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. kubectl diff also gives you a preview of what changes apply will make.

Before you begin

Install kubectl.

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Killercoda
• Play with Kubernetes

To check the version, enter kubectl version.

Trade-offs

The kubectl tool supports three kinds of object management:

• Imperative commands
• Imperative object configuration
• Declarative object configuration

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

Overview

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:

• Managing Kubernetes Objects Using Imperative Commands
• Imperative Management of Kubernetes Objects Using Configuration Files

Following are definitions for terms used in this document:

• object configuration file / configuration file: A file that defines the configuration for a Kubernetes object. This topic shows how to pass configuration files to kubectl apply. Configuration files are typically stored in source control, such as Git.
• live object configuration / live configuration: The live configuration values of an object, as observed by the Kubernetes cluster. These are kept in the Kubernetes cluster storage, typically etcd.
• declarative configuration writer / declarative writer: A person or software component that makes updates to a live object. The live writers referred to in this topic make changes to object configuration files and run kubectl apply to write the changes.

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:1.14.2
        ports:
        - containerPort: 80

Run kubectl diff to print the object that will be created:

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

Note:

diff uses server-side dry-run, which needs to be enabled on kube-apiserver.

Since diff performs a server-side apply request in dry-run mode, it requires granting PATCH, CREATE, and UPDATE permissions. See Dry-Run Authorization for details.

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:1.14.2","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}      
  # ...
spec:
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.14.2
        # ...
        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 diff -f <directory>
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:1.14.2
        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:1.14.2","name":"nginx",
      "ports":[{"containerPort":80}]}]}}}}      
  # ...
spec:
  # ...
  minReadySeconds: 5
  selector:
    matchLabels:
      # ...
      app: nginx
  template:
    metadata:
      # ...
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.14.2
        # ...
        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 deployment nginx-deployment -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:1.14.2","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:1.14.2
        # ...
        name: nginx
        ports:
        - containerPort: 80
      # ...

Update the simple_deployment.yaml configuration file to change the image from nginx:1.14.2 to nginx:1.16.1, 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:1.16.1 # update the image
        ports:
        - containerPort: 80

Apply the changes made to the configuration file:

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

The output shows the following changes to the live configuration:

• The replicas field retains the value of 2 set by kubectl scale. This is possible because it is omitted from the configuration file.
• The image field has been updated to nginx:1.16.1 from nginx:1.14.2.
• The last-applied-configuration annotation has been updated with the new image.
• The minReadySeconds field has been cleared.
• The last-applied-configuration annotation no longer contains the minReadySeconds field.

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    # ...
    # The annotation contains the updated image to nginx 1.16.1,
    # 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:1.16.1","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:1.16.1 # 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.

Recommended: kubectl delete -f <filename>

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

As an alternative to kubectl delete, you can use kubectl apply to identify objects to be deleted after their manifests have been removed from a directory in the local filesystem.

In Kubernetes 1.30, there are two pruning modes available in kubectl apply:

• Allowlist-based pruning: This mode has existed since kubectl v1.5 but is still in alpha due to usability, correctness and performance issues with its design. The ApplySet-based mode is designed to replace it.

• ApplySet-based pruning: An apply set is a server-side object (by default, a Secret) that kubectl can use to accurately and efficiently track set membership across apply operations. This mode was introduced in alpha in kubectl v1.27 as a replacement for allowlist-based pruning.

• Allow list

• Apply set

FEATURE STATE: Kubernetes v1.5 [alpha]

Warning: Take care when using --prune with kubectl apply in allow list mode. Which objects are pruned depends on the values of the --prune-allowlist, --selector and --namespace flags, and relies on dynamic discovery of the objects in scope. Especially if flag values are changed between invocations, this can lead to objects being unexpectedly deleted or retained.

To use allowlist-based pruning, add the following flags to your kubectl apply invocation:

• --prune: Delete previously applied objects that are not in the set passed to the current invocation.
• --prune-allowlist: A list of group-version-kinds (GVKs) to consider for pruning. This flag is optional but strongly encouraged, as its default value is a partial list of both namespaced and cluster-scoped types, which can lead to surprising results.
• --selector/-l: Use a label selector to constrain the set of objects selected for pruning. This flag is optional but strongly encouraged.
• --all: use instead of --selector/-l to explicitly select all previously applied objects of the allowlisted types.

Allowlist-based pruning queries the API server for all objects of the allowlisted GVKs that match the given labels (if any), and attempts to match the returned live object configurations against the object manifest files. If an object matches the query, and it does not have a manifest in the directory, and it has a kubectl.kubernetes.io/last-applied-configuration annotation, it is deleted.

kubectl apply -f <directory> --prune -l <labels> --prune-allowlist=<gvk-list>

Warning: Apply with prune should only be run against the root directory containing the object manifests. Running against sub-directories can cause objects to be unintentionally deleted if they were previously applied, have the labels given (if any), and do not appear in the subdirectory.

FEATURE STATE: Kubernetes v1.27 [alpha]

Caution: kubectl apply --prune --applyset is in alpha, and backwards incompatible changes might be introduced in subsequent releases.

To use ApplySet-based pruning, set the KUBECTL_APPLYSET=true environment variable, and add the following flags to your kubectl apply invocation:

• --prune: Delete previously applied objects that are not in the set passed to the current invocation.
• --applyset: The name of an object that kubectl can use to accurately and efficiently track set membership across apply operations.

KUBECTL_APPLYSET=true kubectl apply -f <directory> --prune --applyset=<name>

By default, the type of the ApplySet parent object used is a Secret. However, ConfigMaps can also be used in the format: --applyset=configmaps/<name>. When using a Secret or ConfigMap, kubectl will create the object if it does not already exist.

It is also possible to use custom resources as ApplySet parent objects. To enable this, label the Custom Resource Definition (CRD) that defines the resource you want to use with the following: applyset.kubernetes.io/is-parent-type: true. Then, create the object you want to use as an ApplySet parent (kubectl does not do this automatically for custom resources). Finally, refer to that object in the applyset flag as follows: --applyset=<resource>.<group>/<name> (for example, widgets.custom.example.com/widget-name).

With ApplySet-based pruning, kubectl adds the applyset.kubernetes.io/part-of=<parentID> label to each object in the set before they are sent to the server. For performance reasons, it also collects the list of resource types and namespaces that the set contains and adds these in annotations on the live parent object. Finally, at the end of the apply operation, it queries the API server for objects of those types in those namespaces (or in the cluster scope, as applicable) that belong to the set, as defined by the applyset.kubernetes.io/part-of=<parentID> label.

Caveats and restrictions:

• Each object may be a member of at most one set.
• The --namespace flag is required when using any namespaced parent, including the default Secret. This means that ApplySets spanning multiple namespaces must use a cluster-scoped custom resource as the parent object.
• To safely use ApplySet-based pruning with multiple directories, use a unique ApplySet name for each.

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:1.16.1 # 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:1.14.2","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:1.14.2
        # ...
        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 1.16.1,
    # 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:1.16.1","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:1.16.1 # 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:

• primitive: A field of type string, integer, or boolean. For example, image and replicas are primitive fields. Action: Replace.

• map, also called object: A field of type map or a complex type that contains subfields. For example, labels, annotations,spec and metadata are all maps. Action: Merge elements or subfields.

• list: A field containing a list of items that can be either primitive types or maps. For example, containers, ports, and args are lists. Action: Varies.

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.

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.

Merging changes for fields of type list

Merging changes to a list uses one of three strategies:

• Replace the list if all its elements are primitives.
• Merge individual elements in a list of complex elements.
• Merge a list of primitive elements.

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

Replace the list if all its elements are primitives

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:1.16
    - 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:1.16
    - 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:1.16
    - 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:1.16
      # 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:

• The container named “nginx-helper-a” was deleted because no container named “nginx-helper-a” appeared in the configuration file.
• The container named “nginx-helper-b” retained the changes to args in the live configuration. kubectl apply was able to identify that “nginx-helper-b” in the live configuration was the same “nginx-helper-b” as in the configuration file, even though their fields had different values (no args in the configuration file). This is because the patchMergeKey field value (name) was identical in both.
• The container named “nginx-helper-c” was added because no container with that name appeared in the live configuration, but one with that name appeared in the configuration file.
• The container named “nginx-helper-d” was retained because no element with that name appeared in the last-applied-configuration.

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:1.14.2
        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 by apiserver
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: nginx
    spec:
      containers:
      - image: nginx:1.14.2
        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:1.14.2
        ports:
        - containerPort: 80
# configuration file
spec:
  strategy:
    type: Recreate # updated value
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        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:1.14.2
        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:1.14.2
        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:

• Selectors and PodTemplate labels on workloads, such as Deployment, StatefulSet, Job, DaemonSet, ReplicaSet, and ReplicationController
• Deployment rollout strategy

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:

• Use kubectl apply.
• Write directly to the live configuration without modifying the configuration file: for example, use kubectl scale.

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:

• Remove the field from the configuration file.
• Remove the field from the kubectl.kubernetes.io/last-applied-configuration annotation on the live object.

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 > <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: "apps/v1/deployment/nginx"
template:
  metadata:
    labels:
      controller-selector: "apps/v1/deployment/nginx"

What’s next

• Managing Kubernetes Objects Using Imperative Commands
• Imperative Management of Kubernetes Objects Using Configuration Files
• Kubectl Command Reference
• Kubernetes API Reference