k8s中设计了job、cronjob和daemonset这样的几个workload resource，应用于一些特殊的任务场景。

job

job顾名思义就是任务，其定义为
A Job creates one or more Pods and will continue to retry execution of the Pods until a specified number of them successfully terminate.
如这个例子

apiVersion: batch/v1
kind: Job
metadata:
  name: pi
spec:
  template:
    spec:
      containers:
      - name: pi
        image: perl
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: Never
  backoffLimit: 4

这个job是计算pi，执行完成后用kubectl get pods查看pod的id，然后使用kubectl logs jobid，可以看到pi的计算结果。

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095505822317253594081284811174502841027019385211055596446229489549303819644288109756659334461284756482337867831652712019091456485669234603486104543266482133936072602491412737245870066063155881748815209209628292540917153643678925903600113305305488204665213841469519415116094330572703657595919530921861173819326117931051185480744623799627495673518857527248912279381830119491298336733624406566430860213949463952247371907021798609437027705392171762931767523846748184676694051320005681271452635608277857713427577896091736371787214684409012249534301465495853710507922796892589235420199561121290219608640344181598136297747713099605187072113499999983729780499510597317328160963185950244594553469083026425223082533446850352619311881710100031378387528865875332083814206171776691473035982534904287554687311595628638823537875937519577818577805321712268066130019278766111959092164201989380952572010654858632788659361533818279682303019520353018529689957736225994138912497217752834791315155748572424541506959508295331168617278558890750983817546374649393192550604009277016711390098488240128583616035637076601047101819429555961989467678374494482553797747268471040475346462080466842590694912933136770289891521047521620569660240580381501935112533824300355876402474964732639141992726042699227967823547816360093417216412199245863150302861829745557067498385054945885869269956909272107975093029553211653449872027559602364806654991198818347977535663698074265425278625518184175746728909777727938000816470600161452491921732172147723501414419735685481613611573525521334757418494684385233239073941433345477624168625189835694855620992192221842725502542568876717904946016534668049886272327917860857843838279679766814541009538837863609506800642251252051173929848960841284886269456042419652850222106611863067442786220391949450471237137869609563643719172874677646575739624138908658326459958133904780275901

job有几个场景化的关键参数

• restartPolicy:Never，表示永不重试
• restartPolicy:Never, backoffLimit: 4，表示如果失败，最大重试次数是4
• activeDeadlineSeconds:100, 表示pod最多可运行100秒
• completions:8,parallelism:2，表示完成8次，同时只能有2个job并行运行

cronjob

看到cronjob，马上就可以对应到到jaba的cron组件，定时任务。
如下例子

apiVersion: batch/v1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            imagePullPolicy: IfNotPresent
            command:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure

这个yaml描述了一个定时任务，定时任务每分钟产生一个pod执行任务，使用kubectl log可以看到打印的日志

Hello from the Kubernetes cluster

这里主要要看到时间怎么定义的

#      ┌────────────────── timezone (optional)
#      |      ┌───────────── minute (0 - 59)
#      |      │ ┌───────────── hour (0 - 23)
#      |      │ │ ┌───────────── day of the month (1 - 31)
#      |      │ │ │ ┌───────────── month (1 - 12)
#      |      │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
#      |      │ │ │ │ │                                   7 is also Sunday on some systems)
#      |      │ │ │ │ │
#      |      │ │ │ │ │
# CRON_TZ=UTC * * * * *

daemonset

daemon负责守护，和linux中的daemon进程是一样的，为每个node启动一个守护pods，如下例子

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-elasticsearch
  namespace: kube-system
  labels:
    k8s-app: fluentd-logging
spec:
  selector:
    matchLabels:
      name: fluentd-elasticsearch
  template:
    metadata:
      labels:
        name: fluentd-elasticsearch
    spec:
      tolerations:
      # this toleration is to have the daemonset runnable on master nodes
      # remove it if your masters can't run pods
      - key: node-role.kubernetes.io/master
        operator: Exists
        effect: NoSchedule
      containers:
      - name: fluentd-elasticsearch
        image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
        resources:
          limits:
            memory: 200Mi
          requests:
            cpu: 100m
            memory: 200Mi
        volumeMounts:
        - name: varlog
          mountPath: /var/log
        - name: varlibdockercontainers
          mountPath: /var/lib/docker/containers
          readOnly: true
      terminationGracePeriodSeconds: 30
      volumes:
      - name: varlog
        hostPath:
          path: /var/log
      - name: varlibdockercontainers
        hostPath:
          path: /var/lib/docker/containers

运行这个yaml，会在node上启动日志收集功能的守护pod

小结

job是任务、cronjob是定时任务、daemonset是为每个node启动守护pod，各有各的使用场景，例如说cronjob在ci中可以执行定义任务，是非常有用的。