Run Memcached persistently on Kubernetes

Oct 15, 2020 · 2 min read · docker kubernetes memcached ·

When running Memcached on the Kubernetes cluster, the cache data will disappear whenever the pod restarts. I know that Redis has the built-in replication and it can be able to have data on persistence disks. However, I have some reasons to run the Memecached on our Kubernetes cluster. So, I was thinking about it and I came across my mind the way using postStart and preStop in lifecycle settings.

Before explaining, the disadvantages of this way are as follows.

Cannot cope with node failures
Need to add a persistent volume

First, create a script file that loads a dump file if it exists, and then output an empty file for the readiness probe. (post_start.sh)

 1#!/bin/ash
 2memc_host="127.0.0.1"
 3memc_port=11211
 4
 5# Load previous prestop data
 6dump_file="${DUMP_DIR}/${HOSTNAME}.dump.prestop"
 7if [ -f $dump_file ];then
 8    echo "Loading previous prestop data..."
 9    cat $dump_file | nc $memc_host $memc_port
10fi
11
12# For readiness probe
13touch /tmp/ready

And then, create a script that dump data before termination.

1#!/bin/ash
2
3# Load previous prestop data
4/memcached-tool ${HOSTNAME} dump > ${DUMP_DIR}/${HOSTNAME}.dump.prestop

After creating the above scripts, build the container image with the scripts by the following Dockerfile.

 1FROM memcached:1.6.6-alpine
 2
 3# Switch to root for installing packages
 4USER root
 5
 6RUN apk --update --no-cache add \
 7        bash \
 8        busybox-extras \
 9        perl
10
11COPY memcached-tool  /
12COPY post_start.sh   /
13COPY pre_stop.sh     /
14
15# Switch to memcache for running
16USER memcache

Finally, create the following YAML file and deploy it to the Kubernetes cluster.

 1apiVersion: apps/v1
 2kind: StatefulSet 
 3metadata:
 4  name: memcached
 5  namespace: app
 6  labels:
 7    app: memcached
 8spec:
 9  replicas: 2
10
11  selector:
12    matchLabels:
13      app: memcached
14
15  serviceName: memcached
16  template:
17    metadata:
18      labels:
19        app: memcached
20    spec:
21      affinity:
22        podAntiAffinity:
23          requiredDuringSchedulingIgnoredDuringExecution:
24          - labelSelector:
25              matchExpressions:
26              - key: app
27                operator: In
28                values:
29                - memcached
30            topologyKey: "kubernetes.io/hostname"
31
32      containers:
33        - name: memcached
34          image: registry/my-memcached:202010051112
35          imagePullPolicy: IfNotPresent
36          args: ["-U", "0", "-B", "ascii"]
37
38          ports:
39            - containerPort: 11211 
40
41          env:
42            - name: DUMP_DIR
43              value: "/var/tmp/memcache-dump"
44
45          resources:
46            requests:
47              cpu: 10m
48              memory: 16Mi
49
50          livenessProbe:
51            tcpSocket:
52              port: 11211
53            initialDelaySeconds: 2 
54            periodSeconds: 5
55
56          readinessProbe:
57            exec:
58              command:
59              - cat
60              - /tmp/ready
61            initialDelaySeconds: 5
62            periodSeconds: 5
63   
64          lifecycle:
65            postStart:
66              exec:
67                command: ["/post_start.sh"]
68            preStop:
69              exec:
70                command: ["/pre_stop.sh"]
71
72          volumeMounts:
73            - name: "memcache-dump"
74              mountPath: "/var/tmp/memcache-dump"
75
76      volumes:
77        - name: "memcache-dump"
78          persistentVolumeClaim:
79            claimName: memcache-efs

I know this is not a sophisticated way but it’s easy to create. So, I think it’s useful in some situations.