Write your first Kubernetes charm for a Go app¶

Imagine you have a Go application backed up by a database such as PostgreSQL and need to deploy it. In a traditional setup, this can be quite a challenge, but with Charmcraft you’ll find yourself packaging and deploying your Go application in no time.

In this tutorial we will build a Kubernetes charm for a Go application using Charmcraft, so we can have a Go application up and running with Juju. Let’s get started!

This tutorial should take 90 minutes for you to complete.

Note

If you’re new to the charming world: Go applications are specifically supported with a template to quickly generate a rock (i.e., a special kind of OCI-compliant container image) and a matching template to quickly generate a charm (i.e., a software operator for cloud operations done with the Juju orchestration engine). The result is Go applications that can be easily deployed, configured, scaled, integrated, etc., on any Kubernetes cluster.

What you’ll need:¶

A local system, e.g., a laptop, with amd64 or arm64 architecture which has sufficient resources to launch a virtual machine with 4 CPUs, 4 GB RAM, and a 50 GB disk.
Familiarity with Linux.

What you’ll do:¶

Create a Go application. Use that to create a rock with rockcraft. Use that to create a charm with charmcraft. Use that to test, deploy, configure, etc., your Go application on a local Kubernetes cloud, microk8s, with juju. All of that multiple times, mimicking a real development process.

Important

Should you get stuck or notice issues, please get in touch on Matrix or Discourse

Set things up¶

Warning

This tutorial requires version 3.2.0 or later of Charmcraft. Check the version of Charmcraft using charmcraft --version.

First, install Multipass.

Create the Go application¶

Start by creating the “Hello, world” Go application that will be used for this tutorial.

Install go and initialize the Go module:

sudo snap install go --classic
go mod init go-hello-world

Create a main.go file using touch main.go. Then, open the file in a text editor using nano main.go, copy the following text into it and then save the file:

main.go¶

package main

import (
  "fmt"
  "log"
  "net/http"
)

func helloWorldHandler(w http.ResponseWriter, req *http.Request) {
  log.Printf("new hello world request")
  fmt.Fprintln(w, "Hello, world!")
}

func main() {
  log.Printf("starting hello world application")
  http.HandleFunc("/", helloWorldHandler)
  http.ListenAndServe(":8080", nil)
}

Run the Go application locally¶

First, we need to build the Go application so it can run:

go build .

Now that we have a binary compiled, let’s run the Go application to verify that it works:

./go-hello-world

Test the Go application by using curl to send a request to the root endpoint. You will need a new terminal for this; use multipass shell charm-dev to open a new terminal in Multipass:

curl localhost:8080

The Go application should respond with Hello, world!.

The Go application looks good, so we can stop it for now from the original terminal using Ctrl + C.

Pack the Go application into a rock¶

First, we’ll need a rockcraft.yaml file. Using the go-framework profile, Rockcraft will automate the creation of rockcraft.yaml and tailor the file for a Go application. From the /go-hello-world directory, initialize the rock:

rockcraft init --profile go-framework

The rockcraft.yaml file will automatically be created and set the name based on your working directory.

Check out the contents of rockcraft.yaml:

cat rockcraft.yaml

The top of the file should look similar to the following snippet:

rockcraft.yaml¶

name: go-hello-world
# see https://documentation.ubuntu.com/rockcraft/en/latest/explanation/bases/
# for more information about bases and using 'bare' bases for chiselled rocks
base: bare # as an alternative, a ubuntu base can be used
build-base: [email protected] # build-base is required when the base is bare
version: '0.1' # just for humans. Semantic versioning is recommended
summary: A summary of your Go application # 79 char long summary
description: |
    This is go-hello-world's description. You have a paragraph or two to tell the
    most important story about it. Keep it under 100 words though,
    we live in tweetspace and your description wants to look good in the
    container registries out there.
# the platforms this rock should be built on and run on.
# you can check your architecture with `dpkg --print-architecture`
platforms:
    amd64:
    # arm64:
    # ppc64el:
    # s390x:

...

Verfiy that the name is go-hello-world.

Ensure that platforms includes the architecture of your host. Check the architecture of your system:

dpkg --print-architecture

If your host uses the ARM architecture, open rockcraft.yaml in a text editor and include arm64 in platforms.

Now let’s pack the rock:

ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true rockcraft pack

Note

ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS is required while the Go extension is experimental.

Depending on your system and network, this step can take several minutes to finish.

Once Rockcraft has finished packing the Go rock, the terminal will respond with something similar to Packed go-hello-world_0.1_amd64.rock.

Note

If you are not on the amd64 platform, the name of the .rock file will be different for you.

The rock needs to be copied to the MicroK8s registry, which stores OCI archives so they can be downloaded and deployed in the Kubernetes cluster. Copy the rock:

rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
  oci-archive:go-hello-world_0.1_$(dpkg --print-architecture).rock \
  docker://localhost:32000/go-hello-world:0.1

Create the charm¶

From the /go-hello-world directory, let’s create a new directory for the charm and change inside it:

mkdir charm
cd charm

Using the go-framework profile, Charmcraft will automate the creation of the files needed for our charm, including a charmcraft.yaml, requirements.txt and source code for the charm. The source code contains the logic required to operate the Go application.

Initialize a charm named go-hello-world:

charmcraft init --profile go-framework --name go-hello-world

The files will automatically be created in your working directory.

Check out the contents of charmcraft.yaml:

cat charmcraft.yaml

The top of the file should look similar to the following snippet:

# This file configures Charmcraft.
# See https://juju.is/docs/sdk/charmcraft-config for guidance.

name: go-hello-world

type: charm

base: [email protected]

# the platforms this charm should be built on and run on.
# you can check your architecture with `dpkg --print-architecture`
platforms:
  amd64:
  # arm64:
  # ppc64el:
  # s390x:

# (Required)
summary: A very short one-line summary of the Go application.

...

Verify that the name is go-hello-world. Ensure that platforms includes the architecture of your host. If your host uses the ARM architecture, open charmcraft.yaml in a text editor and include arm64 in platforms.

Let’s pack the charm:

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft pack

Note

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS is required while the Go extension is experimental.

Depending on your system and network, this step can take several minutes to finish.

Once Charmcraft has finished packing the charm, the terminal will respond with something similar to Packed go-hello-world_ubuntu-24.04-amd64.charm.

Note

If you are not on the amd64 platform, the name of the .charm file will be different for you.

Deploy the Go application¶

A Juju model is needed to handle Kubernetes resources while deploying the Go application. Let’s create a new model:

juju add-model go-hello-world

If you are not on a host with the amd64 architecture, you will need to include to include a constraint to the Juju model to specify your architecture.

Set the Juju model constraints with:

juju set-model-constraints -m go-hello-world \
  arch=$(dpkg --print-architecture)

Now let’s use the OCI image we previously uploaded to deploy the Go application. Deploy using Juju by specifying the OCI image name with the --resource option:

juju deploy \
  ./go-hello-world_$(dpkg --print-architecture).charm \
  go-hello-world --resource \
  app-image=localhost:32000/go-hello-world:0.1

It will take a few minutes to deploy the Go application. You can monitor its progress with:

juju status --watch 2s

It can take a couple of minutes for the app to finish the deployment. Once the status of the App has gone to active, you can stop watching using Ctrl + C.

Configure the Go application¶

To demonstrate how to provide a configuration to the Go application, we will make the greeting configurable. We will expect this configuration option to be available in the Go app configuration under the keyword GREETING. Change back to the /go-hello-world directory using cd .. and replace the code into main.go with the following:

package main

import (
  "fmt"
  "log"
  "os"
  "net/http"
)

func helloWorldHandler(w http.ResponseWriter, req *http.Request) {
  log.Printf("new hello world request")
  greeting, found := os.LookupEnv("APP_GREETING")
  if !found {
        greeting = "Hello, world!"
  }
  fmt.Fprintln(w, greeting)
}

func main() {
  log.Printf("starting hello world application")
  http.HandleFunc("/", helloWorldHandler)
  http.ListenAndServe(":8080", nil)
}

Increment the version in rockcraft.yaml to 0.2 such that the top of the rockcraft.yaml file looks similar to the following:

name: go-hello-world
# see https://documentation.ubuntu.com/rockcraft/en/latest/explanation/bases/
# for more information about bases and using 'bare' bases for chiselled rocks
base: bare # as an alternative, a ubuntu base can be used
build-base: [email protected] # build-base is required when the base is bare
version: '0.2' # just for humans. Semantic versioning is recommended
summary: A summary of your Go application # 79 char long summary
description: |
    This is go-hello-world's description. You have a paragraph or two to tell the
    most important story about it. Keep it under 100 words though,
    we live in tweetspace and your description wants to look good in the
    container registries out there.
# the platforms this rock should be built on and run on.
# you can check your architecture with `dpkg --print-architecture`
platforms:
    amd64:
    # arm64:
    # ppc64el:
    # s390x:

...

Let’s pack and upload the rock:

ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true rockcraft pack
rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
  oci-archive:go-hello-world_0.2_$(dpkg --print-architecture).rock \
  docker://localhost:32000/go-hello-world:0.2

Change back into the charm directory using cd charm.

The go-framework Charmcraft extension supports adding configurations to charmcraft.yaml, which will be passed as environment variables to the Go application. Add the following to the end of the charmcraft.yaml file:

# configuration snippet for Go application

config:
  options:
    greeting:
      description: |
        The greeting to be returned by the Go application.
      default: "Hello, world!"
      type: string

Note

Configuration options are automatically capitalized and - are replaced by _. An APP_ prefix will also be added as a namespace for app configurations.

We can now pack and deploy the new version of the Go app:

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft pack
juju refresh go-hello-world \
  --path=./go-hello-world_$(dpkg --print-architecture).charm \
  --resource app-image=localhost:32000/go-hello-world:0.2

After we wait for a bit monitoring juju status the application should go back to active again. Verify that the new configuration has been added using juju config go-hello-world | grep -A 6 greeting:, which should show the configuration option.

Using curl http://go-hello-world --resolve go-hello-world:80:127.0.0.1 shows that the response is still Hello, world! as expected.

Now let’s change the greeting:

juju config go-hello-world greeting='Hi!'

After we wait for a moment for the app to be restarted, using curl http://go-hello-world --resolve go-hello-world:80:127.0.0.1 should now return the updated Hi! greeting.

Integrate with a database¶

Now let’s keep track of how many visitors your application has received. This will require integration with a database to keep the visitor count. This will require a few changes:

We will need to create a database migration that creates the visitors table.
We will need to keep track how many times the root endpoint has been called in the database.
We will need to add a new endpoint to retrieve the number of visitors from the database.

Let’s start with the database migration to create the required tables. The charm created by the go-framework extension will execute the migrate.sh script if it exists. This script should ensure that the database is initialized and ready to be used by the application. We will create a migrate.sh file containing this logic.

Go back out to the /go-hello-world directory using cd ... Create the migrate.sh file using a text editor and paste the following code into it:

#!/bin/bash

PGPASSWORD="${POSTGRESQL_DB_PASSWORD}" psql -h "${POSTGRESQL_DB_HOSTNAME}" -U "${POSTGRESQL_DB_USERNAME}" "${POSTGRESQL_DB_NAME}" -c "CREATE TABLE IF NOT EXISTS visitors (timestamp TIMESTAMP NOT NULL, user_agent TEXT NOT NULL);"

Note

The charm will pass the Database connection string in the POSTGRESQL_DB_CONNECT_STRING environment variable once PostgreSQL has been integrated with the charm.

Change the permissions of the file migrate.sh so that it is executable:

chmod u+x migrate.sh

For the migrations to work, we need the postgresql-client package installed in the rock. By default, the go-framework uses the base base, so we will also need to install a shell interpreter. Let’s do it as a slice, so that the rock does not include unnecessary files. Open the rockcraft.yaml file using a text editor and add the following to the end of the file:

parts:
  runtime-debs:
    plugin: nil
    stage-packages:
      # Added manually for the migrations
      - postgresql-client
  runtime-slices:
    plugin: nil
    stage-packages:
      # Added manually for the migrations
      - bash_bins

Increment the version in rockcraft.yaml to 0.3 such that the top of the rockcraft.yaml file looks similar to the following:

name: go-hello-world
# see https://documentation.ubuntu.com/rockcraft/en/latest/explanation/bases/
# for more information about bases and using 'bare' bases for chiselled rocks
base: bare # as an alternative, a ubuntu base can be used
build-base: [email protected] # build-base is required when the base is bare
version: '0.3' # just for humans. Semantic versioning is recommended
summary: A summary of your Go application # 79 char long summary
description: |
    This is go-hello-world's description. You have a paragraph or two to tell the
    most important story about it. Keep it under 100 words though,
    we live in tweetspace and your description wants to look good in the
    container registries out there.
# the platforms this rock should be built on and run on.
# you can check your architecture with `dpkg --print-architecture`
platforms:
    amd64:
    # arm64:
    # ppc64el:
    # s390x:

...

To be able to connect to PostgreSQL from the Go app, the library pgx will be used. The app code needs to be updated to keep track of the number of visitors and to include a new endpoint to retrieve the number of visitors. Open main.go in a text editor and replace its content with the following code:

Check all the packages and their dependencies in the Go project with the following command:

go mod tidy

Let’s pack and upload the rock:

ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true rockcraft pack
rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
  oci-archive:go-hello-world_0.3_$(dpkg --print-architecture).rock \
  docker://localhost:32000/go-hello-world:0.3

Change back into the charm directory using cd charm.

The Go app now requires a database which needs to be declared in the charmcraft.yaml file. Open charmcraft.yaml in a text editor and add the following section to the end of the file:

# requires snippet for Go application with a database

requires:
  postgresql:
    interface: postgresql_client
    optional: false

We can now pack and deploy the new version of the Go app:

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft pack
juju refresh go-hello-world \
  --path=./go-hello-world_$(dpkg --print-architecture).charm \
  --resource app-image=localhost:32000/go-hello-world:0.3

Now let’s deploy PostgreSQL and integrate it with the Go application:

juju deploy postgresql-k8s --trust
juju integrate go-hello-world postgresql-k8s

Wait for juju status to show that the App is active again. Running curl http://go-hello-world --resolve go-hello-world:80:127.0.0.1 should still return the Hi! greeting.

To check the local visitors, use curl http://go-hello-world/visitors --resolve go-hello-world:80:127.0.0.1, which should return Number of visitors 1 after the previous request to the root endpoint. This should be incremented each time the root endpoint is requested. If we repeat this process, the output should be as follows:

user@host:~$ curl http://go-hello-world --resolve go-hello-world:80:127.0.0.1

Hi!

user@host:~$ curl http://go-hello-world/visitors --resolve go-hello-world:80:127.0.0.1

Number of visitors 2

Tear things down¶

We’ve reached the end of this tutorial. We went through the entire development process, including:

Creating a Go application
Deploying the application locally
Packaging the application using Rockcraft
Building the application with Ops code using Charmcraft
Deplyoing the application using Juju
Exposing the application using an ingress
Configuring the application
Integrating the application with a database

If you’d like to reset your working environment, you can run the following in the rock directory /go-hello-world for the tutorial:

CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true charmcraft clean
# Back out to main directory for cleanup
cd ..
ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=true rockcraft clean
# exit and delete the charm dir
rm -rf charm
# delete all the files created during the tutorial
rm go-hello-world_0.1_$(dpkg --print-architecture).rock \
  go-hello-world_0.2_$(dpkg --print-architecture).rock \
  go-hello-world_0.3_$(dpkg --print-architecture).rock \
  rockcraft.yaml main.go migrate.sh go-hello-world go.mod go.sum
# Remove the juju model
juju destroy-model go-hello-world --destroy-storage --no-prompt --force

You can also clean up your Multipass instance. Start by exiting it:

exit

And then you can proceed with its deletion:

multipass delete charm-dev
multipass purge

Next steps¶

By the end of this tutorial you will have built a charm and evolved it in a number of typical ways. But there is a lot more to explore:

If you are wondering…	Visit…
“How do I…?”	How-to guides, Ops \| How-to guides
“How do I debug?”	Charm debugging tools
“How do I get in touch?”	Matrix channel
“What is…?”	Reference, Ops \| Reference, Juju \| Reference
“Why…?”, “So what?”	Ops \| Explanation, Juju \| Explanation