k8s で volume があるはずなのに volumeMounts[0].name: Not found になってしまう

まさかの同名 volume が定義できるケースがある


Posted on Fri, Nov 19, 2021
Tags kubernetes

ConfigMap の volumeMounts で思わぬ沼にはまってしまう

検証したバージョン: kubernetes 1.20.11

下記のように、Deployment などで ConfigMap を volumeMounts して利用することがある。

yaml-1

    spec:
      containers:
      - image: httpd
        name: httpd
        volumeMounts:
        - name: configmap-volume
          mountPath: /var/tmp/configmap
      volumes:
      - name: configmap-volume
        configMap:
          name: configmap-sample-data

上記の volume なのだが、通常は同名で重複して apply することはできない。

yaml-2

    spec:
      containers:
      - image: httpd
        name: httpd
        volumeMounts:
        - name: configmap-volume
          mountPath: /var/tmp/configmap
      volumes:
      - name: configmap-volume
        configMap:
          name: configmap-sample-data
      - name: configmap-volume
        configMap:
          name: configmap-sample-data
The Deployment "test" is invalid: spec.template.spec.volumes[1].name: Duplicate value: "configmap-volume"

しかし、yaml-1 を apply した後、yaml-2 を apply すると、同名の volume を apply できてしまう (!!!)

apply すると last-applied-configuration としては configmap-volume が 2つ生成されている。

その後、yaml-1 を apply すると Not found のエラーが発生する

# ここは普通
$ kubectl apply -f ~/Desktop/yaml-1.yml
deployment.apps/test created

# ここが反映できてしまう
$ kubectl apply -f ~/Desktop/yaml-2.yml
deployment.apps/test configured

# Not found になる
$ kubectl apply -f ~/Desktop/yaml-1.yml
The Deployment "test" is invalid:
* spec.template.spec.containers[0].volumeMounts[0].name: Not found: "configmap-volume"

Kubernetes 本家の issue をざっと見たが、仕様なのか、あるいはバグなのか不明(volume が配列で定義されていて、apply 後の追加をどう処理しているか次第)

調査の仕方として、1回目の apply を行ったあとに kubectl get deployment test -oyaml で deployment の中の last-applied-configuration の json を展開して確認する。

よく言われるように、kubernetes は宣言的なシステムというが、宣言的な設定のマスターは etcd の中にあり、それは last-applied-configuration で閲覧することができる。

kustomize や helm で色々試していると出くわすかもしれないケースで、沼にハマってしまった。

minikube での検証

下記の方法で再現が行える。

$ minikube kubectl -- apply -f <(cat <<EOF
apiVersion: v1
kind: Namespace
metadata:
  name: test
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: configmap-sample-data
  namespace: test
data:
  test: "test"
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
  namespace: test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80
        volumeMounts:
        - name: configmap-volume
          mountPath: /var/tmp/configmap
      volumes:
      - name: configmap-volume
        configMap:
          name: configmap-sample-data  
EOF
)
namespace/test created
configmap/configmap-sample-data created
deployment.apps/nginx-deployment created


$ minikube kubectl -- apply -f <(cat <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
  namespace: test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80
        volumeMounts:
        - name: configmap-volume
          mountPath: /var/tmp/configmap
      volumes:
      - name: configmap-volume
        configMap:
          name: configmap-sample-data 
      - name: configmap-volume # これが適用できてしまう
        configMap:
          name: configmap-sample-data
EOF
)
deployment.apps/nginx-deployment configured

$ minikube kubectl -- -ntest get deploy nginx-deployment -oyaml
apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    deployment.kubernetes.io/revision: "1"
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment","metadata":{"annotations":{},"labels":{"app":"nginx"},"name":"nginx-deployment","namespace":"test"},"spec":{"replicas":1,"selector":{"matchLabels":{"app":"nginx"}},"template":{"metadata":{"labels":{"app":"nginx"}},"spec":{"containers":[{"image":"nginx:latest","name":"nginx","ports":[{"containerPort":80}],"volumeMounts":[{"mountPath":"/var/tmp/configmap","name":"configmap-volume"}]}],"volumes":[{"configMap":{"name":"configmap-sample-data"},"name":"configmap-volume"},{"configMap":{"name":"configmap-sample-data"},"name":"configmap-volume"}]}}}}
  creationTimestamp: "2021-11-20T14:27:43Z"
  generation: 2
  labels:
    app: nginx
...

# last-applied-configuration を見てみる
{
    "apiVersion": "apps/v1",
    "kind": "Deployment",
    "metadata": {
        "annotations": {},
        "labels": {
            "app": "nginx"
        },
        "name": "nginx-deployment",
        "namespace": "test"
    },
    "spec": {
        "replicas": 1,
        "selector": {
            "matchLabels": {
                "app": "nginx"
            }
        },
        "template": {
            "metadata": {
                "labels": {
                    "app": "nginx"
                }
            },
            "spec": {
                "containers": [
                    {
                        "image": "nginx:latest",
                        "name": "nginx",
                        "ports": [
                            {
                                "containerPort": 80
                            }
                        ],
                        "volumeMounts": [
                            {
                                "mountPath": "/var/tmp/configmap",
                                "name": "configmap-volume"
                            }
                        ]
                    }
                ],
                "volumes": [
                    {
                        "configMap": {
                            "name": "configmap-sample-data"
                        },
                        "name": "configmap-volume"     # <-- 同名が2つ存在できる...
                    },
                    {
                        "configMap": {
                            "name": "configmap-sample-data"
                        },
                        "name": "configmap-volume"     # <-- 同名が2つ存在できる..
                    }
                ]
            }
        }
    }
}


# こちらが失敗する
$ minikube kubectl -- apply -f <(cat <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
  namespace: test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80
        volumeMounts:
        - name: configmap-volume
          mountPath: /var/tmp/configmap
      volumes:
      - name: configmap-volume
        configMap:
          name: configmap-sample-data
EOF
)
The Deployment "nginx-deployment" is invalid: spec.template.spec.containers[0].volumeMounts[0].name: Not found: "configmap-volume"

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