Kubernetes 健康状态检查liveness和readiness
1 .健康检查
健康检查（Health Check）是让系统知道您的应用实例是否正常工作的简单方法。 如果您的应用实例不再工作，则其他服务不应访问该应用或向其发送请求。 相反，应该将请求发送到已准备好的应用程序实例，或稍后重试。 系统还应该能够使您的应用程序恢复健康状态。
强大的自愈能力是 Kubernetes 这类容器编排引擎的一个重要特性。自愈的默认实现方式是自动重启发生故障的容器。除此之外，用户还可以利用Liveness 和 Readiness 探测机制设置更精细的健康检查，进而实现如下需求：
- 零停机部署。
- 避免部署无效的镜像。
- 更加安全的滚动升级。
2 .探针类型
Liveness存活性探针
Liveness探针让Kubernetes知道你的应用程序是活着还是死了。 如果你的应用程序还活着，那么Kubernetes就不管它了。 如果你的应用程序已经死了，Kubernetes将删除Pod并启动一个新的替换它。
Readiness就绪性探针
Readiness探针旨在让Kubernetes知道您的应用何时准备好其流量服务。 Kubernetes确保Readiness探针检测通过，然后允许服务将流量发送到Pod。 如果Readiness探针开始失败，Kubernetes将停止向该容器发送流量，直到它通过。 判断容器是否处于可用Ready状态, 达到ready状态表示pod可以接受请求, 如果不健康， 从service的后端endpoint列表中把pod隔离出去。
3 .探针执行方式
HTTP
HTTP探针可能是最常见的自定义Liveness探针类型。 即使您的应用程序不是HTTP服务，您也可以在应用程序内创建轻量级HTTP服务以响应Liveness探针。 Kubernetes去ping一个路径，如果它得到的是200或300范围内的HTTP响应，它会将应用程序标记为健康。 否则它被标记为不健康。
httpget配置项
host：连接的主机名，默认连接到pod的IP。你可能想在http header中设置"Host"而不是使用IP。 scheme：连接使用的schema，默认HTTP。 path: 访问的HTTP server的path。 httpHeaders：自定义请求的header。HTTP运行重复的header。 port：访问的容器的端口名字或者端口号。端口号必须介于1和65535之间。
Exec
对于Exec探针，Kubernetes则只是在容器内运行命令。 如果命令以退出代码0返回，则容器标记为健康。 否则，它被标记为不健康。 当您不能或不想运行HTTP服务时，此类型的探针则很有用，但是必须是运行可以检查您的应用程序是否健康的命令。
TCP
最后一种类型的探针是TCP探针，Kubernetes尝试在指定端口上建立TCP连接。 如果它可以建立连接，则容器被认为是健康的；否则被认为是不健康的。
如果您有HTTP探针或Command探针不能正常工作的情况，TCP探测器会派上用场。 例如，gRPC或FTP服务是此类探测的主要候选者。
4 .Liveness-exec样例
执行命令。容器的状态由命令执行完返回的状态码确定。如果返回的状态码是0，则认为pod是健康的，如果返回的是其他状态码，则认为pod不健康，这里不停的重启它。
#cat liveness_exec.yaml
apiVersion: v1
kind: Pod
metadata:
labels:
test: liveness-exec
name: liveness-exec
spec:
containers:
- name: liveness-exec
image: busybox
args:
- /bin/sh
- -c
- touch /tmp/healthy; sleep 30; rm -rf /tmp/healthy; sleep 600
livenessProbe:
exec:
command:
- cat
- /tmp/healthy
initialDelaySeconds: 5
periodSeconds: 5
periodSeconds字段指定kubelet应每5秒执行一次活跃度探测。
initialDelaySeconds字段告诉kubelet它应该在执行第一个探测之前等待5秒。 要执行探测，kubelet将在Container中执行命令cat /tmp/healthy。 如果命令成功，则返回0，并且kubelet认为Container是活动且健康的。 如果该命令返回非零值，则kubelet会终止容器并重新启动它。
5 .readiness-exec样例
apiVersion: apps/v1
kind: Deployment
metadata:
name: busybox-deployment
namespace: default
labels:
app: busybox
spec:
selector:
matchLabels:
app: busybox
replicas: 3
template:
metadata:
labels:
app: busybox
spec:
containers:
- name: busybox
image: busybox:latest
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
args:
- /bin/sh
- -c
- touch /tmp/healthy; sleep 30; rm -rf /tmp/healthy; sleep 600
readinessProbe:
exec:
command:
- cat
- /tmp/healthy
initialDelaySeconds: 5
periodSeconds: 5
pod启动，创建健康检查文件，这个时候是正常的，30s后删除，ready变成0，但pod没有被删除或者重启，k8s只是不管他了，仍然可以登录
[root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE busybox-deployment-6f86ddd894-l9phc 0/1 Running 0 3m10s busybox-deployment-6f86ddd894-lh46t 0/1 Running 0 3m busybox-deployment-6f86ddd894-sz5c2 0/1 Running 0 3m17s
我们再登录进去，手动创建健康检查文件，健康检查通过
[root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE busybox-deployment-6f86ddd894-l9phc 1/1 Running 0 7m44s busybox-deployment-6f86ddd894-lh46t 0/1 Running 0 7m34s busybox-deployment-6f86ddd894-sz5c2 1/1 Running 0 7m51s [root@k8s-master health]# [root@k8s-master health]# [root@k8s-master health]# [root@k8s-master health]# kubectl exec -it busybox-deployment-6f86ddd894-lh46t /bin/sh / # touch tmp/healthy / # [root@k8s-master health]# kubeget pods NAME READY STATUS RESTARTS AGE busybox-deployment-6f86ddd894-l9phc 1/1 Running 0 8m21s busybox-deployment-6f86ddd894-lh46t 1/1 Running 0 8m11s busybox-deployment-6f86ddd894-sz5c2 1/1 Running 0 8m28s [root@k8s-master health]#
6.iveness-http样例
#cat liveness_http.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
namespace: default
labels:
app: nginx
spec:
selector:
matchLabels:
app: nginx
replicas: 2
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
livenessProbe:
httpGet:
path: /index.html
port: 80
httpHeaders:
- name: X-Custom-Header
value: hello
initialDelaySeconds: 5
periodSeconds: 3
7 readiness-http样例
创建一个2个副本的deployment
# cat readiness_http.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
namespace: default
labels:
app: nginx
spec:
selector:
matchLabels:
app: nginx
replicas: 2
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
readinessProbe:
httpGet:
path: /index.html
port: 80
httpHeaders:
- name: X-Custom-Header
value: hello
initialDelaySeconds: 5
periodSeconds: 3
创建一个svc能访问
# cat readiness_http_svc.yaml
apiVersion: v1
kind: Service
metadata:
name: nginx
spec:
type: NodePort
ports:
- port: 80
nodePort: 30001
selector: #标签选择器
app: nginx
服务可以访问
[root@k8s-master health]# kubectl get pods -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES nginx-deployment-7db8445987-9wplj 1/1 Running 0 57s 10.254.1.81 k8s-node-1 <none> <none> nginx-deployment-7db8445987-mlc6d 1/1 Running 0 57s 10.254.2.65 k8s-node-2 <none> <none> [root@k8s-master health]# kubectl get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 5d3h nginx NodePort 10.108.167.58 <none> 80:30001/TCP 27m [root@k8s-master health]# [root@k8s-master health]# curl -I 10.6.76.24:30001/index.html HTTP/1.1 200 OK Server: nginx/1.17.3 Date: Tue, 03 Sep 2019 04:40:05 GMT Content-Type: text/html Content-Length: 612 Last-Modified: Tue, 13 Aug 2019 08:50:00 GMT Connection: keep-alive ETag: "5d5279b8-264" Accept-Ranges: bytes [root@k8s-master health]# curl -I 10.6.76.23:30001/index.html HTTP/1.1 200 OK Server: nginx/1.17.3 Date: Tue, 03 Sep 2019 04:40:11 GMT Content-Type: text/html Content-Length: 612 Last-Modified: Tue, 13 Aug 2019 08:50:00 GMT Connection: keep-alive ETag: "5d5279b8-264" Accept-Ranges: bytes
修改Nginx pod
[root@k8s-master health]# kubectl exec -it nginx-deployment-7db8445987-9wplj /bin/bash root@nginx-deployment-7db8445987-9wplj:/# cd /usr/share/nginx/html/ root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# ls 50x.html index.html root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# rm -f index.html root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# nginx -s reload 2019/09/03 03:58:52 [notice] 14#14: signal process started root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# root@nginx-deployment-7db8445987-9wplj:/usr/share/nginx/html# exit [root@k8s-master health]# kubectl get pods -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES nginx-deployment-7db8445987-9wplj 0/1 Running 0 110s 10.254.1.81 k8s-node-1 <none> <none> nginx-deployment-7db8445987-mlc6d 1/1 Running 0 110s 10.254.2.65 k8s-node-2 <none> <none> [root@k8s-master health]# curl -I 10.254.1.81/index.html HTTP/1.1 404 Not Found Server: nginx/1.17.3 Date: Tue, 03 Sep 2019 03:59:16 GMT Content-Type: text/html Content-Length: 153 Connection: keep-alive [root@k8s-master health]# kubectl describe pod nginx-deployment-7db8445987-9wplj Events: Type Reason Age From Message ---- ------ ---- ---- ------- Normal Scheduled 43m default-scheduler Successfully assigned default/nginx-deployment-7db8445987-9wplj tok8s-node-1 Normal Pulled 43m kubelet, k8s-node-1 Container image "nginx" already present on machine Normal Created 43m kubelet, k8s-node-1 Created container nginx Normal Started 43m kubelet, k8s-node-1 Started container nginx Warning Unhealthy 3m47s (x771 over 42m) kubelet, k8s-node-1 Readiness probe failed: HTTP probe failed with statuscode: 404
不再分发流量
我们把Nginx pod的index这个健康检查文件删除，并且把Nginx reload，k8s根据readiness把ready变成0，把它从集群摘除，不再分发流量，我们查看 一下两个pod的日志
[root@k8s-master health]# kubectl logs nginx-deployment-7db8445987-mlc6d | tail -10 10.254.1.0 - - [03/Sep/2019:04:43:44 +0000] "HEAD /index.html HTTP/1.1" 200 0 "-" "curl/7.29.0" "-" 10.254.1.0 - - [03/Sep/2019:04:43:45 +0000] "HEAD /index.html HTTP/1.1" 200 0 "-" "curl/7.29.0" "-" 10.254.1.0 - - [03/Sep/2019:04:43:45 +0000] "HEAD /index.html HTTP/1.1" 200 0 "-" "curl/7.29.0" "-" 10.254.2.1 - - [03/Sep/2019:04:43:46 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:43:49 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:43:52 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:43:55 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:43:58 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:44:01 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" 10.254.2.1 - - [03/Sep/2019:04:44:04 +0000] "GET /index.html HTTP/1.1" 200 612 "-" "kube-probe/1.15" "-" [root@k8s-master health]# kubectl logs nginx-deployment-7db8445987- | tail -10 nginx-deployment-7db8445987-9wplj nginx-deployment-7db8445987-mlc6d [root@k8s-master health]# kubectl logs nginx-deployment-7db8445987-9wplj | tail -10 2019/09/03 04:44:11 [error] 15#15: *939 open() "/usr/share/nginx/html/index.html" failed (2: No such file or directory), client: 10.254.1.1, server: localhost, request: "GET /index.html HTTP/1.1", host: "10.254.1.81:80" 10.254.1.1 - - [03/Sep/2019:04:44:11 +0000] "GET /index.html HTTP/1.1" 404 153 "-" "kube-probe/1.15" "-" 10.254.1.1 - - [03/Sep/2019:04:44:14 +0000] "GET /index.html HTTP/1.1" 404 153 "-" "kube-probe/1.15" "-" 2019/09/03 04:44:14 [error] 15#15: *940 open() "/usr/share/nginx/html/index.html" failed (2: No such file or directory), client: 10.254.1.1, server: localhost, request: "GET /index.html HTTP/1.1", host: "10.254.1.81:80" 2019/09/03 04:44:17 [error] 15#15: *941 open() "/usr/share/nginx/html/index.html" failed (2: No such file or directory), client: 10.254.1.1, server: localhost, request: "GET /index.html HTTP/1.1", host: "10.254.1.81:80" 10.254.1.1 - - [03/Sep/2019:04:44:17 +0000] "GET /index.html HTTP/1.1" 404 153 "-" "kube-probe/1.15" "-" 10.254.1.1 - - [03/Sep/2019:04:44:20 +0000] "GET /index.html HTTP/1.1" 404 153 "-" "kube-probe/1.15" "-" 2019/09/03 04:44:20 [error] 15#15: *942 open() "/usr/share/nginx/html/index.html" failed (2: No such file or directory), client: 10.254.1.1, server: localhost, request: "GET /index.html HTTP/1.1", host: "10.254.1.81:80" 2019/09/03 04:44:23 [error] 15#15: *943 open() "/usr/share/nginx/html/index.html" failed (2: No such file or directory), client: 10.254.1.1, server: localhost, request: "GET /index.html HTTP/1.1", host: "10.254.1.81:80" 10.254.1.1 - - [03/Sep/2019:04:44:23 +0000] "GET /index.html HTTP/1.1" 404 153 "-" "kube-probe/1.15" "-" [root@k8s-master health]#
8 TCP liveness和readiness探针
TCP检查的配置与HTTP检查非常相似，主要对于没有http接口的pod，像MySQL，Redis，等等
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
namespace: default
labels:
app: nginx
spec:
selector:
matchLabels:
app: nginx
replicas: 1
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
livenessProbe:
tcpSocket:
port: 80
initialDelaySeconds: 5
periodSeconds: 3
readinessProbe:
tcpSocket:
port: 80
initialDelaySeconds: 5
periodSeconds: 3
9 Probe详细配置
initialDelaySeconds：容器启动后第一次执行探测是需要等待多少秒。
periodSeconds：执行探测的频率。默认是10秒，最小1秒。
timeoutSeconds：探测超时时间。默认1秒，最小1秒。
successThreshold：探测失败后，最少连续探测成功多少次才被认定为成功。默认是1。对于liveness必须是1。最小值是1。
failureThreshold：探测成功后，最少连续探测失败多少次才被认定为失败。默认是3。最小值是1。
HTTP probe 中可以给 httpGet设置其他配置项：
使用Liveness探针时需要配置一个非常重要的设置，就是initialDelaySeconds设置。
Liveness探针失败会导致Pod重新启动。 在应用程序准备好之前，您需要确保探针不会启动。 否则，应用程序将不断重启，永远不会准备好！
10 健康检查在扩容中的应用readiness
对于多副本应用，当执行 Scale Up 操作时，新副本会作为 backend 被添加到 Service 的负责均衡中，与已有副本一起处理客户的请求。考虑到应用启动通常都需要一个准备阶段，比如加载缓存数据，连接数据库等，从容器启动到正真能够提供服务是需要一段时间的。我们可以通过 Readiness 探测判断容器是否就绪，避免将请求发送到还没有 ready 的 backend。
以上面readiness-http为例
# cat liveness_http.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
namespace: default
labels:
app: nginx
spec:
selector:
matchLabels:
app: nginx
replicas: 2
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
livenessProbe:
httpGet:
path: /index.html
port: 80
httpHeaders:
- name: X-Custom-Header
value: hello
initialDelaySeconds: 5
periodSeconds: 3
# cat readiness_http_svc.yaml
apiVersion: v1
kind: Service
metadata:
name: nginx
spec:
type: NodePort
ports:
- port: 80
nodePort: 30001
selector: #标签选择器
app: nginx
- 容器启动 5 秒之后开始探测。
- 如果 http://[container_ip]:80/index.html 返回代码不是 200-400，表示容器没有就绪，不接收 Service web-svc 的请求。
- 每隔 3 秒再探测一次。
- 直到返回代码为 200-400，表明容器已经就绪，然后将其加入到 web-svc 的负责均衡中，开始处理客户请求。
- 探测会继续以 5 秒的间隔执行，如果连续发生 3 次失败，容器又会从负载均衡中移除，直到下次探测成功重新加入。
- 对于生产环境中重要的应用都建议配置 Health Check，保证处理客户请求的容器都是准备就绪的 Service backend。
我们手动扩容一下，在pod的健康检查没有通过之前，新起的pod就不加入集群
[root@k8s-master health]# kubectl scale deployment nginx-deployment --replicas=5 deployment.extensions/nginx-deployment scaled [root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE nginx-deployment-7db8445987-k9df8 0/1 ContainerCreating 0 3s nginx-deployment-7db8445987-mlc6d 1/1 Running 0 3h14m nginx-deployment-7db8445987-q5d9k 0/1 ContainerCreating 0 3s nginx-deployment-7db8445987-w2w2t 0/1 ContainerCreating 0 3s nginx-deployment-7db8445987-zwj8t 1/1 Running 0 8m4s
11.健康检查在滚动更新中的应用
现有一个正常运行的多副本应用，接下来对应用进行更新（比如使用更高版本的 image），Kubernetes 会启动新副本，然后发生了如下事件：
l 正常情况下新副本需要 10 秒钟完成准备工作，在此之前无法响应业务请求。
l 但由于人为配置错误，副本始终无法完成准备工作（比如无法连接后端数据库）。
因为新副本本身没有异常退出，默认的 Health Check 机制会认为容器已经就绪，进而会逐步用新副本替换现有副本，其结果就是：当所有旧副本都被替换后，整个应用将无法处理请求，无法对外提供服务。如果这是发生在重要的生产系统上，后果会非常严重。
如果正确配置了 Health Check，新副本只有通过了 Readiness 探测，才会被添加到 Service；如果没有通过探测，现有副本不会被全部替换，业务仍然正常进行。
app.v1模拟一个5个副本的应用
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: app
spec:
replicas: 5
template:
metadata:
labels:
run: app
spec:
containers:
- name: app
image: busybox
args:
- /bin/sh
- -c
- sleep 10; touch /tmp/healthy; sleep 30000
readinessProbe:
exec:
command:
- cat
- /tmp/healthy
initialDelaySeconds: 10
periodSeconds: 5
10 秒后副本能够通过 Readiness 探测
[root@k8s-master health]# kubectl apply -f app_v1.yaml deployment.extensions/app unchanged [root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE app-6dd7f876c4-5hvdl 1/1 Running 0 6m17s app-6dd7f876c4-9vcp7 1/1 Running 0 6m17s app-6dd7f876c4-k59mm 1/1 Running 0 6m17s app-6dd7f876c4-trw8f 1/1 Running 0 6m17s app-6dd7f876c4-wrhz8 1/1 Running 0 6m17s [root@k8s-master health]#
接下来滚动更新应用
# cat app_v2.yaml
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: app
spec:
replicas: 5
template:
metadata:
labels:
run: app
spec:
containers:
- name: app
image: busybox
args:
- /bin/sh
- -c
- sleep 30000
readinessProbe:
exec:
command:
- cat
- /tmp/healthy
initialDelaySeconds: 10
periodSeconds: 5
[root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE app-6dd7f876c4-5hvdl 1/1 Running 0 14m app-6dd7f876c4-9vcp7 1/1 Running 0 14m app-6dd7f876c4-k59mm 1/1 Running 0 14m app-6dd7f876c4-trw8f 1/1 Running 0 14m app-6dd7f876c4-wrhz8 1/1 [root@k8s-master health]# kubectl apply -f app_v2.yaml deployment.extensions/app configured [root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE app-6dd7f876c4-5hvdl 1/1 Running 0 14m app-6dd7f876c4-9vcp7 1/1 Running 0 14m app-6dd7f876c4-k59mm 1/1 Terminating 0 14m app-6dd7f876c4-trw8f 1/1 Running 0 14m app-6dd7f876c4-wrhz8 1/1 Running 0 14m app-7fbf9d8fb7-g99hn 0/1 ContainerCreating 0 2s app-7fbf9d8fb7-ltlv5 0/1 ContainerCreating 0 3s [root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE app-6dd7f876c4-5hvdl 1/1 Running 0 14m app-6dd7f876c4-9vcp7 1/1 Running 0 14m app-6dd7f876c4-k59mm 1/1 Terminating 0 14m app-6dd7f876c4-trw8f 1/1 Running 0 14m app-6dd7f876c4-wrhz8 1/1 Running 0 14m app-7fbf9d8fb7-g99hn 0/1 ContainerCreating 0 9s app-7fbf9d8fb7-ltlv5 0/1 Running 0 10s [root@k8s-master health]# [root@k8s-master health]# kubectl get pods NAME READY STATUS RESTARTS AGE app-6dd7f876c4-5hvdl 1/1 Running 0 15m app-6dd7f876c4-9vcp7 1/1 Running 0 15m app-6dd7f876c4-trw8f 1/1 Running 0 15m app-6dd7f876c4-wrhz8 1/1 Running 0 15m app-7fbf9d8fb7-g99hn 0/1 Running 0 68s app-7fbf9d8fb7-ltlv5 0/1 Running 0 69s [root@k8s-master health]#
- 从 Pod 的 AGE 栏可判断，最后 2 个 Pod 是新副本，目前处于 NOT READY 状态。
- 旧副本从最初 5个减少到 4 个。
[root@k8s-master health]# kubectl get deployment app NAME READY UP-TO-DATE AVAILABLE AGE app 4/5 2 4 16m
l DESIRED 5 表示期望的状态是 5个 READY 的副本。
l UP-TO-DATE 2 表示当前已经完成更新的副本数：即 2 个新副本。
l AVAILABLE 4 表示当前处于 READY 状态的副本数：即 4个旧副本。
在我们的设定中，新副本始终都无法通过 Readiness 探测，所以这个状态会一直保持下去。
上面我们模拟了一个滚动更新失败的场景。不过幸运的是：Health Check 帮我们屏蔽了有缺陷的副本，同时保留了大部分旧副本，业务没有因更新失败受到影响。
滚动更新可以通过参数 maxSurge 和 maxUnavailable 来控制副本替换的数量。
