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Replace master/slave with primary/secondary where possible

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This commit is contained in:
Christopher De Vries
2020-08-13 08:59:24 -04:00
parent 8cb0b199ae
commit 044d11de32
4 changed files with 37 additions and 37 deletions
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content/en/docs/tasks/run-application/run-replicated-stateful-application.md
+27 -27
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@@ -15,7 +15,7 @@ weight: 30
This page shows how to run a replicated stateful application using a
[StatefulSet](/docs/concepts/workloads/controllers/statefulset/) controller.
The example is a MySQL single-master topology with multiple slaves running
The example is a MySQL single-primary topology with multiple secondaries running
asynchronous replication.
{{< 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
configuration on the MySQL master and 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.
configuration on the MySQL primary and secondaries.
In this case, you want the primary to be able to serve replication logs to secondaries
and you want secondaries 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.
@@ -96,12 +96,12 @@ cluster and namespace.
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 MySQL master and all
slaves.
being Ready. The set of potential endpoints includes the MySQL primary and all
secondaries.
Note that only read queries can use the load-balanced Client Service.
Because there is only one MySQL master, clients should connect directly to the
MySQL master Pod (through its DNS entry within the Headless Service) to execute
Because there is only one MySQL primary, clients should connect directly to the
MySQL primary Pod (through its DNS entry within the Headless Service) to execute
writes.
### StatefulSet
@@ -167,33 +167,33 @@ This translates the unique, stable identity provided by the StatefulSet
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 ConfigMap by copying the contents into `conf.d`.
Because the example topology consists of a single MySQL master and any number of
slaves, the script simply assigns ordinal `0` to be the master, and everyone
else to be slaves.
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`.
Because the example topology consists of a single MySQL primary and any number of
secondaries, the script simply assigns ordinal `0` to be the primary, and everyone
else to be secondaries.
Combined with the StatefulSet controller's
[deployment order guarantee](/docs/concepts/workloads/controllers/statefulset/#deployment-and-scaling-guarantees/),
this ensures the MySQL master is Ready before creating slaves, so they can begin
this ensures the MySQL primary is Ready before creating secondaries, so they can begin
replicating.
### Cloning existing data
In general, when a new Pod joins the set as a slave, it must assume the MySQL
master might already have data on it. It also must assume that the replication
In general, when a new Pod joins the set as a secondary, it must assume the MySQL
primary 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.
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
a slave Pod the first time it starts up on an empty PersistentVolume.
a secondary 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 master.
so its local state is consistent enough to begin replicating from the primary.
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 might suffer reduced performance.
To minimize impact on the MySQL master, the script instructs each Pod to clone
To minimize impact on the MySQL primary, the script instructs each Pod to clone
from the Pod whose ordinal index is one lower.
This works because the StatefulSet controller always ensures Pod `N` is
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).
The `xtrabackup` sidecar looks at the cloned data files and determines if
it's necessary to initialize MySQL replication on the slave.
it's necessary to initialize MySQL replication on the secondary.
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, it remembers its MySQL master and
Once a secondary begins replication, it remembers its MySQL primary and
reconnects automatically if the server restarts or the connection dies.
Also, because slaves look for the master at its stable DNS name
(`mysql-0.mysql`), they automatically find the master even if it gets a new
Also, because secondaries look for the primary at its stable DNS name
(`mysql-0.mysql`), they automatically find the primary even if it gets a new
Pod IP due to being rescheduled.
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
You can send test queries to the MySQL master (hostname `mysql-0.mysql`)
You can send test queries to the MySQL primary (hostname `mysql-0.mysql`)
by running a temporary container with the `mysql:5.7` image and running the
`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
To demonstrate the increased availability of reading from the pool of slaves
To demonstrate the increased availability of reading from the pool of secondaries
instead of a single server, keep the `SELECT @@server_id` loop from above
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>
```
## Scaling the number of slaves
## Scaling the number of secondaries
With MySQL replication, you can scale your read query capacity by adding slaves.
With MySQL replication, you can scale your read query capacity by adding secondaries.
With StatefulSet, you can do this with a single command:
```shell
content/en/examples/application/mysql/mysql-configmap.yaml
+4 -4
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@@ -5,12 +5,12 @@ metadata:
labels:
app: mysql
data:
master.cnf: |
# Apply this config only on the master.
primary.cnf: |
# Apply this config only on the primary.
[mysqld]
log-bin
slave.cnf: |
# Apply this config only on slaves.
secondary.cnf: |
# Apply this config only on secondaries.
[mysqld]
super-read-only
content/en/examples/application/mysql/mysql-services.yaml
+1 -1
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@@ -14,7 +14,7 @@ spec:
app: mysql
---
# Client service for connecting to any MySQL instance for reads.
# For writes, you must instead connect to the master: mysql-0.mysql.
# For writes, you must instead connect to the primary: mysql-0.mysql.
apiVersion: v1
kind: Service
metadata:
content/en/examples/application/mysql/mysql-statefulset.yaml
+5 -5
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@@ -29,9 +29,9 @@ spec:
echo server-id=$((100 + $ordinal)) >> /mnt/conf.d/server-id.cnf
# Copy appropriate conf.d files from config-map to emptyDir.
if [[ $ordinal -eq 0 ]]; then
cp /mnt/config-map/master.cnf /mnt/conf.d/
cp /mnt/config-map/primary.cnf /mnt/conf.d/
else
cp /mnt/config-map/slave.cnf /mnt/conf.d/
cp /mnt/config-map/secondary.cnf /mnt/conf.d/
fi
volumeMounts:
- name: conf
@@ -47,7 +47,7 @@ spec:
set -ex
# Skip the clone if data already exists.
[[ -d /var/lib/mysql/mysql ]] && exit 0
# Skip the clone on master (ordinal index 0).
# Skip the clone on primary (ordinal index 0).
[[ `hostname` =~ -([0-9]+)$ ]] || exit 1
ordinal=${BASH_REMATCH[1]}
[[ $ordinal -eq 0 ]] && exit 0
@@ -108,12 +108,12 @@ spec:
# Determine binlog position of cloned data, if any.
if [[ -f xtrabackup_slave_info && "x$(<xtrabackup_slave_info)" != "x" ]]; then
# XtraBackup already generated a partial "CHANGE MASTER TO" query
# because we're cloning from an existing slave. (Need to remove the tailing semicolon!)
# because we're cloning from an existing secondary. (Need to remove the tailing semicolon!)
cat xtrabackup_slave_info | sed -E 's/;$//g' > change_master_to.sql.in
# Ignore xtrabackup_binlog_info in this case (it's useless).
rm -f xtrabackup_slave_info xtrabackup_binlog_info
elif [[ -f xtrabackup_binlog_info ]]; then
# We're cloning directly from master. Parse binlog position.
# We're cloning directly from primary. Parse binlog position.
[[ `cat xtrabackup_binlog_info` =~ ^(.*?)[[:space:]]+(.*?)$ ]] || exit 1
rm -f xtrabackup_binlog_info xtrabackup_slave_info
echo "CHANGE MASTER TO MASTER_LOG_FILE='${BASH_REMATCH[1]}',\