Suppose an application needs a configuration file, but changing that file is the only customization you need. A ConfigMap lets you define the file in a Kubernetes manifest and present it to a container through a mounted volume, rather than building a custom container image. section 9.1.1 section 9.2.2
A ConfigMap holds configuration, not the application's persistent data. In a Redis StatefulSet, the container reads its configuration from /redis/conf, while a separate persistent volume holds data in /redis/data. An emptyDir has another job: it provides temporary space that containers in a Pod can share. section 9.2.1 section 9.1.1
One file for a Redis Pod
A ConfigMap defines redis.conf with settings such as the port and data directory. The StatefulSet mounts that ConfigMap at /redis/conf, then starts redis-server with /redis/conf/redis.conf as its argument. section 9.2.1
Listing 9.9 Chapter09/9.2.2_StatefulSet_Redis_Multi/redis-configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: redis-role-config
data:
primary.conf: | #1
bind 0.0.0.0
port 6379
protected-mode no
appendonly yes
dir /redis/data
replica.conf: | #2
replicaof redis-0.redis-service 6379 #3
bind 0.0.0.0
port 6379
protected-mode no
appendonly yes
dir /redis/data
#1 The first file in the config map, to configure the primary role
#2 The second file in the config map, to configure the replica role
#3 Configure the Redis replica to reference the primary Pod by its name
The manifest defines two files in one ConfigMap: primary.conf and replica.conf. The replica file names the primary Pod, giving each Redis role the configuration it needs. section 9.2.2
command:
- bash
- "-c"
- |
set -ex
# Generate server-id from Pod ordinal index.
[[ `hostname` =~ -([0-9]+)$ ]] || exit 1
ordinal=${BASH_REMATCH[1]}
# Copy appropriate config files from config-map to emptyDir.
mkdir -p /redis/conf/
if [[ $ordinal -eq 0 ]]; then
cp /mnt/redis-configmap/primary.conf /redis/conf/redis.conf
else
cp /mnt/redis-configmap/replica.conf /redis/conf/redis.conf
fi
An init container uses the StatefulSet Pod's ordinal to copy the appropriate ConfigMap file into a shared emptyDir as redis.conf: primary configuration for ordinal 0, replica configuration for the others. section 9.2.2
A ConfigMap supplies settings; it does not make Redis replicas coordinate by itself. The role-specific configuration and init-container step turn separate StatefulSet Pods into a primary-and-replica setup. The demonstrated Redis configuration has no authentication, so it needs further consideration before production use. section 9.2.2 section 9.2.1
