容器探测用于检测容器中的应用实例是否正常工作，是保障业务可用性的一种传统机制。如果经过探测，实例的状态不符合预期，那么kubernetes就会把该问题实例" 摘除 "，不承担业务流量。kubernetes提供了两种探针来实现容器探测，分别是：

• liveness probes：存活性探针，用于检测应用实例当前是否处于正常运行状态，如果不是，k8s会重启容器
• readiness probes：就绪性探针，用于检测应用实例当前是否可以接收请求，如果不能，k8s不会转发流量

livenessProbe 决定是否重启容器，readinessProbe 决定是否将请求转发给容器。

上面两种探针目前均支持三种探测方式：

• Exec命令：在容器内执行一次命令，如果命令执行的退出码为0，则认为程序正常，否则不正常
• TCPSocket：将会尝试访问一个用户容器的端口，如果能够建立这条连接，则认为程序正常，否则不正常
• HTTPGet：调用容器内Web应用的URL，如果返回的状态码在200和399之间，则认为程序正常，否则不正常

下面以liveness probes为例，做几个演示：

方式一：Exec

#cat >pod-test.yaml <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: pod-liveness-exec
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    ports: 
    - name: nginx-port
      containerPort: 80
    livenessProbe:
      exec:
        command: ["/bin/cat","/tmp/hello.txt"] # 执行一个查看文件的命令
        #command: ["echo", "hello"]            # 改成能成功运行的命令就不会重启了
#EOF

'''
创建pod，发现pod不停重启，describe后发现如下log，
原因是pod启动之后就进行了健康检查，检查失败之后，容器被kill掉，然后尝试进行重启
Events:
  Type     Reason     Age                From               Message
  ----     ------     ----               ----               -------
  Normal   Scheduled  82s                default-scheduler  Successfully assigned dev/pod-liveness-exec to node2
  Normal   Pulled     30s (x3 over 82s)  kubelet, node2     Container image "nginx:1.17.1" already present on machine
  Normal   Created    30s (x3 over 82s)  kubelet, node2     Created container nginx
  Normal   Started    30s (x3 over 82s)  kubelet, node2     Started container nginx
  Warning  Unhealthy  0s (x9 over 80s)   kubelet, node2     Liveness probe failed: /bin/cat: /tmp/hello.txt: No such file or directory
  Normal   Killing    0s (x3 over 60s)   kubelet, node2     Container nginx failed liveness probe, will be restarted
'''

方式二：TCPSocket

#cat >pod-test.yaml <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: pod-liveness-tcpsocket
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    ports: 
    - name: nginx-port
      containerPort: 80
    livenessProbe:
      tcpSocket:
        port: 81 # 尝试访问81端口
        #port: 80 # 尝试访问80端口
#EOF

'''
尝试访问81端口,但是失败了，所以重启。改成80端口即可。这里tcp访问的是pod ip:port
Events:
  Type     Reason     Age                From               Message
  ----     ------     ----               ----               -------
  Normal   Scheduled  39s                default-scheduler  Successfully assigned dev/pod-liveness-tcpsocket to node2
  Normal   Pulled     10s (x2 over 39s)  kubelet, node2     Container image "nginx:1.17.1" already present on machine
  Normal   Created    10s (x2 over 39s)  kubelet, node2     Created container nginx
  Warning  Unhealthy  10s (x3 over 30s)  kubelet, node2     Liveness probe failed: dial tcp 10.244.2.55:81: connect: connection refused
  Normal   Killing    10s                kubelet, node2     Container nginx failed liveness probe, will be restarted
'''

方式三：HTTPGet

#cat >pod-test.yaml <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: pod-liveness-httpget
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    ports:
    - name: nginx-port
      containerPort: 80
    livenessProbe:
      httpGet:  # 其实就是访问http://127.0.0.1:80/hello  
        scheme: HTTP #支持的协议，http或者https
        port: 80 #端口号
        path: /hello #URI地址
        #path: / #URI地址
#EOF

'''
访问http://127.0.0.1:80/hello失败，重启pod。改个可访问的路径即可
Events:
  Type     Reason     Age               From               Message
  ----     ------     ----              ----               -------
  Normal   Scheduled  28s               default-scheduler  Successfully assigned dev/pod-liveness-httpget to node2
  Normal   Pulled     27s               kubelet, node2     Container image "nginx:1.17.1" already present on machine
  Normal   Created    27s               kubelet, node2     Created container nginx
  Normal   Started    27s               kubelet, node2     Started container nginx
  Warning  Unhealthy  9s (x2 over 19s)  kubelet, node2     Liveness probe failed: HTTP probe failed with statuscode: 404

'''