---
title: "How Velero Works"
layout: docs
---

Each Velero operation -- on-demand backup, scheduled backup, restore -- is a custom resource, defined with a Kubernetes [Custom Resource Definition (CRD)][20] and stored in [etcd][22]. Velero also includes controllers that process the custom resources to perform backups, restores, and all related operations.

You can back up or restore all objects in your cluster, or you can filter objects by type, namespace, and/or label.

Velero is ideal for the disaster recovery use case, as well as for snapshotting your application state, prior to performing system operations on your cluster, like upgrades.

## On-demand backups

The **backup** operation:

1. Uploads a tarball of copied Kubernetes objects into cloud object storage.

1. Calls the cloud provider API to make disk snapshots of persistent volumes, if specified.

You can optionally specify backup hooks to be executed during the backup. For example, you might
need to tell a database to flush its in-memory buffers to disk before taking a snapshot. [More about backup hooks][10].

Note that cluster backups are not strictly atomic. If Kubernetes objects are being created or edited at the time of backup, they might not be included in the backup. The odds of capturing inconsistent information are low, but it is possible.

## Scheduled backups

The **schedule** operation allows you to back up your data at recurring intervals. You can create a scheduled backup at any time, and the first backup is then performed at the schedule's specified interval. These intervals are specified by a Cron expression.

Velero saves backups created from a schedule with the name `<SCHEDULE NAME>-<TIMESTAMP>`, where `<TIMESTAMP>` is formatted as *YYYYMMDDhhmmss*. For more information see the [Backup Reference documentation](backup-reference.md).

## Restores

The **restore** operation allows you to restore all of the objects and persistent volumes from a previously created backup. You can also restore only a filtered subset of objects and persistent volumes. Velero supports multiple namespace remapping--for example, in a single restore, objects in namespace "abc" can be recreated under namespace "def", and the objects in namespace "123" under "456".

The default name of a restore is `<BACKUP NAME>-<TIMESTAMP>`, where `<TIMESTAMP>` is formatted as *YYYYMMDDhhmmss*. You can also specify a custom name. A restored object also includes a label with key `velero.io/restore-name` and value `<RESTORE NAME>`.

By default, backup storage locations are created in read-write mode. However, during a restore, you can configure a backup storage location to be in read-only mode, which disables backup creation and deletion for the storage location. This is useful to ensure that no backups are inadvertently created or deleted during a restore scenario.

You can optionally specify restore hooks to be executed during a restore or after resources are restored. For example, you might need to perform a custom database restore operation before the database application containers start. [More about restore hooks][11].

## Backup workflow

When you run `velero backup create test-backup`:

1. The Velero client makes a call to the Kubernetes API server to create a `Backup` object.

1. The `BackupController` notices the new `Backup` object and performs validation.

1. The `BackupController` begins the backup process. It collects the data to back up by querying the API server for resources.

1. The `BackupController` makes a call to the object storage service -- for example, AWS S3 -- to upload the backup file.

By default, `velero backup create` makes disk snapshots of any persistent volumes. You can adjust the snapshots by specifying additional flags. Run `velero backup create --help` to see available flags. Snapshots can be disabled with the option `--snapshot-volumes=false`.

![19]

## Backed-up API versions

Velero backs up resources using the Kubernetes API server's *preferred version* for each group/resource. When restoring a resource, this same API group/version must exist in the target cluster in order for the restore to be successful.

For example, if the cluster being backed up has a `gizmos` resource in the `things` API group, with group/versions `things/v1alpha1`, `things/v1beta1`, and `things/v1`, and the server's preferred group/version is `things/v1`, then all `gizmos` will be backed up from the `things/v1` API endpoint. When backups from this cluster are restored, the target cluster **must** have the `things/v1` endpoint in order for `gizmos` to be restored. Note that `things/v1` **does not** need to be the preferred version in the target cluster; it just needs to exist.

## Set a backup to expire

When you create a backup, you can specify a TTL (time to live) by adding the flag `--ttl <DURATION>`. If Velero sees that an existing backup resource is expired, it removes:

* The backup resource
* The backup file from cloud object storage
* All PersistentVolume snapshots
* All associated Restores

The TTL flag allows the user to specify the backup retention period with the value specified in hours, minutes and seconds in the form `--ttl 24h0m0s`. If not specified, a default TTL value of 30 days will be applied.

## Object storage sync

Velero treats object storage as the source of truth. It continuously checks to see that the correct backup resources are always present. If there is a properly formatted backup file in the storage bucket, but no corresponding backup resource in the Kubernetes API, Velero synchronizes the information from object storage to Kubernetes.

This allows restore functionality to work in a cluster migration scenario, where the original backup objects do not exist in the new cluster.

Likewise, if a backup object exists in Kubernetes but not in object storage, it will be deleted from Kubernetes since the backup tarball no longer exists.

[10]: backup-hooks.md
[11]: restore-hooks.md
[19]: /docs/main/img/backup-process.png
[20]: https://kubernetes.io/docs/concepts/api-extension/custom-resources/#customresourcedefinitions
[21]: https://kubernetes.io/docs/concepts/api-extension/custom-resources/#custom-controllers
[22]: https://github.com/coreos/etcd
