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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.

Environment VariableUsage
namespaceNamespace of backup or restore job/workload
backupSessionName of the respective BackupSession object
restoreSessionName of the respective RestoreSession object
enableCacheSpecify whether to use cache while backup or restore
snapshotName of the respective Snapshot object

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:

FieldUsage
resourcesCompute resources required by sidecar container or backup job. To know how to manage resources for containers, please visit here.
livenessProbePeriodic probe of backup sidecar/job container’s liveness. Container will be restarted if the probe fails.
readinessProbePeriodic probe of backup sidecar/job container’s readiness. Container will be removed from service endpoints if the probe fails.
lifecycleActions that the management system should take in response to container lifecycle events.
securityContextSecurity options that backup sidecar/job’s container should run with. For more details, please visit here.
niceSet CPU scheduling priority for the backup process. For more details about nice, please visit here.
ioniceSet I/O scheduling class and priority for the backup process. For more details about ionice, please visit here.
envA list of the environment variables to set in the container that will be created for this function.
envFromThis allows to set environment variables to the container that will be created for this function from a Secret or ConfigMap.

Next Steps

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