Flavio Castelli

Debugging my life

Feb 27, 2020 - 6 minute read - Comments - docker containers kubernetes

Semantic versioning and containers

Developers are used to express the dependencies of their programs using semantic versioning constraints.

For example a Node.js application relying on left-pad could force only certain versions of this library to be used by specifying a constraint like >= 1.1.0 < 1.2.0. This would force npm to install the latest version of the library that satisfies the constraint.

How does that translates to containers?

Imagine the following scenario: a developer deploys a containerized application that requires a Redi database. The developer deploys the latest version of the redis container (eg: redis:4.0.5), ensures his application works fine and then moves to do other things.

After some weeks a security issue/bug is found inside of Redis and a new patched release takes place. Suddenly the deployed container is outdated. How can the developer be aware a new v4 release of Redis is available? Wouldn’t be even better to have some automated tool taking care of this upgrade?

After some more weeks a new minor release of Redis is released (eg: 4.1.0). Is it safe to automatically update to a new minor release of Redis, is the developer application going to work as expected?

Some container images have special tags like v4 or v4.1 and the developer could just leverage them to kinda pinpoint the redis container to a more delimited set of versions. However using these tags reduces reproducibility and debuggability.

Let’s imagine the redis image being deployed is redis:v4.1 and everything is working as expected. Assume after some time the developer (or some automated tool) pulls a new version of the redis:v4.1 image and suddenly the application has some issues. How can the developer understand what really changed? Wouldn’t it be great to be able to say something like “everything worked fine with redis:4.1.0 but it broke when I upgraded to redis:4.1.9”?

There are some tools that can be used to find and automatically update old container images: Watchtower and ouroboros. However none of them allows the flexibility I was looking for (in terms of checks), plus they are both tailored to work only against docker.

Because of that, during the 2020 edition of SUSE Hackweek, I spent some time working on a different solution to this use case.

Introducing fresh-container

fresh-container is a tool that can be used to see if a container can be updated to a more recent release.

fresh-container is different compared to Watchtower and ouroboros because it relies on semantic versioning to process container image tags.

Semantic versioning is used to express the version constraints a container version must satisfy. This gives more flexibility, for example take a look at the following scenarios:

  • I’m fine with any release of Redis that is part of the v4 code stream: >= 4.0.0 < 5.0.0
  • I’m fine only with patch releases of Redis that belong to the 4.1 code stream: >= 4.1.0 < 4.2.0
  • I’m don’t want any release of Redis after v6: < 6.0.0

CLI mode

fresh-container can be run as a standalone program:

$ fresh-container check --constraint ">= 1.9.0 < 1.10.0" nginx:1.9.0

The 'docker.io/library/nginx' container image can be upgraded from the '1.9.0' tag to the '1.9.15' one and still satisfy the '>= 1.9.0 < 1.10.0' constraint.

Behind the scenes fresh-container will query the container registry hosting the image to gather the list of all the available tags. The tags that do not respect semantic versioning will be ignored and finally the tool will evaluate the constraint provided by the user.

It can also generate computer parsable output by producing a JSON response:

$ fresh-container check -o json --constraint ">= 1.9.0 < 1.10.0" nginx:1.9.0

  "image": "docker.io/library/nginx",
  "constraint": ">= 1.9.0 < 1.10.0",
  "current_version": "1.9.0",
  "next_version": "1.9.15",
  "stale": true

Server mode

Querying the remote container registries to fetch all the available tags of a container image is an expensive operation. That gets even worse when multiple containers have to be inspected on a regular basis.

The fresh-container binary can operate in a server mode to alleviate this issue:

$ fresh-container server

This will start a web server offering a simple REST API that can be used to perform queries. The remote tags of the container images are cached inside of an in-memory database to speed up constraint resolution.

It’s possible to run fresh-container check against a fresh-container server to perform faster queries by using the --server <http://fresh-container-server> flag.

Kubernetes integration

fresh-container is a tool built to serve one specific use case: your provide some data as input and, as output, it will tell you if the container image can be updated to a more recent version.

It’s main goal is to be leveraged by other tools to build something bigger like fresh-container-operator.

This is a kubernetes operator that, once deployed inside of a kubernetes cluster, will look at all the kubernetes deployments running inside of it and finds the ones having stale containers.

The operator can also automatically update these outdated deployments to use the latest version of the container images that satisfy their requirements.


How does it work? First of all you have to enrich your deployment definition by adding some ad-hoc annotations.

For each container image used by the deployment you have to specify the semantic versioning constraint that has to be used to evaluate their “freshness”.

Take a look at the following example:

apiVersion: apps/v1
kind: Deployment
  name: nginx-deployment
    fresh-container.autopilot: "false"
      app: nginx
  replicas: 1
        app: nginx
        fresh-container.constraint/nginx: ">= 1.9.0 < 1.10.0"
      - name: nginx
        image: nginx:1.9.0
        - containerPort: 80

In this case the operator will look at the version of the nginx container in use and evaluate it against the >= 1.9.0 < 1.10.0 constraint.

Note well: deployments that do not have any fresh-container.constraint/<container name> will be ignored by the operator.

Find stale deployments

The operator adds the special label fresh-container.hasOutdatedContainers=true to all the deployments that have one or more stale containers inside of them.

This allows quick searches against all the deployments:

$ kubectl get deployments --all-namespaces -l fresh-container.hasOutdatedContainers=true
default     nginx-deployment   1/1     1            1           19m

Why is a deployment stale?

The details about the stale containers are added by the operator as annotations of the deployment:

kubectl describe deployments.apps nginx-deployment
Name:                   nginx-deployment
Namespace:              default
CreationTimestamp:      Thu, 27 Feb 2020 10:32:55 +0100
Labels:                 fresh-container.hasOutdatedContainers=true
Annotations:            deployment.kubernetes.io/revision: 1
                        fresh-container.autopilot: false
                        fresh-container.lastChecked: 2020-02-27T09:45:07Z
                        fresh-container.nextTag/nginx: 1.9.15

For each stale container the operator adds an annotation with fresh-container.nextTag/<container name> as key and the tag of the most recent container that satisfies the constraint as value.

In the example above you can see that the nginx container inside of the deployment can be updated to the 1.9.15 tag while still satisfying the >= 1.9.0 < 1.10.0 constraint.

Automatic upgrades

The next step is to allow fresh-container-operator to update all the deployments that have stale containers.

This is not done by default, but can be enable on a per-deployment basis by adding the fresh-container.autopilot=true annotation inside of the deployment metadata.

What comes next

As I stated in the beginning I created these projects during the 2020 edition of SUSE Hackweek. They are early prototypes that need more love.

I would be happy to hear what you think about them. Feel free to leave a comment below or open an issue on their GitHub projects: