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Declarative Management of Kubernetes Objects Using Configuration Files

Kubernetes objects can be created, updated, and deleted by storing multiple object configuration files in a directory and using kubectl apply to recursively create and update those objects as needed. This method retains writes made to live objects without merging the changes back into the object configuration files.

Trade-offs

The kubectl tool supports three kinds of object management:

See Kubernetes Object Management for a discussion of the advantages and disadvantage of each kind of object management.

Before you begin

Declarative object configuration requires a firm understanding of the Kubernetes object definitions and configuration. Read and complete the following documents if you have not already:

Following are definitions for terms used in this document:

How to create objects

Use kubectl apply to create all objects, except those that already exist, defined by configuration files in a specified directory:

kubectl apply -f <directory>/

This sets the kubectl.kubernetes.io/last-applied-configuration: '{...}' annotation on each object. The annotation contains the contents of the object configuration file that was used to create the object.

Note: Add the -R flag to recursively process directories.

Here’s an example of an object configuration file:

application/simple_deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  minReadySeconds: 5
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

Create the object using kubectl apply:

kubectl apply -f https://k8s.io/examples/application/simple_deployment.yaml

Print the live configuration using kubectl get:

kubectl get -f https://k8s.io/examples/application/simple_deployment.yaml -o yaml

The output shows that the kubectl.kubernetes.io/last-applied-configuration annotation was written to the live configuration, and it matches the configuration file:

kind: Deployment
metadata:
  annotations:
    # ...
    # This is the json representation of simple_deployment.yaml
    # It was written by kubectl apply when the object was created
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"minReadySeconds":5,"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.7.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
  # ...
spec:
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.7.9
        # ...
        name: nginx
        ports:
        - containerPort: 80
        # ...
      # ...
    # ...
  # ...

How to update objects

You can also use kubectl apply to update all objects defined in a directory, even if those objects already exist. This approach accomplishes the following:

  1. Sets fields that appear in the configuration file in the live configuration.
  2. Clears fields removed from the configuration file in the live configuration.
kubectl apply -f <directory>/
Note: Add the -R flag to recursively process directories.

Here’s an example configuration file:

application/simple_deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  minReadySeconds: 5
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

Create the object using kubectl apply:

kubectl apply -f https://k8s.io/examples/application/simple_deployment.yaml
Note: For purposes of illustration, the preceding command refers to a single configuration file instead of a directory.

Print the live configuration using kubectl get:

kubectl get -f https://k8s.io/examples/application/simple_deployment.yaml -o yaml

The output shows that the kubectl.kubernetes.io/last-applied-configuration annotation was written to the live configuration, and it matches the configuration file:

kind: Deployment
metadata:
  annotations:
    # ...
    # This is the json representation of simple_deployment.yaml
    # It was written by kubectl apply when the object was created
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"minReadySeconds":5,"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.7.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
  # ...
spec:
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.7.9
        # ...
        name: nginx
        ports:
        - containerPort: 80
        # ...
      # ...
    # ...
  # ...

Directly update the replicas field in the live configuration by using kubectl scale. This does not use kubectl apply:

kubectl scale deployment/nginx-deployment --replicas=2

Print the live configuration using kubectl get:

kubectl get -f https://k8s.io/examples/application/simple_deployment.yaml -o yaml

The output shows that the replicas field has been set to 2, and the last-applied-configuration annotation does not contain a replicas field:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    # ...
    # note that the annotation does not contain replicas
    # because it was not updated through apply
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"minReadySeconds":5,"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.7.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
  # ...
spec:
  replicas: 2 # written by scale
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.7.9
        # ...
        name: nginx
        ports:
        - containerPort: 80
      # ...

Update the simple_deployment.yaml configuration file to change the image from nginx:1.7.9 to nginx:1.11.9, and delete the minReadySeconds field:

application/update_deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.11.9 # update the image
        ports:
        - containerPort: 80

Apply the changes made to the configuration file:

kubectl apply -f https://k8s.io/examples/application/update_deployment.yaml

Print the live configuration using kubectl get:

kubectl get -f https://k8s.io/examples/application/simple_deployment.yaml -o yaml

The output shows the following changes to the live configuration:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    # ...
    # The annotation contains the updated image to nginx 1.11.9,
    # but does not contain the updated replicas to 2
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.11.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
    # ...
spec:
  replicas: 2 # Set by `kubectl scale`.  Ignored by `kubectl apply`.
  # minReadySeconds cleared by `kubectl apply`
  # ...
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.11.9 # Set by `kubectl apply`
        # ...
        name: nginx
        ports:
        - containerPort: 80
        # ...
      # ...
    # ...
  # ...
Warning: Mixing kubectl apply with the imperative object configuration commands create and replace is not supported. This is because create and replace do not retain the kubectl.kubernetes.io/last-applied-configuration that kubectl apply uses to compute updates.

How to delete objects

There are two approaches to delete objects managed by kubectl apply.

Manually deleting objects using the imperative command is the recommended approach, as it is more explicit about what is being deleted, and less likely to result in the user deleting something unintentionally:

kubectl delete -f <filename>

Alternative: kubectl apply -f <directory/> --prune -l your=label

Only use this if you know what you are doing.

Warning: kubectl apply --prune is in alpha, and backwards incompatible changes might be introduced in subsequent releases.
Warning: You must be careful when using this command, so that you do not delete objects unintentionally.

As an alternative to kubectl delete, you can use kubectl apply to identify objects to be deleted after their configuration files have been removed from the directory. Apply with --prune queries the API server for all objects matching a set of labels, and attempts to match the returned live object configurations against the object configuration files. If an object matches the query, and it does not have a configuration file in the directory, and it has a last-applied-configuration annotation, it is deleted.

kubectl apply -f <directory/> --prune -l <labels>
Warning: Apply with prune should only be run against the root directory containing the object configuration files. Running against sub-directories can cause objects to be unintentionally deleted if they are returned by the label selector query specified with -l <labels> and do not appear in the subdirectory.

How to view an object

You can use kubectl get with -o yaml to view the configuration of a live object:

kubectl get -f <filename|url> -o yaml

How apply calculates differences and merges changes

Caution: A patch is an update operation that is scoped to specific fields of an object instead of the entire object. This enables updating only a specific set of fields on an object without reading the object first.

When kubectl apply updates the live configuration for an object, it does so by sending a patch request to the API server. The patch defines updates scoped to specific fields of the live object configuration. The kubectl apply command calculates this patch request using the configuration file, the live configuration, and the last-applied-configuration annotation stored in the live configuration.

Merge patch calculation

The kubectl apply command writes the contents of the configuration file to the kubectl.kubernetes.io/last-applied-configuration annotation. This is used to identify fields that have been removed from the configuration file and need to be cleared from the live configuration. Here are the steps used to calculate which fields should be deleted or set:

  1. Calculate the fields to delete. These are the fields present in last-applied-configuration and missing from the configuration file.
  2. Calculate the fields to add or set. These are the fields present in the configuration file whose values don’t match the live configuration.

Here’s an example. Suppose this is the configuration file for a Deployment object:

application/update_deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.11.9 # update the image
        ports:
        - containerPort: 80

Also, suppose this is the live configuration for the same Deployment object:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    # ...
    # note that the annotation does not contain replicas
    # because it was not updated through apply
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"minReadySeconds":5,"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.7.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
  # ...
spec:
  replicas: 2 # written by scale
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.7.9
        # ...
        name: nginx
        ports:
        - containerPort: 80
      # ...

Here are the merge calculations that would be performed by kubectl apply:

  1. Calculate the fields to delete by reading values from last-applied-configuration and comparing them to values in the configuration file. Clear fields explicitly set to null in the local object configuration file regardless of whether they appear in the last-applied-configuration. In this example, minReadySeconds appears in the last-applied-configuration annotation, but does not appear in the configuration file. Action: Clear minReadySeconds from the live configuration.
  2. Calculate the fields to set by reading values from the configuration file and comparing them to values in the live configuration. In this example, the value of image in the configuration file does not match the value in the live configuration. Action: Set the value of image in the live configuration.
  3. Set the last-applied-configuration annotation to match the value of the configuration file.
  4. Merge the results from 1, 2, 3 into a single patch request to the API server.

Here is the live configuration that is the result of the merge:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    # ...
    # The annotation contains the updated image to nginx 1.11.9,
    # but does not contain the updated replicas to 2
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"apps/v1","kind":"Deployment",
      "metadata":{"annotations":{},"name":"nginx-deployment","namespace":"default"},
      "spec":{"selector":{"matchLabels":{"app":nginx}},"template":{"metadata":{"labels":{"app":"nginx"}},
      "spec":{"containers":[{"image":"nginx:1.11.9","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}
    # ...
spec:
  selector:
    matchLabels:
      # ...
      app: nginx
  replicas: 2 # Set by `kubectl scale`.  Ignored by `kubectl apply`.
  # minReadySeconds cleared by `kubectl apply`
  # ...
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.11.9 # Set by `kubectl apply`
        # ...
        name: nginx
        ports:
        - containerPort: 80
        # ...
      # ...
    # ...
  # ...

How different types of fields are merged

How a particular field in a configuration file is merged with the live configuration depends on the type of the field. There are several types of fields:

When kubectl apply updates a map or list field, it typically does not replace the entire field, but instead updates the individual subelements. For instance, when merging the spec on a Deployment, the entire spec is not replaced. Instead the subfields of spec, such as replicas, are compared and merged.

Merging changes to primitive fields

Primitive fields are replaced or cleared.

Note: - is used for “not applicable” because the value is not used.
Field in object configuration file Field in live object configuration Field in last-applied-configuration Action
Yes Yes - Set live to configuration file value.
Yes No - Set live to local configuration.
No - Yes Clear from live configuration.
No - No Do nothing. Keep live value.

Merging changes to map fields

Fields that represent maps are merged by comparing each of the subfields or elements of the map:

Note: - is used for “not applicable” because the value is not used.
Key in object configuration file Key in live object configuration Field in last-applied-configuration Action
Yes Yes - Compare sub fields values.
Yes No - Set live to local configuration.
No - Yes Delete from live configuration.
No - No Do nothing. Keep live value.

Merging changes for fields of type list

Merging changes to a list uses one of three strategies:

The choice of strategy is made on a per-field basis.

Replace the list

Treat the list the same as a primitive field. Replace or delete the entire list. This preserves ordering.

Example: Use kubectl apply to update the args field of a Container in a Pod. This sets the value of args in the live configuration to the value in the configuration file. Any args elements that had previously been added to the live configuration are lost. The order of the args elements defined in the configuration file is retained in the live configuration.

# last-applied-configuration value
    args: ["a", "b"]

# configuration file value
    args: ["a", "c"]

# live configuration
    args: ["a", "b", "d"]

# result after merge
    args: ["a", "c"]

Explanation: The merge used the configuration file value as the new list value.

Merge individual elements of a list of complex elements:

Treat the list as a map, and treat a specific field of each element as a key. Add, delete, or update individual elements. This does not preserve ordering.

This merge strategy uses a special tag on each field called a patchMergeKey. The patchMergeKey is defined for each field in the Kubernetes source code: types.go When merging a list of maps, the field specified as the patchMergeKey for a given element is used like a map key for that element.

Example: Use kubectl apply to update the containers field of a PodSpec. This merges the list as though it was a map where each element is keyed by name.

# last-applied-configuration value
    containers:
    - name: nginx
      image: nginx:1.10
    - name: nginx-helper-a # key: nginx-helper-a; will be deleted in result
      image: helper:1.3
    - name: nginx-helper-b # key: nginx-helper-b; will be retained
      image: helper:1.3

# configuration file value
    containers:
    - name: nginx
      image: nginx:1.10
    - name: nginx-helper-b
      image: helper:1.3
    - name: nginx-helper-c # key: nginx-helper-c; will be added in result
      image: helper:1.3

# live configuration
    containers:
    - name: nginx
      image: nginx:1.10
    - name: nginx-helper-a
      image: helper:1.3
    - name: nginx-helper-b
      image: helper:1.3
      args: ["run"] # Field will be retained
    - name: nginx-helper-d # key: nginx-helper-d; will be retained
      image: helper:1.3

# result after merge
    containers:
    - name: nginx
      image: nginx:1.10
      # Element nginx-helper-a was deleted
    - name: nginx-helper-b
      image: helper:1.3
      args: ["run"] # Field was retained
    - name: nginx-helper-c # Element was added
      image: helper:1.3
    - name: nginx-helper-d # Element was ignored
      image: helper:1.3

Explanation:

Merge a list of primitive elements

As of Kubernetes 1.5, merging lists of primitive elements is not supported.

Note: Which of the above strategies is chosen for a given field is controlled by the patchStrategy tag in types.go If no patchStrategy is specified for a field of type list, then the list is replaced.

Default field values

The API server sets certain fields to default values in the live configuration if they are not specified when the object is created.

Here’s a configuration file for a Deployment. The file does not specify strategy:

application/simple_deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  minReadySeconds: 5
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

Create the object using kubectl apply:

kubectl apply -f https://k8s.io/examples/application/simple_deployment.yaml

Print the live configuration using kubectl get:

kubectl get -f https://k8s.io/examples/application/simple_deployment.yaml -o yaml

The output shows that the API server set several fields to default values in the live configuration. These fields were not specified in the configuration file.

apiVersion: apps/v1
kind: Deployment
# ...
spec:
  selector:
    matchLabels:
      app: nginx
  minReadySeconds: 5
  replicas: 1 # defaulted by apiserver
  strategy:
    rollingUpdate: # defaulted by apiserver - derived from strategy.type
      maxSurge: 1
      maxUnavailable: 1
    type: RollingUpdate # defaulted apiserver
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.7.9
        imagePullPolicy: IfNotPresent # defaulted by apiserver
        name: nginx
        ports:
        - containerPort: 80
          protocol: TCP # defaulted by apiserver
        resources: {} # defaulted by apiserver
        terminationMessagePath: /dev/termination-log # defaulted by apiserver
      dnsPolicy: ClusterFirst # defaulted by apiserver
      restartPolicy: Always # defaulted by apiserver
      securityContext: {} # defaulted by apiserver
      terminationGracePeriodSeconds: 30 # defaulted by apiserver
# ...

In a patch request, defaulted fields are not re-defaulted unless they are explicitly cleared as part of a patch request. This can cause unexpected behavior for fields that are defaulted based on the values of other fields. When the other fields are later changed, the values defaulted from them will not be updated unless they are explicitly cleared.

For this reason, it is recommended that certain fields defaulted by the server are explicitly defined in the configuration file, even if the desired values match the server defaults. This makes it easier to recognize conflicting values that will not be re-defaulted by the server.

Example:

# last-applied-configuration
spec:
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

# configuration file
spec:
  strategy:
    type: Recreate # updated value
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

# live configuration
spec:
  strategy:
    type: RollingUpdate # defaulted value
    rollingUpdate: # defaulted value derived from type
      maxSurge : 1
      maxUnavailable: 1
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

# result after merge - ERROR!
spec:
  strategy:
    type: Recreate # updated value: incompatible with rollingUpdate
    rollingUpdate: # defaulted value: incompatible with "type: Recreate"
      maxSurge : 1
      maxUnavailable: 1
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

Explanation:

  1. The user creates a Deployment without defining strategy.type.
  2. The server defaults strategy.type to RollingUpdate and defaults the strategy.rollingUpdate values.
  3. The user changes strategy.type to Recreate. The strategy.rollingUpdate values remain at their defaulted values, though the server expects them to be cleared. If the strategy.rollingUpdate values had been defined initially in the configuration file, it would have been more clear that they needed to be deleted.
  4. Apply fails because strategy.rollingUpdate is not cleared. The strategy.rollingupdate field cannot be defined with a strategy.type of Recreate.

Recommendation: These fields should be explicitly defined in the object configuration file:

How to clear server-defaulted fields or fields set by other writers

Fields that do not appear in the configuration file can be cleared by setting their values to null and then applying the configuration file. For fields defaulted by the server, this triggers re-defaulting the values.

How to change ownership of a field between the configuration file and direct imperative writers

These are the only methods you should use to change an individual object field:

Changing the owner from a direct imperative writer to a configuration file

Add the field to the configuration file. For the field, discontinue direct updates to the live configuration that do not go through kubectl apply.

Changing the owner from a configuration file to a direct imperative writer

As of Kubernetes 1.5, changing ownership of a field from a configuration file to an imperative writer requires manual steps:

Changing management methods

Kubernetes objects should be managed using only one method at a time. Switching from one method to another is possible, but is a manual process.

Note: It is OK to use imperative deletion with declarative management.

Migrating from imperative command management to declarative object configuration

Migrating from imperative command management to declarative object configuration involves several manual steps:

  1. Export the live object to a local configuration file:

     kubectl get <kind>/<name> -o yaml --export > <kind>_<name>.yaml

  2. Manually remove the status field from the configuration file.

    Note: 
    This step is optional, as `kubectl apply` does not update the status field
even if it is present in the configuration file.

  3. Set the kubectl.kubernetes.io/last-applied-configuration annotation on the object:

    kubectl replace --save-config -f <kind>_<name>.yaml

  4. Change processes to use kubectl apply for managing the object exclusively.

Migrating from imperative object configuration to declarative object configuration

  1. Set the kubectl.kubernetes.io/last-applied-configuration annotation on the object:

    kubectl replace --save-config -f <kind>_<name>.yaml

  2. Change processes to use kubectl apply for managing the object exclusively.

Defining controller selectors and PodTemplate labels

Warning: Updating selectors on controllers is strongly discouraged.

The recommended approach is to define a single, immutable PodTemplate label used only by the controller selector with no other semantic meaning.

Example:

selector:
  matchLabels:
      controller-selector: "extensions/v1beta1/deployment/nginx"
template:
  metadata:
    labels:
      controller-selector: "extensions/v1beta1/deployment/nginx"

What's next

Feedback