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Change secondary to replica and use primary as an adjective consistently

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This commit is contained in:
Christopher De Vries
2020-08-14 08:04:50 -04:00
parent 044d11de32
commit 4bb76cd0fb
3 changed files with 30 additions and 30 deletions
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content/en/docs/tasks/run-application/run-replicated-stateful-application.md
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@@ -15,7 +15,7 @@ weight: 30
This page shows how to run a replicated stateful application using a This page shows how to run a replicated stateful application using a
[StatefulSet](/docs/concepts/workloads/controllers/statefulset/) controller. [StatefulSet](/docs/concepts/workloads/controllers/statefulset/) controller.
The example is a MySQL single-primary topology with multiple secondaries running The example is a MySQL single-primary topology with multiple replicas running
asynchronous replication. asynchronous replication.
{{< note >}} {{< note >}}
@@ -69,9 +69,9 @@ kubectl apply -f https://k8s.io/examples/application/mysql/mysql-configmap.yaml
``` ```
This ConfigMap provides `my.cnf` overrides that let you independently control This ConfigMap provides `my.cnf` overrides that let you independently control
configuration on the MySQL primary and secondaries. configuration on the primary MySQL server and replicas.
In this case, you want the primary to be able to serve replication logs to secondaries In this case, you want the primary server to be able to serve replication logs to replicas
and you want secondaries to reject any writes that don't come via replication. and you want replicas to reject any writes that don't come via replication.
There's nothing special about the ConfigMap itself that causes different There's nothing special about the ConfigMap itself that causes different
portions to apply to different Pods. portions to apply to different Pods.
@@ -96,12 +96,12 @@ 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 cluster IP that distributes connections across all MySQL Pods that report
being Ready. The set of potential endpoints includes the MySQL primary and all being Ready. The set of potential endpoints includes the primary MySQL server and all
secondaries. 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 MySQL primary, clients should connect directly to the Because there is only one primary MySQL server, clients should connect directly to the
MySQL primary 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. writes.
### StatefulSet ### StatefulSet
@@ -168,32 +168,32 @@ controller into the domain of MySQL server IDs, which require the same
properties. properties.
The script in the `init-mysql` container also applies either `primary.cnf` or The script in the `init-mysql` container also applies either `primary.cnf` or
`secondary.cnf` from the ConfigMap by copying the contents into `conf.d`. `replica.cnf` from the ConfigMap by copying the contents into `conf.d`.
Because the example topology consists of a single MySQL primary and any number of Because the example topology consists of a single primary MySQL server and any number of
secondaries, the script simply assigns ordinal `0` to be the primary, and everyone replicas, the script simply assigns ordinal `0` to be the primary server, and everyone
else to be secondaries. else to be replicas.
Combined with the StatefulSet controller's Combined with the StatefulSet controller's
[deployment order guarantee](/docs/concepts/workloads/controllers/statefulset/#deployment-and-scaling-guarantees/), [deployment order guarantee](/docs/concepts/workloads/controllers/statefulset/#deployment-and-scaling-guarantees/),
this ensures the MySQL primary is Ready before creating secondaries, so they can begin this ensures the primary MySQL server is Ready before creating replicas, so they can begin
replicating. replicating.
### Cloning existing data ### Cloning existing data
In general, when a new Pod joins the set as a secondary, it must assume the MySQL In general, when a new Pod joins the set as a replica, it must assume the primary MySQL
primary might already have data on it. It also must assume that the replication server might already have data on it. It also must assume that the replication
logs might not go all the way back to the beginning of time. logs might not go all the way back to the beginning of time.
These conservative assumptions are the key to allow a running StatefulSet 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. 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 secondary Pod the first time it starts up on an empty PersistentVolume. a replica Pod the first time it starts up on an empty PersistentVolume.
That means it copies all existing data from another running Pod, That means it copies all existing data from another running Pod,
so its local state is consistent enough to begin replicating from the primary. so its local state is consistent enough to begin replicating from the primary server.
MySQL itself does not provide a mechanism to do this, so the example uses a MySQL itself does not provide a mechanism to do this, so the example uses a
popular open-source tool called Percona XtraBackup. popular open-source tool called Percona XtraBackup.
During the clone, the source MySQL server might suffer reduced performance. During the clone, the source MySQL server might suffer reduced performance.
To minimize impact on the MySQL primary, the script instructs each Pod to clone To minimize impact on the primary MySQL server, the script instructs each Pod to clone
from the Pod whose ordinal index is one lower. from the Pod whose ordinal index is one lower.
This works because the StatefulSet controller always ensures Pod `N` is This works because the StatefulSet controller always ensures Pod `N` is
Ready before starting Pod `N+1`. Ready before starting Pod `N+1`.
@@ -206,15 +206,15 @@ server, and an `xtrabackup` container that acts as a
[sidecar](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns). [sidecar](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns).
The `xtrabackup` sidecar looks at the cloned data files and determines if The `xtrabackup` sidecar looks at the cloned data files and determines if
it's necessary to initialize MySQL replication on the secondary. it's necessary to initialize MySQL replication on the replica.
If so, it waits for `mysqld` to be ready and then executes the If so, it waits for `mysqld` to be ready and then executes the
`CHANGE MASTER TO` and `START SLAVE` commands with replication parameters `CHANGE MASTER TO` and `START SLAVE` commands with replication parameters
extracted from the XtraBackup clone files. extracted from the XtraBackup clone files.
Once a secondary begins replication, it remembers its MySQL primary and Once a replica begins replication, it remembers its primary MySQL server and
reconnects automatically if the server restarts or the connection dies. reconnects automatically if the server restarts or the connection dies.
Also, because secondaries look for the primary at its stable DNS name Also, because replicas look for the primary server at its stable DNS name
(`mysql-0.mysql`), they automatically find the primary even if it gets a new (`mysql-0.mysql`), they automatically find the primary server even if it gets a new
Pod IP due to being rescheduled. Pod IP due to being rescheduled.
Lastly, after starting replication, the `xtrabackup` container listens for Lastly, after starting replication, the `xtrabackup` container listens for
@@ -224,7 +224,7 @@ case the next Pod loses its PersistentVolumeClaim and needs to redo the clone.
## Sending client traffic ## Sending client traffic
You can send test queries to the MySQL primary (hostname `mysql-0.mysql`) You can send test queries to the primary MySQL server (hostname `mysql-0.mysql`)
by running a temporary container with the `mysql:5.7` image and running the by running a temporary container with the `mysql:5.7` image and running the
`mysql` client binary. `mysql` client binary.
@@ -291,7 +291,7 @@ it running in another window so you can see the effects of the following steps.
## Simulating Pod and Node downtime ## Simulating Pod and Node downtime
To demonstrate the increased availability of reading from the pool of secondaries 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 instead of a single server, keep the `SELECT @@server_id` loop from above
running while you force a Pod out of the Ready state. running while you force a Pod out of the Ready state.
@@ -409,9 +409,9 @@ Now uncordon the Node to return it to a normal state:
kubectl uncordon <node-name> kubectl uncordon <node-name>
``` ```
## Scaling the number of secondaries ## Scaling the number of replicas
With MySQL replication, you can scale your read query capacity by adding secondaries. With MySQL replication, you can scale your read query capacity by adding replicas.
With StatefulSet, you can do this with a single command: With StatefulSet, you can do this with a single command:
```shell ```shell
content/en/examples/application/mysql/mysql-configmap.yaml
+2 -2
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@@ -9,8 +9,8 @@ data:
# Apply this config only on the primary. # Apply this config only on the primary.
[mysqld] [mysqld]
log-bin log-bin
secondary.cnf: | replica.cnf: |
# Apply this config only on secondaries. # Apply this config only on replicas.
[mysqld] [mysqld]
super-read-only super-read-only
content/en/examples/application/mysql/mysql-statefulset.yaml
+2 -2
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@@ -31,7 +31,7 @@ spec:
if [[ $ordinal -eq 0 ]]; then if [[ $ordinal -eq 0 ]]; then
cp /mnt/config-map/primary.cnf /mnt/conf.d/ cp /mnt/config-map/primary.cnf /mnt/conf.d/
else else
cp /mnt/config-map/secondary.cnf /mnt/conf.d/ cp /mnt/config-map/replica.cnf /mnt/conf.d/
fi fi
volumeMounts: volumeMounts:
- name: conf - name: conf
@@ -108,7 +108,7 @@ spec:
# Determine binlog position of cloned data, if any. # Determine binlog position of cloned data, if any.
if [[ -f xtrabackup_slave_info && "x$(<xtrabackup_slave_info)" != "x" ]]; then if [[ -f xtrabackup_slave_info && "x$(<xtrabackup_slave_info)" != "x" ]]; then
# XtraBackup already generated a partial "CHANGE MASTER TO" query # XtraBackup already generated a partial "CHANGE MASTER TO" query
# because we're cloning from an existing secondary. (Need to remove the tailing semicolon!) # because we're cloning from an existing replica. (Need to remove the tailing semicolon!)
cat xtrabackup_slave_info | sed -E 's/;$//g' > change_master_to.sql.in cat xtrabackup_slave_info | sed -E 's/;$//g' > change_master_to.sql.in
# Ignore xtrabackup_binlog_info in this case (it's useless). # Ignore xtrabackup_binlog_info in this case (it's useless).
rm -f xtrabackup_slave_info xtrabackup_binlog_info rm -f xtrabackup_slave_info xtrabackup_binlog_info