K8up Architecture
K8up consists of two parts:
-
Kubernetes Operator:
k8up operator -
Restic wrapper:
k8up restic
High Level Description
k8up operator is deployed as a cluster wide Kubernetes Operator.
It is responsible for processing the Backup resources and to satisfy the defined Schedule resources.
When it’s time to do a backup, the operator scans the namespace for matching PVCs and creates a backup job in the corresponding namespace.
It mounts the matching PVCs under /data/<pvcname>.
Restic then backups the data from this location to the configured endpoint.
Operator Architecture
The following diagram shows a high-level architecture of K8up internals. This is mainly aimed at developers helping to understand the code. But it could be interesting for anyone trying to understand an issue they may face with K8up.
Schedules
The Schedule is defined by the user.
It contains rules, when to archive, backup, check or prune.
Whenever it’s time to do one of these actions, the operator will create a corresponding object in the cluster.
For example, if a backup needs to be performed, a new instance of the Backup custom resource is created.
There is an EffectiveSchedule object for each Schedule object.
It is not meant to be edited by users.
It contains the effective times of the scheduled actions.
The reason is that the schedule specification can contain such schedules as @yearly.
The EffectiveSchedule stores the actual time when such schedules actually happen.
Backup, Prune, Archive, Check, Restore
These custom resources are usually created by a Schedule, but users may create them as well.
The exception is the Restore custom resource, which is always created manually by a user.
Whenever a new instance of one of these custom resources is detected, a corresponding Job is created.
The Job will contain at least one Pod.
The Pod will contain a single container with the k8up image.
The image is invoked as k8up restic.
All the relevant parameters are extracted by the k8up operator from the specific action objects, from the schedule or from the global configuration of k8up operator.
These parameters are then passed to k8up restic as command line arguments and as environment variables.
The k8up operator also ensures that the relevant PVC is mounted into the Job, usually as /data/<pvc-name>.
Restic Job
The Job that runs k8up restic contains the logic to perform the requested actions using the underlying restic binary.
It uses the same Docker image as the k8up operator.
While k8up operator is usually running for a long time, k8up restic will do the job it is tasked to do and then exit.
It also does not modify any resources in the Kubernetes cluster or leave permanent traces.
But it will look for Pod resources that are annotated.
And it will also create one-off containers in the Pod resources it finds that are annotated with k8up.io/backupcommand in order to run the respective backup command and collect its output.