Expose the version of the application behind your charm

From Zero to Hero: Write your first Kubernetes charm > Expose the version of the application behind your charm

See previous: Make your charm configurable

git clone https://github.com/canonical/juju-sdk-tutorial-k8s.git
cd juju-sdk-tutorial-k8s
git checkout 02_make_your_charm_configurable
git checkout -b 03_set_workload_version 

In this chapter of the tutorial you will learn how to expose the version of the application (workload) run by the charm – something that a charm user might find it useful to know.

Contents:

1. Define functions to collect the workload application version and set it in the charm
2. Declare Python dependencies
3. Validate your charm
4. Review the final code

Define functions to collect the workload application version and set it in the charm

As a first step we need to add two helper functions that will send an HTTP request to our application to get its version. If the container is available, we can send a request using the requests Python library and then add class methods to parse the JSON output to get a version string, as shown below:

• Import the requests Python library:

import requests

• Add the following class methods:

@property
def version(self) -> str:
    """Reports the current workload (FastAPI app) version."""
    try:
        if self.container.get_services(self.pebble_service_name):
            return self._request_version()
    # Catching Exception is not ideal, but we don't care much for the error here, and just
    # default to setting a blank version since there isn't much the admin can do!
    except Exception as e:
        logger.warning("unable to get version from API: %s", str(e), exc_info=True)
    return ""

def _request_version(self) -> str:
    """Helper for fetching the version from the running workload using the API."""
    resp = requests.get(f"http://localhost:{self.config['server-port']}/version", timeout=10)
    return resp.json()["version"]

Next, we need to update the _update_layer_and_restart method to set our workload version. Insert the following lines before setting ActiveStatus:

# Add workload version in Juju status.
self.unit.set_workload_version(self.version)

Declare Python dependencies

Since we’ve added a third party Python dependency into our project, we need to list it in requirements.txt. Edit the file to add the following line:

requests~=2.28

Next time you run charmcraft it will fetch this new dependency into the charm package.

Validate your charm

We’ve exposed the workload version behind our charm. Let’s test that it’s working!

First, repack and refresh your charm:

charmcraft pack
juju refresh \
  --path="./demo-api-charm_ubuntu-22.04-amd64.charm" \
  demo-api-charm --force-units --resource \
  demo-server-image=ghcr.io/canonical/api_demo_server:1.0.1

Our charm should fetch the application version and forward it to juju. Run juju status to check:

juju status

Indeed, the version of our workload is now displayed – see the App block, the Version column:

Model        Controller           Cloud/Region        Version  SLA          Timestamp
charm-model  tutorial-controller  microk8s/localhost  3.0.0    unsupported  12:37:27+01:00

App             Version  Status  Scale  Charm           Channel  Rev  Address         Exposed  Message
demo-api-charm  1.0.1    active      1  demo-api-charm             0  10.152.183.233  no       

Unit               Workload  Agent  Address      Ports  Message
demo-api-charm/0*  active    idle   10.1.157.75   

Review the final code

For the full code see: 03_set_workload_version

For a comparative view of the code before and after this doc see: Comparison

See next: Integrate your charm with PostgreSQL

Contributors: @beliaev-maksim