// denisdefreyne.com / notes
Last revised:
April 2021

Use Docker tags to document the lifecycle of an image

A Docker image typically goes through a lifecycle:

  1. built: the image is successfully built
  2. tested: the image has successfully passed tests (e.g. integration tests, security tests)
  3. deployed to staging: the image is deployed into the staging environment
  4. deployed to production: the image is deployed into the production environment

It’s advantageous to add a new tag when the image progresses to a new stage in the lifecycle.

As an example, take the Git commit SHA 0b3da7f:

  1. If an image is built from this Git commit, the Docker image can be tagged as built-0b3da7f.
  2. Once tests have passed, it can be tagged additionally as tested-0b3da7f (note that previously added tags are kept).
  3. Once deployed to staging, it can be tagged as deployed-staging-0b3da7f.
  4. Once deployed to production, it can be tagged as deployed-production-0b3da7f.

Advantages to cleanup

The list of tags can be used to be clean up stale Docker images. For each Docker image, the following can be checked:

Advantages to safety

When deploying a Git commit, the deploy script can construct the appropriate Docker tag to deploy. For example, to perform a staging deploy, concatenate tested- with the (short) Git SHA, e.g. tested-0b3da7f. Pull the Docker image with that tag and deploy it.

The deployment will fail if no Docker image with such a tag exists. In other words, the deployment will fail if the Docker image did not pass tests.

Some cautionary notes:

Use outside of Docker

This approach is not limited to Docker. While I have not done so myself, I imagine it could work for publishing builds on an S3 bucket or similar.

How it could work (assuming 0b3da7f is the Git commit SHA of the build, which itself is a .tar.gz file):

  1. Create a bucket with directories builds and tags.
  2. When a build is published, store it in builds/0b3da7f.tar.gz. Create the file tags/built-0b3da7f and store in it the string builds/0b3da7f.tar.gz.
  3. When a build is tested, create the file tags/tested-0b3da7f and store in it the string builds/0b3da7f.tar.gz.
  4. When a build is deployed to staging, create the file tags/deployed-staging-0b3da7f and store in it the string builds/0b3da7f.tar.gz.
  5. When a build is deployed to production, create the file tags/deployed-production-0b3da7f and store in it the string builds/0b3da7f.tar.gz.

To facilitate cleanup, it might be necessary to add the date to each tag, e.g. tags/deployed-production-20210222-0b3da7f, with 20210222 being the current date (February 22nd, 2021). With the date in the tag name, it becomes possible to parse out the date and delete stale builds.