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Add tutorial: Running a Replicated Stateful Application

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Anthony Yeh 2016-11-23 14:19:13 -08:00
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_data/tutorials.yml
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section:
- title: Running a Single-Instance Stateful Application
path: /docs/tutorials/stateful-application/run-stateful-application/
- title: Running a Replicated Stateful Application
path: /docs/tutorials/replicated-stateful-application/run-replicated-stateful-application/

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You need to either have a dynamic Persistent Volume provisioner with a default
[Storage Class](/docs/user-guide/persistent-volumes/#storageclasses),
or [statically provision Persistent Volumes](/docs/user-guide/persistent-volumes/#provisioning)
yourself to satisfy the Persistent Volume Claims used here.

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docs/tutorials/index.md
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@ -21,6 +21,7 @@ each of which has a sequence of steps.
#### Stateful Applications
* [Running a Single-Instance Stateful Application](/docs/tutorials/stateful-application/run-stateful-application/)
* [Running a Replicated Stateful Application](/docs/tutorials/replicated-stateful-application/run-replicated-stateful-application/)
### What's next

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docs/tutorials/replicated-stateful-application/run-replicated-stateful-application.md Normal file
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---
assignees:
- bprashanth
- enisoc
- erictune
- foxish
- janetkuo
- kow3ns
- smarterclayton
---
{% capture overview %}
This page shows how to run a replicated stateful application using a
[Stateful Set](/docs/concepts/controllers/statefulsets/) controller.
The example is a MySQL single-master topology with multiple slaves running
asynchronous replication.
Note that **this is not a production configuration**.
In particular, MySQL settings remain on insecure defaults to keep the focus
on general patterns for running stateful applications in Kubernetes.
{% endcapture %}
{% capture prerequisites %}
* {% include task-tutorial-prereqs.md %}
* {% include default-storage-class-prereqs.md %}
* This tutorial assumes you are familiar with
[Persistent Volumes](/docs/user-guide/persistent-volumes/)
and [Stateful Sets](/docs/concepts/controllers/statefulsets/),
as well as other core concepts like Pods, Services and Config Maps.
* Some familiarity with MySQL will help, but this tutorial aims to present
general patterns that should be useful for other systems.
{% endcapture %}
{% capture objectives %}
* Deploy a replicated MySQL topology with a Stateful Set controller.
* Send MySQL client traffic.
* Observe resistance to downtime.
* Scale the Stateful Set up and down.
{% endcapture %}
{% capture lessoncontent %}
### Deploying MySQL
The example MySQL deployment consists of a Config Map, two Services,
and a Stateful Set.
#### Config Map
Create the Config Map by saving the following manifest to `mysql-configmap.yaml`
and running:
```shell
kubectl create -f mysql-configmap.yaml
```
{% include code.html language="yaml" file="mysql-configmap.yaml" ghlink="/docs/tutorials/replicated-stateful-application/mysql-configmap.yaml" %}
This Config Map provides `my.cnf` overrides that let you independently control
configuration on the master and the slaves.
In this case, you want the master to be able to serve replication logs to slaves
and you want slaves to reject any writes that don't come via replication.
There's nothing special about the ConfigMap itself that causes different
portions to apply to different Pods.
Each Pod will decide which portion to look at as it's initializing,
based on information provided by the Stateful Set controller.
#### Services
Create the Services by saving the following manifest to `mysql-services.yaml`
and running:
```shell
kubectl create -f mysql-services.yaml
```
{% include code.html language="yaml" file="mysql-services.yaml" ghlink="/docs/tutorials/replicated-stateful-application/mysql-services.yaml" %}
The Headless Service provides a home for the DNS entries that the Stateful Set
controller will create for each Pod that's part of the set.
Since the Headless Service is named `mysql`, the Pods will be accessible by
resolving `<pod-name>.mysql` from within any other Pod in the same Kubernetes
cluster and namespace.
The Client Service, called `mysql-read`, is a normal Service with its own
cluster IP that will distribute connections across all MySQL Pods that report
being Ready. The set of endpoints will include the master and all slaves.
Note that only read queries can use the load-balanced Client Service.
Since there is only one master, clients should connect directly to the master
Pod (through its DNS entry within the Headless Service) to execute writes.
#### Stateful Set
Finally, create the Stateful Set by saving the following manifest to
`mysql-statefulset.yaml` and running:
```shell
kubectl create -f mysql-statefulset.yaml
```
{% include code.html language="yaml" file="mysql-statefulset.yaml" ghlink="/docs/tutorials/replicated-stateful-application/mysql-statefulset.yaml" %}
You can watch the startup progress by running:
```shell
kubectl get pods -l app=mysql --watch
```
After a while, you should see all 3 Pods become Running:
```
NAME READY STATUS RESTARTS AGE
mysql-0 2/2 Running 0 2m
mysql-1 2/2 Running 0 1m
mysql-2 2/2 Running 0 1m
```
Press **Ctrl+C** to cancel the watch.
If you don't see any progress, make sure you have a dynamic Persistent Volume
provisioner enabled as mentioned in the [prerequisites](#before-you-begin).
This manifest uses a variety of techniques for managing stateful Pods as part of
a Stateful Set. The next section highlights some of these techniques to explain
what happens as the Stateful Set creates Pods.
### Understanding stateful Pod initialization
The Stateful Set controller starts Pods one at a time, in order by their
ordinal index.
It waits until each Pod reports being Ready before starting the next one.
In addition, the controller assigns each Pod a unique, stable name of the form
`<statefulset-name>-<ordinal-index>`.
In this case, that results in Pods named `mysql-0`, `mysql-1`, and `mysql-2`.
The Pod template in the above Stateful Set manifest takes advantage of these
properties to perform orderly startup of MySQL replication.
#### Generating configuration
Before starting any of the containers in the Pod spec, the Pod first runs any
[Init Containers](/docs/user-guide/production-pods/#handling-initialization)
in the order defined.
In the Stateful Set manifest, you will find these defined within the
`pod.beta.kubernetes.io/init-containers` annotation.
The first Init Container, named `init-mysql`, generates special MySQL config
files based on the ordinal index.
The script determines its own ordinal index by extracting it from the end of
the Pod name, which is returned by the `hostname` command.
Then it saves the ordinal (with a numeric offset to avoid reserved values)
into a file called `server-id.cnf` in the MySQL `conf.d` directory.
This translates the unique, stable identity provided by the Stateful Set
controller into the domain of MySQL server IDs, which require the same
properties.
The script in the `init-mysql` container also applies either `master.cnf` or
`slave.cnf` from the Config Map by copying the contents into `conf.d`.
Since the example topology consists of a single master and any number of slaves,
the script simply assigns ordinal `0` to be the master, and everyone else to be
slaves.
#### Cloning existing data
In general, when a new Pod joins the set as a slave, it must assume the master
may already have data on it. It also must assume that the replication logs may
not go all the way back to the beginning of time.
These conservative assumptions are the key to allowing a running Stateful Set
to scale up and down over time, rather than being fixed at its initial size.
The second Init Container, named `clone-mysql`, performs a clone operation on
a slave Pod the first time it starts up on an empty Persistent Volume.
That means it copies all existing data from another running Pod,
so its local state is consistent enough to begin replicating from the master.
MySQL itself does not provide a mechanism to do this, so the example uses a
popular open-source tool called Percona XtraBackup.
During the clone, the source MySQL server may suffer reduced performance.
To minimize impact on the master, the script instructs each Pod to clone from
the Pod whose ordinal index is one lower.
This works because the Stateful Set controller will always ensure Pod `N` is
Ready before starting Pod `N+1`.
#### Starting replication
After the Init Containers complete successfully, the regular containers run.
The MySQL Pods consist of a `mysql` container that runs the actual `mysqld`
server, and an `xtrabackup` container that acts as a
[sidecar](http://blog.kubernetes.io/2015/06/the-distributed-system-toolkit-patterns.html).
The `xtrabackup` sidecar looks at the cloned data files and determines if
it's necessary to initialize MySQL replication on the slave.
If so, it waits for `mysqld` to be ready and then executes the
`CHANGE MASTER TO` and `START SLAVE` commands with replication parameters
extracted from the XtraBackup clone files.
Once a slave begins replication, by default it will remember its master and
reconnect automatically if the server is restarted or the connection dies.
Also, since slaves look for the master at its stable DNS name (`mysql-0.mysql`),
they will automatically find the master even if it gets a new Pod IP due to
being rescheduled.
Lastly, after starting replication, the `xtrabackup` container listens for
connections from other Pods requesting a data clone.
This server remains up indefinitely in case the Stateful Set scales up, or in
case the next Pod loses its Persistent Volume Claim and needs to redo the clone.
### Sending client traffic
You can send test queries to the master (hostname `mysql-0.mysql`)
by running a temporary container with the `mysql:5.7` image and running the
`mysql` client binary.
```shell
kubectl run mysql-client --image=mysql:5.7 -i -t --rm --restart=Never --\
mysql -h mysql-0.mysql <<EOF
CREATE DATABASE test;
CREATE TABLE test.messages (message VARCHAR(250));
INSERT INTO test.messages VALUES ('hello');
EOF
```
Use the hostname `mysql-read` to send test queries to any server that reports
being Ready:
```shell
kubectl run mysql-client --image=mysql:5.7 -i -t --rm --restart=Never --\
mysql -h mysql-read -e "SELECT * FROM test.messages"
```
You should get output like this:
```
Waiting for pod default/mysql-client to be running, status is Pending, pod ready: false
+---------+
| message |
+---------+
| hello |
+---------+
pod "mysql-client" deleted
```
To demonstrate that the `mysql-read` Service distributes connections across
servers, you can run `SELECT @@server_id` in a loop:
```shell
kubectl run mysql-client-loop --image=mysql:5.7 -i -t --rm --restart=Never --\
bash -ic "while sleep 1; do mysql -h mysql-read -e 'SELECT @@server_id,NOW()'; done"
```
You should see the reported `@@server_id` change randomly, since a different
endpoint may be selected upon each connection attempt:
```
+-------------+---------------------+
| @@server_id | NOW() |
+-------------+---------------------+
| 100 | 2006-01-02 15:04:05 |
+-------------+---------------------+
+-------------+---------------------+
| @@server_id | NOW() |
+-------------+---------------------+
| 102 | 2006-01-02 15:04:06 |
+-------------+---------------------+
+-------------+---------------------+
| @@server_id | NOW() |
+-------------+---------------------+
| 101 | 2006-01-02 15:04:07 |
+-------------+---------------------+
```
You can press **Ctrl+C** when you want to stop the loop, but it's useful to keep
it running in another window so you can see the effects of the following steps.
### Simulating Pod and Node downtime
To demonstrate the increased availability of reading from the pool of slaves
instead of a single server, keep the `SELECT @@server_id` loop from above
running while you force a Pod out of the Ready state.
#### Break the Readiness Probe
The [readiness probe](/docs/user-guide/production-pods/#liveness-and-readiness-probes-aka-health-checks)
for the `mysql` container runs the command `mysql -h 127.0.0.1 -e 'SELECT 1'`
to make sure the server is up and able to execute queries.
One way to force this readiness probe to fail is to break that command:
```shell
kubectl exec mysql-2 -c mysql -- mv /usr/bin/mysql /usr/bin/mysql.off
```
This reaches into the actual container's filesystem for Pod `mysql-2` and
renames the `mysql` command so the readiness probe can't find it.
After a few seconds, the Pod should report one of its containers as not Ready,
which you can check by running:
```shell
kubectl get pod mysql-2
```
Look for `1/2` in the `READY` column:
```
NAME READY STATUS RESTARTS AGE
mysql-2 1/2 Running 0 3m
```
At this point, you should see your `SELECT @@server_id` loop continue to run,
although it never reports `102` anymore.
Recall that the `init-mysql` script defined `server-id` as `100 + $ordinal`,
so server ID `102` corresponds to Pod `mysql-2`.
Now repair the Pod and it should reappear in the loop output
after a few seconds:
```shell
kubectl exec mysql-2 -c mysql -- mv /usr/bin/mysql.off /usr/bin/mysql
```
#### Delete Pods
The Stateful Set will also recreate Pods if they're deleted, similar to what a
Replica Set does for stateless Pods.
```shell
kubectl delete pod mysql-2
```
The Stateful Set controller will notice that no `mysql-2` Pod exists anymore,
and will create a new one with the same name and linked to the same
Persistent Volume Claim.
You should see server ID `102` disappear from the loop output for a while
and then return on its own.
#### Drain a Node
If your Kubernetes cluster has multiple Nodes, you can simulate Node downtime
(such as when Nodes are upgraded) by issuing a
[drain](http://kubernetes.io/docs/user-guide/kubectl/kubectl_drain/).
First determine which Node one of the MySQL Pods is on:
```shell
kubectl get pod mysql-2 -o wide
```
The Node name should show up in the last column:
```
NAME READY STATUS RESTARTS AGE IP NODE
mysql-2 2/2 Running 0 15m 10.244.5.27 kubernetes-minion-group-9l2t
```
Then drain the Node by running the following command, which will cordon it so
no new Pods may schedule there, and then evict any existing Pods.
Replace `<node-name>` with the name of the Node you found in the last step.
This may impact other applications on the Node, so it's best to
**only do this in a test cluster**.
```shell
kubectl drain <node-name> --force --delete-local-data --ignore-daemonsets
```
Now you can watch as the Pod reschedules on a different Node:
```shell
kubectl get pod mysql-2 -o wide --watch
```
It should look something like this:
```
NAME READY STATUS RESTARTS AGE IP NODE
mysql-2 2/2 Terminating 0 15m 10.244.1.56 kubernetes-minion-group-9l2t
[...]
mysql-2 0/2 Pending 0 0s <none> kubernetes-minion-group-fjlm
mysql-2 0/2 Init:0/2 0 0s <none> kubernetes-minion-group-fjlm
mysql-2 0/2 Init:1/2 0 20s 10.244.5.32 kubernetes-minion-group-fjlm
mysql-2 0/2 PodInitializing 0 21s 10.244.5.32 kubernetes-minion-group-fjlm
mysql-2 1/2 Running 0 22s 10.244.5.32 kubernetes-minion-group-fjlm
mysql-2 2/2 Running 0 30s 10.244.5.32 kubernetes-minion-group-fjlm
```
And again, you should see server ID `102` disappear from the
`SELECT @@server_id` loop output for a while and then return.
Now uncordon the Node to return it to a normal state:
```shell
kubectl uncordon <node-name>
```
### Scaling the number of slaves
With MySQL replication, you can scale your read query capacity by adding slaves.
With Stateful Set, you can do this with a single command:
```shell
kubectl scale --replicas=5 statefulset mysql
```
Watch the new Pods come up by running:
```shell
kubectl get pods -l app=mysql --watch
```
Once they're up, you should see server IDs `103` and `104` start appearing in
the `SELECT @@server_id` loop output.
You can also verify that these new servers have the data you added before they
existed:
```shell
kubectl run mysql-client --image=mysql:5.7 -i -t --rm --restart=Never --\
mysql -h mysql-3.mysql -e "SELECT * FROM test.messages"
```
```
Waiting for pod default/mysql-client to be running, status is Pending, pod ready: false
+---------+
| message |
+---------+
| hello |
+---------+
pod "mysql-client" deleted
```
Scaling back down is also seamless:
```shell
kubectl scale --replicas=3 statefulset mysql
```
Note, however, that while scaling up creates new Persistent Volume Claims
automatically, scaling down does not automatically delete these PVCs.
This gives you the choice to keep those initialized PVCs around to make
scaling back up quicker, or to extract data before deleting them.
You can see this by running:
```shell
kubectl get pvc -l app=mysql
```
Which will show that all 5 PVCs still exist, despite having scaled the
Stateful Set down to 3:
```
NAME STATUS VOLUME CAPACITY ACCESSMODES AGE
data-mysql-0 Bound pvc-8acbf5dc-b103-11e6-93fa-42010a800002 10Gi RWO 20m
data-mysql-1 Bound pvc-8ad39820-b103-11e6-93fa-42010a800002 10Gi RWO 20m
data-mysql-2 Bound pvc-8ad69a6d-b103-11e6-93fa-42010a800002 10Gi RWO 20m
data-mysql-3 Bound pvc-50043c45-b1c5-11e6-93fa-42010a800002 10Gi RWO 2m
data-mysql-4 Bound pvc-500a9957-b1c5-11e6-93fa-42010a800002 10Gi RWO 2m
```
If you don't intend to reuse the extra PVCs, you can delete them:
```shell
kubectl delete pvc data-mysql-3
kubectl delete pvc data-mysql-4
```
{% endcapture %}
{% capture cleanup %}
* Cancel the `SELECT @@server_id` loop by pressing **Ctrl+C** in its terminal,
or running the following from another terminal:
```shell
kubectl delete pod mysql-client-loop --now
```
* Delete the Stateful Set. This will also begin terminating the Pods.
```shell
kubectl delete statefulset mysql
```
* Verify that the Pods disappear. They may take some time to finish terminating.
```shell
kubectl get pods -l app=mysql
```
You'll know the Pods have terminated when the above returns:
```
No resources found.
```
* Delete the ConfigMap, Services, and Persistent Volume Claims.
```shell
kubectl delete configmap,service,pvc -l app=mysql
```
{% endcapture %}
{% capture whatsnext %}
* Look in the [Helm Charts repository](https://github.com/kubernetes/charts)
for other stateful application examples.
{% endcapture %}
{% include templates/tutorial.md %}