---
title: "Backup Hooks"
layout: docs
---

Velero supports executing commands in containers in pods during a backup.

## Backup Hooks

When performing a backup, you can specify one or more commands to execute in a container in a pod
when that pod is being backed up. The commands can be configured to run *before* any custom action
processing ("pre" hooks), or after all custom actions have been completed and any additional items
specified by custom action have been backed up ("post" hooks). Note that hooks are _not_ executed within a shell
on the containers.

As of Velero 1.15, related items that must be backed up together are grouped into ItemBlocks, and pod hooks run before and after the ItemBlock is backed up.
In particular, this means that if an ItemBlock contains more than one pod (such as in a scenario where an RWX volume is mounted by multiple pods), pre hooks are run for all pods in the ItemBlock, then the items are backed up, then all post hooks are run.

There are two ways to specify hooks: annotations on the pod itself, and in the Backup spec.

### Specifying Hooks As Pod Annotations

You can use the following annotations on a pod to make Velero execute a hook when backing up the pod:

#### Pre hooks

* `pre.hook.backup.velero.io/container`
  * The container where the command should be executed. Defaults to the first container in the pod. Optional.
* `pre.hook.backup.velero.io/command`
  * The command to execute. This command is not executed within a shell by default. If a shell is needed to run your command, include a shell command, like `/bin/sh`, that is supported by the container at the beginning of your command. If you need multiple arguments, specify the command as a JSON array, such as `["/usr/bin/uname", "-a"]`. See [examples of using pre hook commands](#backup-hook-commands-examples). Optional.
* `pre.hook.backup.velero.io/on-error`
  * What to do if the command returns a non-zero exit code.  Defaults is `Fail`. Valid values are Fail and Continue. Optional.
* `pre.hook.backup.velero.io/timeout`
  * How long to wait for the command to execute. The hook is considered in error if the command exceeds the timeout. Defaults is 30s. Optional.


#### Post hooks

* `post.hook.backup.velero.io/container`
  * The container where the command should be executed. Default is the first container in the pod. Optional.
* `post.hook.backup.velero.io/command`
  * The command to execute. This command is not executed within a shell by default. If a shell is needed to run your command, include a shell command, like `/bin/sh`, that is supported by the container at the beginning of your command. If you need multiple arguments, specify the command as a JSON array, such as `["/usr/bin/uname", "-a"]`. See [examples of using pre hook commands](#backup-hook-commands-examples). Optional.
* `post.hook.backup.velero.io/on-error`
  * What to do if the command returns a non-zero exit code.  Defaults is `Fail`. Valid values are Fail and Continue. Optional.
* `post.hook.backup.velero.io/timeout`
  * How long to wait for the command to execute. The hook is considered in error if the command exceeds the timeout. Defaults is 30s. Optional.

### Specifying Hooks in the Backup Spec

Please see the documentation on the [Backup API Type][1] for how to specify hooks in the Backup
spec.

## Hook Example with fsfreeze

This examples walks you through using both pre and post hooks for freezing a file system. Freezing the
file system is useful to ensure that all pending disk I/O operations have completed prior to taking a snapshot.

### Annotations

The Velero [example/nginx-app/with-pv.yaml][2] serves as an example of adding the pre and post hook annotations directly
to your declarative deployment. Below is an example of what updating an object in place might look like.

```shell
kubectl annotate pod -n nginx-example -l app=nginx \
    pre.hook.backup.velero.io/command='["/sbin/fsfreeze", "--freeze", "/var/log/nginx"]' \
    pre.hook.backup.velero.io/container=fsfreeze \
    post.hook.backup.velero.io/command='["/sbin/fsfreeze", "--unfreeze", "/var/log/nginx"]' \
    post.hook.backup.velero.io/container=fsfreeze
```

Now test the pre and post hooks by creating a backup. You can use the Velero logs to verify that the pre and post
hooks are running and exiting without error.

```shell
velero backup create nginx-hook-test

velero backup get nginx-hook-test
velero backup logs nginx-hook-test | grep hookCommand
```

## Backup hook commands examples

### Multiple commands

To use multiple commands, wrap your target command in a shell and separate them with `;`, `&&`, or other shell conditional constructs.

```shell
    pre.hook.backup.velero.io/command='["/bin/bash", "-c", "echo hello > hello.txt && echo goodbye > goodbye.txt"]'
```

#### Using environment variables

You are able to use environment variables from your pods in your pre and post hook commands by including a shell command before using the environment variable. For example, `MYSQL_ROOT_PASSWORD` is an environment variable defined in pod called `mysql`. To use `MYSQL_ROOT_PASSWORD` in your pre-hook, you'd include a shell, like `/bin/sh`, before calling your environment variable:

```
pre:
- exec:
    container: mysql
    command:
      - /bin/sh
      - -c
      - mysql --password=$MYSQL_ROOT_PASSWORD -e "FLUSH TABLES WITH READ LOCK"
    onError: Fail
```

Note that the container must support the shell command you use. 

## Backup Hook Execution Results
### Viewing Results

Velero records the execution results of hooks, allowing users to obtain this information by running the following command:

```bash
$ velero backup describe <backup name>
```

The displayed results include the number of hooks that were attempted to be executed and the number of hooks that failed execution. Any detailed failure reasons will be present in `Errors` section if applicable. 

```bash
HooksAttempted:   1
HooksFailed:      0
```


[1]: api-types/backup.md
[2]: https://github.com/vmware-tanzu/velero/blob/v1.15.0/examples/nginx-app/with-pv.yaml
