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Reword in line with style guide

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Co-authored-by: ZSC <zacharysarah@users.noreply.github.com>
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Tim Bannister
2020-01-09 01:12:43 +00:00
parent 664de5d7a8
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content/en/docs/tasks/run-application/run-replicated-stateful-application.md
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@@ -14,7 +14,7 @@ weight: 30
<!-- overview -->
This page shows how to run a replicated stateful application using a
[StatefulSet](/docs/concepts/workloads/controllers/statefulset/) controller.
{{< glossary_tooltip term_id="statefulset" >}}.
This application is a replicated MySQL database. The example topology has a
single primary server and multiple replicas, using asynchronous row-based
replication.
@@ -27,7 +27,7 @@ on general patterns for running stateful applications in Kubernetes.
## {{% heading "prerequisites" %}}
* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
* {{< include "task-tutorial-prereqs.md" >}}
* {{< include "default-storage-class-prereqs.md" >}}
* This tutorial assumes you are familiar with
[PersistentVolumes](/docs/concepts/storage/persistent-volumes/)
@@ -44,7 +44,7 @@ on general patterns for running stateful applications in Kubernetes.
## {{% heading "objectives" %}}
* Deploy a replicated MySQL topology with a StatefulSet controller.
* Deploy a replicated MySQL topology with a StatefulSet.
* Send MySQL client traffic.
* Observe resistance to downtime.
* Scale the StatefulSet up and down.
@@ -58,7 +58,7 @@ on general patterns for running stateful applications in Kubernetes.
The example MySQL deployment consists of a ConfigMap, two Services,
and a StatefulSet.
### ConfigMap
### Create a ConfigMap {#configmap}
Create the ConfigMap from the following YAML configuration file:
@@ -78,7 +78,7 @@ portions to apply to different Pods.
Each Pod decides which portion to look at as it's initializing,
based on information provided by the StatefulSet controller.
### Services
### Create Services {#services}
Create the Services from the following YAML configuration file:
@@ -88,23 +88,24 @@ Create the Services from the following YAML configuration file:
kubectl apply -f https://k8s.io/examples/application/mysql/mysql-services.yaml
```
The Headless Service provides a home for the DNS entries that the StatefulSet
controller creates for each Pod that's part of the set.
Because the Headless Service is named `mysql`, the Pods are accessible by
The headless Service provides a home for the DNS entries that the StatefulSet
{{< glossary_tooltip text="controllers" term_id="controller" >}} creates for each
Pod that's part of the set.
Because the headless Service is named `mysql`, the Pods are 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
The client Service, called `mysql-read`, is a normal Service with its own
cluster IP that distributes connections across all MySQL Pods that report
being Ready. The set of potential endpoints includes the primary MySQL server and all
replicas.
Note that only read queries can use the load-balanced Client Service.
Note that only read queries can use the load-balanced client Service.
Because there is only one primary MySQL server, clients should connect directly to the
primary MySQL Pod (through its DNS entry within the Headless Service) to execute
primary MySQL Pod (through its DNS entry within the headless Service) to execute
writes.
### StatefulSet
### Create the StatefulSet {#statefulset}
Finally, create the StatefulSet from the following YAML configuration file:
@@ -120,7 +121,7 @@ You can watch the startup progress by running:
kubectl get pods -l app=mysql --watch
```
After a while, you should see all 3 Pods become Running:
After a while, you should see all 3 Pods become `Running`:
```
NAME READY STATUS RESTARTS AGE
@@ -156,10 +157,10 @@ 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/concepts/workloads/pods/init-containers/)
[init containers](/docs/concepts/workloads/pods/init-containers/)
in the order defined.
The first Init Container, named `init-mysql`, generates special MySQL config
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
@@ -167,8 +168,7 @@ 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 StatefulSet
controller into the domain of MySQL server IDs, which require the same
properties.
into the domain of MySQL server IDs, which require the same properties.
The script in the `init-mysql` container also applies either `primary.cnf` or
`replica.cnf` from the ConfigMap by copying the contents into `conf.d`.
@@ -188,7 +188,7 @@ logs might not go all the way back to the beginning of time.
These conservative assumptions are the key to allow a running StatefulSet
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
The second init container, named `clone-mysql`, performs a clone operation on
a replica Pod the first time it starts up on an empty PersistentVolume.
That means it copies all existing data from another running Pod,
so its local state is consistent enough to begin replicating from the primary server.
@@ -203,7 +203,7 @@ Ready before starting Pod `N+1`.
### Starting replication
After the Init Containers complete successfully, the regular containers run.
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](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns).
@@ -292,13 +292,13 @@ endpoint might be selected upon each connection attempt:
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
## Simulate Pod and Node failure {#simulate-pod-and-node-downtime}
To demonstrate the increased availability of reading from the pool of replicas
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
### Break the Readiness probe
The [readiness probe](/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-readiness-probes)
for the `mysql` container runs the command `mysql -h 127.0.0.1 -e 'SELECT 1'`
@@ -372,13 +372,13 @@ NAME READY STATUS RESTARTS AGE IP NODE
mysql-2 2/2 Running 0 15m 10.244.5.27 kubernetes-node-9l2t
```
Then drain the Node by running the following command, which cordons it so
Then, drain the Node by running the following command, which cordons it so
no new Pods may schedule there, and then evicts any existing Pods.
Replace `<node-name>` with the name of the Node you found in the last step.
{{< caution >}}
Draining a Node can impact other workloads and applications that are
running on the same node, so only do the following step in a test
Draining a Node can impact other workloads and applications
running on the same node. Only perform the following step in a test
cluster.
{{< /caution >}}