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Function

What is Function

In KubeStash, a complete task like backup or restore is referred to as a Function.

A Function is a Kubernetes CustomResourceDefinition(CRD) which basically specifies a template for a container that performs only a specific action or task. When backing up or restoring an application, there can be multiple tasks involved. For example, with a PostgreSQL database, you might need to do a manifest backup and a logical backup. In KubeStash, each of these tasks is performed using a dedicated Function.

When you install KubeStash, some Functions will be pre-installed for supported targets like workloads, pvc etc. However, you can create your own function to customize or extend the backup/restore process.

Function CRD Specification

Like any official Kubernetes resource, a Function has TypeMeta, ObjectMeta and Spec sections. However, unlike other Kubernetes resources, it does not have a Status section.

A sample Function object to backup a PostgreSQL is shown below,

apiVersion: addons.kubestash.com/v1alpha1
kind: Function
metadata:
  annotations:
    meta.helm.sh/release-name: kubedb
    meta.helm.sh/release-namespace: kubedb
  creationTimestamp: "2023-12-12T12:55:59Z"
  generation: 1
  labels:
    app.kubernetes.io/instance: kubedb
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/name: kubedb-kubestash-catalog
    app.kubernetes.io/version: v2023.12.11
    helm.sh/chart: kubedb-kubestash-catalog-v2023.12.11
  name: postgres-backup
  resourceVersion: "277644"
  uid: f64449f8-1111-4a4d-8c6e-96c5b877aef6
spec:
  args:
    - backup
    - --namespace=${namespace:=default}
    - --backupsession=${backupSession:=}
    - --enable-cache=${enableCache:=}
    - --scratch-dir=${scratchDir:=}
    - --wait-timeout=${waitTimeout:=300}
    - --pg-args=${args:=}
    - --backup-cmd=${backupCmd:=}
    - --user=${user:=}
  image: ghcr.io/kubedb/postgres-restic-plugin:v0.5.0

Here, we are going to describe the various sections of a Function crd.

Function Spec

A Function object has the following fields in the spec section:

spec.image

spec.image specifies the docker image to use to create a container using the template specified in this Function.

spec.command

spec.command specifies the commands to be executed by the container. Docker image’s ENTRYPOINT will be executed if no commands are specified.

spec.args

spec.args specifies a list of arguments that will be passed to the entrypoint. You can templatize this section using envsubst style variables. KubeStash will resolve all the variables before creating the respective container. A variable should follow the following patterns:

• ${variableName:=default-value}
• ${variableName:=}

In the first case, if KubeStash can’t resolve the variable, the default value will be used in place of this variable. In the second case, if KubeStash can’t resolve the variable, an empty string will be used to replace the variable.

KubeStash Provided Variables

KubeStash operator provides the following built-in variables based on BackupSession, RestoreSession, Function etc.

spec.workingDir

spec.workingDir specifies the container’s working directory. If this field is not specified, the container’s runtime default will be used.

spec.ports

spec.ports specifies a list of the ports to expose from the respective container that will be created for this function.

spec.volumeMounts

spec.volumeMounts specifies a list of volume names and their mountPath that will be mounted into the container that will be created for this function.

spec.volumeDevices

spec.volumeDevices specifies a list of the block devices to be used by the container that will be created for this function.

spec.runtimeSettings

spec.runtimeSettings allows to configure runtime environment of a backup job at container level. You can configure the following container level parameters:

Next Steps

• Learn how KubeStash backup stand-alone PVC using Function-Addon model from here.