Server-Side Apply

FEATURE STATE: Kubernetes v1.22 [stable]

Kubernetes supports multiple appliers collaborating to manage the fields of a single object.

Server-Side Apply provides an optional mechanism for your cluster’s control plane to track changes to an object’s fields. At the level of a specific resource, Server-Side Apply records and tracks information about control over the fields of that object.

Server-Side Apply helps users and controllers manage their resources through declarative configuration. Clients can create and modify objects declaratively by submitting their fully specified intent.

A fully specified intent is a partial object that only includes the fields and values for which the user has an opinion. That intent either creates a new object (using default values for unspecified fields), or is combined, by the API server, with the existing object.

Comparison with Client-Side Apply explains how Server-Side Apply differs from the original, client-side kubectl apply implementation.

Field management

The Kubernetes API server tracks managed fields for all newly created objects.

When trying to apply an object, fields that have a different value and are owned by another manager will result in a conflict. This is done in order to signal that the operation might undo another collaborator’s changes. Writes to objects with managed fields can be forced, in which case the value of any conflicted field will be overridden, and the ownership will be transferred.

Whenever a field’s value does change, ownership moves from its current manager to the manager making the change.

Apply checks if there are any other field managers that also own the field. If the field is not owned by any other field managers, that field is set to its default value (if there is one), or otherwise is deleted from the object. The same rule applies to fields that are lists, associative lists, or maps.

For a user to manage a field, in the Server-Side Apply sense, means that the user relies on and expects the value of the field not to change. The user who last made an assertion about the value of a field will be recorded as the current field manager. This can be done by changing the field manager details explicitly using HTTP POST (create), PUT (update), or non-apply PATCH (patch). You can also declare and record a field manager by including a value for that field in a Server-Side Apply operation.

A Server-Side Apply patch request requires the client to provide its identity as a field manager. When using Server-Side Apply, trying to change a field that is controlled by a different manager results in a rejected request unless the client forces an override. For details of overrides, see Conflicts.

When two or more appliers set a field to the same value, they share ownership of that field. Any subsequent attempt to change the value of the shared field, by any of the appliers, results in a conflict. Shared field owners may give up ownership of a field by making a Server-Side Apply patch request that doesn’t include that field.

Field management details are stored in a managedFields field that is part of an object’s metadata.

If you remove a field from a manifest and apply that manifest, Server-Side Apply checks if there are any other field managers that also own the field. If the field is not owned by any other field managers, it is either deleted from the live object or reset to its default value, if it has one. The same rule applies to associative list or map items.

Compared to the (legacy) kubectl.kubernetes.io/last-applied-configuration annotation managed by kubectl, Server-Side Apply uses a more declarative approach, that tracks a user’s (or client’s) field management, rather than a user’s last applied state. As a side effect of using Server-Side Apply, information about which field manager manages each field in an object also becomes available.

Example

A simple example of an object created using Server-Side Apply could look like this:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: test-cm
  namespace: default
  labels:
    test-label: test
  managedFields:
  - manager: kubectl
    operation: Apply # note capitalization: "Apply" (or "Update")
    apiVersion: v1
    time: "2010-10-10T0:00:00Z"
    fieldsType: FieldsV1
    fieldsV1:
      f:metadata:
        f:labels:
          f:test-label: {}
      f:data:
        f:key: {}
data:
  key: some value

That example ConfigMap object contains a single field management record in .metadata.managedFields. The field management record consists of basic information about the managing entity itself, plus details about the fields being managed and the relevant operation (Apply or Update). If the request that last changed that field was a Server-Side Apply patch then the value of operation is Apply; otherwise, it is Update.

There is another possible outcome. A client could submit an invalid request body. If the fully specified intent does not produce a valid object, the request fails.

It is however possible to change .metadata.managedFields through an update, or through a patch operation that does not use Server-Side Apply. Doing so is highly discouraged, but might be a reasonable option to try if, for example, the .metatadata.managedFields get into an inconsistent state (which should not happen in normal operations).

The format of managedFields is described in the Kubernetes API reference.

Caution: The .metadata.managedFields field is managed by the API server. You should avoid updating it manually.

Conflicts

A conflict is a special status error that occurs when an Apply operation tries to change a field that another manager also claims to manage. This prevents an applier from unintentionally overwriting the value set by another user. When this occurs, the applier has 3 options to resolve the conflicts:

• Overwrite value, become sole manager: If overwriting the value was intentional (or if the applier is an automated process like a controller) the applier should set the force query parameter to true (for kubectl apply, you use the --force-conflicts command line parameter), and make the request again. This forces the operation to succeed, changes the value of the field, and removes the field from all other managers’ entries in managedFields.

• Don’t overwrite value, give up management claim: If the applier doesn’t care about the value of the field any more, the applier can remove it from their local model of the resource, and make a new request with that particular field omitted. This leaves the value unchanged, and causes the field to be removed from the applier’s entry in managedFields.

• Don’t overwrite value, become shared manager: If the applier still cares about the value of a field, but doesn’t want to overwrite it, they can change the value of that field in their local model of the resource so as to match the value of the object on the server, and then make a new request that takes into account that local update. Doing so leaves the value unchanged, and causes that field’s management to be shared by the applier along with all other field managers that already claimed to manage it.

Field managers

Managers identify distinct workflows that are modifying the object (especially useful on conflicts!), and can be specified through the fieldManager query parameter as part of a modifying request. When you Apply to a resource, the fieldManager parameter is required For other updates, the API server infers a field manager identity from the “User-Agent:” HTTP header (if present).

When you use the kubectl tool to perform a Server-Side Apply operation, kubectl sets the manager identity to "kubectl" by default.

Serialization

At the protocol level, Kubernetes represents Server-Side Apply message bodies as YAML, with the media type application/apply-patch+yaml.

Note:

Whether you are submitting JSON data or YAML data, use application/apply-patch+yaml as the Content-Type header value.

All JSON documents are valid YAML.

The serialization is the same as for Kubernetes objects, with the exception that clients are not required to send a complete object.

Here’s an example of a Server-Side Apply message body (fully specified intent):

{
  "apiVersion": "v1",
  "kind": "ConfigMap"
}

(this would make a no-change update, provided that it was sent as the body of a patch request to a valid v1/configmaps resource, and with the appropriate request Content-Type).

Server-Side Apply for custom resources

By default, Server-Side Apply treats custom resources as unstructured data. All keys are treated the same as if they were struct fields for a built-in API, and all lists are considered atomic.

If the CustomResourceDefinition defines a schema that contains annotations as defined in Merge strategy, then these annotations will be used when merging objects of this type.

Operations in scope for field management

The Kubernetes API operations where field management is considered are:

1. Server-Side Apply (HTTP PATCH, with content type application/apply-patch+yaml)
2. Replacing an existing object (update to Kubernetes; PUT at the HTTP level)

Both operations update .metadata.managedFields, but behave a little differently.

Unless you specify a forced override, an apply operation that encounters field-level conflicts always fails; by contrast, if you make a change using update that would affect a managed field, a conflict never provokes failure of the operation.

All Server-Side Apply patch requests are required to identify themselves by providing a fieldManager query parameter, while the query parameter is optional for update operations. Finally, when using the Apply operation you cannot define managedFields in the body of the request that you submit.

An example object with multiple managers could look like this:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: test-cm
  namespace: default
  labels:
    test-label: test
  managedFields:
  - manager: kubectl
    operation: Apply
    apiVersion: v1
    fields:
      f:metadata:
        f:labels:
          f:test-label: {}
  - manager: kube-controller-manager
    operation: Update
    apiVersion: v1
    time: '2019-03-30T16:00:00.000Z'
    fields:
      f:data:
        f:key: {}
data:
  key: new value

In this example, a second operation was run as an update by the manager called kube-controller-manager. The update request succeeded and changed a value in the data field, which caused that field’s management to change to the kube-controller-manager.

If this update has instead been attempted using Server-Side Apply, the request would have failed due to conflicting ownership.

Merge strategy

The merging strategy, implemented with Server-Side Apply, provides a generally more stable object lifecycle. Server-Side Apply tries to merge fields based on the actor who manages them instead of overruling based on values. This way multiple actors can update the same object without causing unexpected interference.

When a user sends a fully-specified intent object to the Server-Side Apply endpoint, the server merges it with the live object favoring the value from the request body if it is specified in both places. If the set of items present in the applied config is not a superset of the items applied by the same user last time, each missing item not managed by any other appliers is removed. For more information about how an object’s schema is used to make decisions when merging, see sigs.k8s.io/structured-merge-diff.

The Kubernetes API (and the Go code that implements that API for Kubernetes) allows defining merge strategy markers. These markers describe the merge strategy supported for fields within Kubernetes objects. For a CustomResourceDefinition, you can set these markers when you define the custom resource.

If listType is missing, the API server interprets a patchMergeStrategy=merge marker as a listType=map and the corresponding patchMergeKey marker as a listMapKey.

The atomic list type is recursive.

(In the Go code for Kubernetes, these markers are specified as comments and code authors need not repeat them as field tags).

Custom resources and Server-Side Apply

By default, Server-Side Apply treats custom resources as unstructured data. All keys are treated the same as struct fields, and all lists are considered atomic.

If the CustomResourceDefinition defines a schema that contains annotations as defined in the previous Merge Strategy section, these annotations will be used when merging objects of this type.

Compatibility across topology changes

On rare occurrences, the author for a CustomResourceDefinition (CRD) or built-in may want to change the specific topology of a field in their resource, without incrementing its API version. Changing the topology of types, by upgrading the cluster or updating the CRD, has different consequences when updating existing objects. There are two categories of changes: when a field goes from map/set/granular to atomic, and the other way around.

When the listType, mapType, or structType changes from map/set/granular to atomic, the whole list, map, or struct of existing objects will end-up being owned by actors who owned an element of these types. This means that any further change to these objects would cause a conflict.

When a listType, mapType, or structType changes from atomic to map/set/granular, the API server is unable to infer the new ownership of these fields. Because of that, no conflict will be produced when objects have these fields updated. For that reason, it is not recommended to change a type from atomic to map/set/granular.

Take for example, the custom resource:

---
apiVersion: example.com/v1
kind: Foo
metadata:
  name: foo-sample
  managedFields:
  - manager: "manager-one"
    operation: Apply
    apiVersion: example.com/v1
    fields:
      f:spec:
        f:data: {}
spec:
  data:
    key1: val1
    key2: val2

Before spec.data gets changed from atomic to granular, manager-one owns the field spec.data, and all the fields within it (key1 and key2). When the CRD gets changed to make spec.data granular, manager-one continues to own the top-level field spec.data (meaning no other managers can delete the map called data without a conflict), but it no longer owns key1 and key2, so another manager can then modify or delete those fields without conflict.

Using Server-Side Apply in a controller

As a developer of a controller, you can use Server-Side Apply as a way to simplify the update logic of your controller. The main differences with a read-modify-write and/or patch are the following:

• the applied object must contain all the fields that the controller cares about.
• there is no way to remove fields that haven’t been applied by the controller before (controller can still send a patch or update for these use-cases).
• the object doesn’t have to be read beforehand; resourceVersion doesn’t have to be specified.

It is strongly recommended for controllers to always force conflicts on objects that the own and manage, since they might not be able to resolve or act on these conflicts.

Transferring ownership

In addition to the concurrency controls provided by conflict resolution, Server-Side Apply provides ways to perform coordinated field ownership transfers from users to controllers.

This is best explained by example. Let’s look at how to safely transfer ownership of the replicas field from a user to a controller while enabling automatic horizontal scaling for a Deployment, using the HorizontalPodAutoscaler resource and its accompanying controller.

Say a user has defined Deployment with replicas set to the desired value:

application/ssa/nginx-deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2

And the user has created the Deployment using Server-Side Apply, like so:

kubectl apply -f https://k8s.io/examples/application/ssa/nginx-deployment.yaml --server-side

Then later, automatic scaling is enabled for the Deployment; for example:

kubectl autoscale deployment nginx-deployment --cpu-percent=50 --min=1 --max=10

Now, the user would like to remove replicas from their configuration, so they don’t accidentally fight with the HorizontalPodAutoscaler (HPA) and its controller. However, there is a race: it might take some time before the HPA feels the need to adjust .spec.replicas; if the user removes .spec.replicas before the HPA writes to the field and becomes its owner, then the API server would set .spec.replicas to 1 (the default replica count for Deployment). This is not what the user wants to happen, even temporarily - it might well degrade a running workload.

There are two solutions:

• (basic) Leave replicas in the configuration; when the HPA eventually writes to that field, the system gives the user a conflict over it. At that point, it is safe to remove from the configuration.

• (more advanced) If, however, the user doesn’t want to wait, for example because they want to keep the cluster legible to their colleagues, then they can take the following steps to make it safe to remove replicas from their configuration:

First, the user defines a new manifest containing only the replicas field:

# Save this file as 'nginx-deployment-replicas-only.yaml'.
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3

Note: The YAML file for SSA in this case only contains the fields you want to change. You are not supposed to provide a fully compliant Deployment manifest if you only want to modify the spec.replicas field using SSA.

The user applies that manifest using a private field manager name. In this example, the user picked handover-to-hpa:

kubectl apply -f nginx-deployment-replicas-only.yaml \
  --server-side --field-manager=handover-to-hpa \
  --validate=false

If the apply results in a conflict with the HPA controller, then do nothing. The conflict indicates the controller has claimed the field earlier in the process than it sometimes does.

At this point the user may remove the replicas field from their manifest:

application/ssa/nginx-deployment-no-replicas.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2

Note that whenever the HPA controller sets the replicas field to a new value, the temporary field manager will no longer own any fields and will be automatically deleted. No further clean up is required.

Transferring ownership between managers

Field managers can transfer ownership of a field between each other by setting the field to the same value in both of their applied configurations, causing them to share ownership of the field. Once the managers share ownership of the field, one of them can remove the field from their applied configuration to give up ownership and complete the transfer to the other field manager.

Comparison with Client-Side Apply

Server-Side Apply is meant both as a replacement for the original client-side implementation of the kubectl apply subcommand, and as simple and effective mechanism for controllers to enact their changes.

Compared to the last-applied annotation managed by kubectl, Server-Side Apply uses a more declarative approach, which tracks an object’s field management, rather than a user’s last applied state. This means that as a side effect of using Server-Side Apply, information about which field manager manages each field in an object also becomes available.

A consequence of the conflict detection and resolution implemented by Server-Side Apply is that an applier always has up to date field values in their local state. If they don’t, they get a conflict the next time they apply. Any of the three options to resolve conflicts results in the applied configuration being an up to date subset of the object on the server’s fields.

This is different from Client-Side Apply, where outdated values which have been overwritten by other users are left in an applier’s local config. These values only become accurate when the user updates that specific field, if ever, and an applier has no way of knowing whether their next apply will overwrite other users’ changes.

Another difference is that an applier using Client-Side Apply is unable to change the API version they are using, but Server-Side Apply supports this use case.

Migration between client-side and server-side apply

Upgrading from client-side apply to server-side apply

Client-side apply users who manage a resource with kubectl apply can start using server-side apply with the following flag.

kubectl apply --server-side [--dry-run=server]

By default, field management of the object transfers from client-side apply to kubectl server-side apply, without encountering conflicts.

Caution:

Keep the last-applied-configuration annotation up to date. The annotation infers client-side applies managed fields. Any fields not managed by client-side apply raise conflicts.

For example, if you used kubectl scale to update the replicas field after client-side apply, then this field is not owned by client-side apply and creates conflicts on kubectl apply --server-side.

This behavior applies to server-side apply with the kubectl field manager. As an exception, you can opt-out of this behavior by specifying a different, non-default field manager, as seen in the following example. The default field manager for kubectl server-side apply is kubectl.

kubectl apply --server-side --field-manager=my-manager [--dry-run=server]

Downgrading from server-side apply to client-side apply

If you manage a resource with kubectl apply --server-side, you can downgrade to client-side apply directly with kubectl apply.

Downgrading works because kubectl Server-Side Apply keeps the last-applied-configuration annotation up-to-date if you use kubectl apply.

This behavior applies to Server-Side Apply with the kubectl field manager. As an exception, you can opt-out of this behavior by specifying a different, non-default field manager, as seen in the following example. The default field manager for kubectl server-side apply is kubectl.

kubectl apply --server-side --field-manager=my-manager [--dry-run=server]

API implementation

The PATCH verb for a resource that supports Server-Side Apply can accepts the unofficial application/apply-patch+yaml content type. Users of Server-Side Apply can send partially specified objects as YAML as the body of a PATCH request to the URI of a resource. When applying a configuration, you should always include all the fields that are important to the outcome (such as a desired state) that you want to define.

All JSON messages are valid YAML. Some clients specify Server-Side Apply requests using YAML request bodies that are also valid JSON.

Access control and permissions

Since Server-Side Apply is a type of PATCH, a principal (such as a Role for Kubernetes RBAC) requires the patch permission to edit existing resources, and also needs the create verb permission in order to create new resources with Server-Side Apply.

Clearing managedFields

It is possible to strip all managedFields from an object by overwriting them using a patch (JSON Merge Patch, Strategic Merge Patch, JSON Patch), or through an update (HTTP PUT); in other words, through every write operation other than apply. This can be done by overwriting the managedFields field with an empty entry. Two examples are:

PATCH /api/v1/namespaces/default/configmaps/example-cm
Accept: application/json
Content-Type: application/merge-patch+json
{
  "metadata": {
    "managedFields": [
      {}
    ]
  }
}

PATCH /api/v1/namespaces/default/configmaps/example-cm
Accept: application/json
Content-Type: application/json-patch+json
If-Match: 1234567890123456789
[{"op": "replace", "path": "/metadata/managedFields", "value": [{}]}]

This will overwrite the managedFields with a list containing a single empty entry that then results in the managedFields being stripped entirely from the object. Note that setting the managedFields to an empty list will not reset the field. This is on purpose, so managedFields never get stripped by clients not aware of the field.

In cases where the reset operation is combined with changes to other fields than the managedFields, this will result in the managedFields being reset first and the other changes being processed afterwards. As a result the applier takes ownership of any fields updated in the same request.

Note: Server-Side Apply does not correctly track ownership on sub-resources that don’t receive the resource object type. If you are using Server-Side Apply with such a sub-resource, the changed fields may not be tracked.

What’s next

You can read about managedFields within the Kubernetes API reference for the metadata top level field.