPu)

* **When RollingUpdate ends**: Kubernetes signals termination clearly. A rolling update completes when all of a set's Pods have been updated to the updateRevision. The status's currentRevision is set to updateRevision and its updateRevision is set to the empty string. The status's currentReplicas is set to updateReplicas and its updateReplicas are set to 0.

###  Controller revision in WQ-RDS

Revision information is stored in MysqlCluster.Status and is no different than Statefulset.Status.

```

root@k8s-master ~]# kubectl get mysqlcluster -o yaml clustershard-c

apiVersion: v1

items:

\- apiVersion: mysql.orain.com/v1

 kind: MysqlCluster

 metadata:

   creationTimestamp: 2017-10-20T08:19:41Z

   labels:

     AppName: clustershard-crm

     Createdby: orain.com

     DBType: MySQL

   name: clustershard-c

   namespace: default

   resourceVersion: "415852"

   uid: 6bb089bb-b56f-11e7-ae02-525400e717a6

 spec:



     dbresourcespec:

       limitedcpu: 1200m

       limitedmemory: 400Mi
Title: Implementation Details of RollingUpdate in WQ-RDS and Controller Revisions
Summary
This section details the implementation of the RollingUpdate strategy in WQ-RDS, focusing on how it handles rolling updates and ControllerRevisions. It explains how WQ-RDS leverages the Kubernetes repository for learning and adapts the StatefulSet's ControllerRevision concept to track changes in MySQL Sharding Clusters. It also clarifies how WQ-RDS updates shards instead of recreating pods, as StatefulSet does, and describes the conditions under which a RollingUpdate is considered complete. Finally, it discusses how revision information is stored in MysqlCluster.Status, similar to Statefulset.Status, and provides a YAML example of a MysqlCluster object.