Automatically purge images from an Azure container registry

When you use an Azure container registry as part of a development workflow, the registry can quickly fill up with images or other artifacts that aren't needed after a short period. You might want to delete all tags that are older than a certain duration or match a specified name filter. To delete multiple artifacts quickly, this article introduces the acr purge command you can run as an on-demand or scheduled ACR Task.

The acr purge command is currently distributed in a public container image (mcr.microsoft.com/acr/acr-cli:0.5), built from source code in the acr-cli repo in GitHub. acr purge is currently in preview.

You can use a local installation of the Azure CLI to run the ACR task examples in this article. If you'd like to use it locally, version 2.0.76 or later is required. Run az --version to find the version. If you need to install or upgrade, see Install Azure CLI.

If you want to delete single image tags or manifests using Azure CLI commands, see Delete container images in Azure Container Registry.

Use the purge command

The acr purge container command deletes images by tag in a repository that match a name filter and that are older than a specified duration. By default, only tag references are deleted, not the underlying manifests and layer data. The command has an option to also delete manifests.

acr purge is designed to run as a container command in an ACR Task, so that it authenticates automatically with the registry where the task runs and performs actions there. The task examples in this article use the acr purge command alias in place of a fully qualified container image command.

• The standard command to execute the acr purge is az acr run --registry <YOUR_REGISTRY> --cmd 'acr purge --optional parameter' /dev/null.
• We recommend running the complete acr purge command to use the ACR Purge. For example, run the acr purge --help as az acr run --registry <YOUR_REGISTRY> --cmd 'acr purge --help' /dev/null.

At a minimum, specify the following when you run acr purge:

• --filter - A repository name regular expression and a tag name regular expression to filter images in the registry. Examples: --filter "hello-world:.*" matches all tags in the hello-world repository, --filter "hello-world:^1.*" matches tags beginning with 1 in the hello-world repository, and --filter ".*/cache:.*" matches all tags in the repositories ending in /cache. You can also pass multiple --filter parameters.
• --ago - A Go-style duration string to indicate a duration beyond which images are deleted. The duration consists of a sequence of one or more decimal numbers, each with a unit suffix. Valid time units include "d" for days, "h" for hours, and "m" for minutes. For example, --ago 2d3h6m selects all filtered images last modified more than two days, 3 hours, and 6 minutes ago, and --ago 1.5h selects images last modified more than 1.5 hours ago.

acr purge supports several optional parameters. The following two are used in examples in this article:

• --untagged - Specifies that all manifests that don't have associated tags (untagged manifests) are deleted. This parameter also deletes untagged manifests in addition to tags that are already being deleted. Remove all tags associated with a manifest to purge it; only then you can purge a tag free manifest using --untagged.
• --dry-run - Specifies that no data is deleted, but the output is the same as if the command is run without this flag. This parameter is useful for testing a purge command to make sure it does not inadvertently delete data you intend to preserve.
• --keep - Specifies that the latest x number of to-be-deleted tags are retained. The latest tags are determined by the last modified time of the tag.
• --concurrency - Specifies a number of purge tasks to process concurrently. A default value is used if this parameter is not provided.

For additional parameters, run acr purge --help.

acr purge supports other features of ACR Tasks commands including run variables and task run logs that are streamed and also saved for later retrieval.

Run in an on-demand task

The following example uses the az acr run command to run the acr purge command on-demand. This example deletes all image tags and manifests in the hello-world repository in myregistry that were modified more than 1 day ago and all untagged manifests. The container command is passed using an environment variable. The task runs without a source context.

# Environment variable for container command line
PURGE_CMD="acr purge --filter 'hello-world:.*' \
  --untagged --ago 1d"

az acr run \
  --cmd "$PURGE_CMD" \
  --registry myregistry \
  /dev/null

Run in a scheduled task

The following example uses the az acr task create command to create a daily scheduled ACR task. The task purges tags modified more than 7 days ago in the hello-world repository. The container command is passed using an environment variable. The task runs without a source context.

# Environment variable for container command line
PURGE_CMD="acr purge --filter 'hello-world:.*' \
  --ago 7d"

az acr task create --name purgeTask \
  --cmd "$PURGE_CMD" \
  --schedule "0 0 * * *" \
  --registry myregistry \
  --context /dev/null

Run the az acr task show command to see that the timer trigger is configured.

Purge large numbers of tags and manifests

Purging a large number of tags and manifests could take several minutes or longer. To purge thousands of tags and manifests, the command might need to run longer than the default timeout time of 600 seconds for an on-demand task, or 3600 seconds for a scheduled task. If the timeout time is exceeded, only a subset of tags and manifests are deleted. To ensure that a large-scale purge is complete, pass the --timeout parameter to increase the value.

For example, the following on-demand task sets a timeout time of 3600 seconds (1 hour):

# Environment variable for container command line
PURGE_CMD="acr purge --filter 'hello-world:.*' \
  --ago 1d --untagged"

az acr run \
  --cmd "$PURGE_CMD" \
  --registry myregistry \
  --timeout 3600 \
  /dev/null

Example: Scheduled purge of multiple repositories in a registry

This example walks through using acr purge to periodically clean up multiple repositories in a registry. For example, you might have a development pipeline that pushes images to the samples/devimage1 and samples/devimage2 repositories. You periodically import development images into a production repository for your deployments, so you no longer need the development images. On a weekly basis, you purge the samples/devimage1 and samples/devimage2 repositories, in preparation for the coming week's work.

Preview the purge

Before deleting data, we recommend running an on-demand purge task using the --dry-run parameter. This option allows you to see the tags and manifests that the command will purge, without removing any data.

In the following example, the filter in each repository selects all tags. The --ago 0d parameter matches images of all ages in the repositories that match the filters. Modify the selection criteria as needed for your scenario. The --untagged parameter indicates to delete manifests in addition to tags. The container command is passed to the az acr run command using an environment variable.

# Environment variable for container command line
PURGE_CMD="acr purge \
  --filter 'samples/devimage1:.*' --filter 'samples/devimage2:.*' \
  --ago 0d --untagged --dry-run"

az acr run \
  --cmd "$PURGE_CMD" \
  --registry myregistry \
  /dev/null

Review the command output to see the tags and manifests that match the selection parameters. Because the command is run with --dry-run, no data is deleted.

Sample output:

[...]
Deleting tags for repository: samples/devimage1
myregistry.azurecr.cn/samples/devimage1:232889b
myregistry.azurecr.cn/samples/devimage1:a21776a
Deleting manifests for repository: samples/devimage1
myregistry.azurecr.cn/samples/devimage1@sha256:81b6f9c92844bbbb5d0a101b22f7c2a7949e40f8ea90c8b3bc396879d95e788b
myregistry.azurecr.cn/samples/devimage1@sha256:3ded859790e68bd02791a972ab0bae727231dc8746f233a7949e40f8ea90c8b3
Deleting tags for repository: samples/devimage2
myregistry.azurecr.cn/samples/devimage2:5e788ba
myregistry.azurecr.cn/samples/devimage2:f336b7c
Deleting manifests for repository: samples/devimage2
myregistry.azurecr.cn/samples/devimage2@sha256:8d2527cde610e1715ad095cb12bc7ed169b60c495e5428eefdf336b7cb7c0371
myregistry.azurecr.cn/samples/devimage2@sha256:ca86b078f89607bc03ded859790e68bd02791a972ab0bae727231dc8746f233a

Number of deleted tags: 4
Number of deleted manifests: 4
[...]

Schedule the purge

After you've verified the dry run, create a scheduled task to automate the purge. The following example schedules a weekly task on Sunday at 1:00 UTC to run the previous purge command:

# Environment variable for container command line
PURGE_CMD="acr purge \
  --filter 'samples/devimage1:.*' --filter 'samples/devimage2:.*' \
  --ago 0d --untagged"

az acr task create --name weeklyPurgeTask \
  --cmd "$PURGE_CMD" \
  --schedule "0 1 * * Sun" \
  --registry myregistry \
  --context /dev/null

Run the az acr task show command to see that the timer trigger is configured.

Next steps

Learn about other options to delete image data in Azure Container Registry.

For more information about image storage, see Container image storage in Azure Container Registry.