Actions

Starting with v0.8 Kopia supports running custom user-provided commands or scripts before and after snapshot root and also before/after individual folders as they get snapshotted. This supports scenarios such as:

  • creating snapshots of filesystems that support it (e.g. ZFS)
  • snapshotting databases or virtual machines as part of taking a snapshot
  • sending notifications to users, etc.

Actions can optionally modify the directory to be snapshotted or redirect upload to another directory (typically a mountpoint representing filesystem snapshot).

Enabling Actions

To reduce the security risk, actions are an opt-in feature and are not enabled by default.

Actions can be enabled globally at connection time or individually per snapshot:

  1. When connecting to repository you can pass --enable-actions which will enable actions globally for the client.
  2. You can override that decision when taking snapshot by passing --force-enable-actions or --force-disable-actions to enable or disable actions for the single snapshot session.

Configuring actions

To set the script for a directory we can use kopia policy set on a directory.

For example:

$ kopia policy set /some/dir --before-folder-action /path/to/command
$ kopia policy set /some/dir --after-folder-action /path/to/command
$ kopia policy set /some/dir --before-snapshot-root-action /path/to/command
$ kopia policy set /some/dir --after-snapshot-root-action /path/to/command

NOTE: Unlike all other policy options, --before-folder-action and --after-folder-action are not inherited and must be set explicitly on target folders, while --before-snapshot-root-action and --after-snapshot-root-action are inherited from their parents and can be set at global, host, user or directory level.

Actions can be essential (must succeed, default behavior), optional (failures are tolerated) or async (kopia will start the action but not wait for it to finish). This can be set using --action-command-mode, for example:

$ kopia policy set /some/dir  --action-command-mode=async \
   --before-folder-action /usr/local/bin/notifier.sh

Each action has an associated timeout (by default 5 minutes), which specifies how long it will be allowed to run before Kopia kills the process. This can be overridden using --action-command-timeout:

$ kopia policy set /some/dir  --action-command-timeout=180s \
   --before-folder-action /usr/local/bin/notifier.sh

Finally, the action command itself can be stored in a repository, when --persist-action-script is passed. To prevent binaries from being stored, the maximum script length can be up to 32000 characters.

Scripts stored like this will be temporarily extracted to a local directory and executed using a shell command, which is:

  • On Linux and macOS: sh -e /path/to/temporary/script/file.sh
  • On Windows: cmd.exe /c C:\path\to\temporary\script\file.cmd

On Unix, if the script has #! prefix, it will be executed directly, bypassing the /bin/sh shell.

Action Environment

When kopia invokes Before actions, it passes the following parameters:

Variable Before After Description
KOPIA_SNAPSHOT_ID Yes Yes Random 64-bit number
KOPIA_SOURCE_PATH Yes Yes Path being snapshotted

The action command can modify the contents source directory in place or it can request other directory be snapshotted instead by printing a line to standard output:

KOPIA_SNAPSHOT_PATH=<new-directory>

This can be used to create point-in-time snapshots - see examples below.

The After action will receive the same parameters as Before plus the actual directory that was snapshotted (either KOPIA_SOURCE_PATH or KOPIA_SNAPSHOT_PATH if returned by the Before script).

Examples

Dumping SQL databases before snapshotting:

This script invokes mysqldump to create a file called dump.sql in the directory that’s being snapshotted. This can be used to automate database backups.

#!/bin/sh
set -e
mysqldump SomeDatabase --result-file=$KOPIA_SOURCE_PATH/dump.sql

ZFS point-in-time snapshotting:

When snapshotting ZFS pools, we must first create a snapshot using zfs snapshot, mount it somewhere and tell kopia to snapshot the mounted directory instead of the current one.

After snapshotting, we need to unmount and destroy the temporary snapshot using zfs destroy command:

Before:

#!/bin/sh
set -e
ZPOOL_NAME=tank
zfs snapshot $ZPOOL_NAME@$KOPIA_SNAPSHOT_ID
mkdir -p /mnt/$KOPIA_SNAPSHOT_ID
mount -t zfs $ZPOOL_NAME@$KOPIA_SNAPSHOT_ID /mnt/$KOPIA_SNAPSHOT_ID
echo KOPIA_SNAPSHOT_PATH=/mnt/$KOPIA_SNAPSHOT_ID

After:

#!/bin/sh
ZPOOL_NAME=tank
umount /mnt/$KOPIA_SNAPSHOT_ID
rmdir /mnt/$KOPIA_SNAPSHOT_ID
zfs destroy $ZPOOL_NAME@$KOPIA_SNAPSHOT_ID

Windows shadow copy

When backing up files opened with exclusive lock in Windows, Kopia would fail the snapshot task because it can’t read the file content. One of the popular solutions is taking a shadow copy of the storage volume and ask Kopia to backup that instead.

In this example, we will use PowerShell to take a shadow copy in the “before” action of the target directory and clean everything up in the “after” action.

before.ps1:

$sourceDrive = Split-Path -Qualifier $env:KOPIA_SOURCE_PATH
$sourcePath = Split-Path -NoQualifier $env:KOPIA_SOURCE_PATH
# use Kopia snapshot ID as mount point name for extra caution for duplication
$mountPoint = "${PSScriptRoot}\${env:KOPIA_SNAPSHOT_ID}"

$shadowId = (Invoke-CimMethod -ClassName Win32_ShadowCopy -MethodName Create -Arguments @{ Volume = "${sourceDrive}\" }).ShadowID
$shadowDevice = (Get-CimInstance -ClassName Win32_ShadowCopy | Where-Object { $_.ID -eq $shadowId }).DeviceObject
if (-not $shadowDevice) {
    # fail the Kopia snapshot early if shadow copy was not created
    exit 1
}

cmd /c mklink /d $mountPoint "${shadowDevice}\"
Write-Output "KOPIA_SNAPSHOT_PATH=${mountPoint}${sourcePath}"

after.ps1:

$mountPoint = Get-Item "${PSScriptRoot}\${env:KOPIA_SNAPSHOT_ID}"
$mountedVolume = $mountPoint.Target
Remove-Item $mountPoint
Get-CimInstance -ClassName Win32_ShadowCopy | Where-Object { "$($_.DeviceObject)\" -eq "\\?\${mountedVolume}" } | Remove-CimInstance

To install the actions:

kopia policy set <target_dir> --before-folder-action 'pwsh -WindowStyle Hidden <path_to_script>\before.ps1'
kopia policy set <target_dir> --after-folder-action  'pwsh -WindowStyle Hidden <path_to_script>\after.ps1'

Contributions Welcome

Those are just some initial ideas, we’re certain more interesting types of actions will be developed using this mechanism, including LVM snapshots, BTRFS Snapshots, notifications and more.

If you have ideas for extending this mechanism, definitely file an Issue on Github.

If you develop a useful action script that you’d like to share with the community, we encourage you to do so by sending us a pull request to add to this web page or you can put them in your own repository and we’ll be happy to link it from here.

To get started, click ‘Edit This Page’ link.