Write your first Kubernetes charm for an Express app¶

Imagine you have an Express app 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 Express app in no time. In this tutorial we will build a Kubernetes charm for an Express app using Charmcraft, so we can have an Express app up and running with Juju. Let’s get started!

This tutorial should take 90 minutes for you to complete.

If you’re new to the charming world, Express apps are specifically supported with a template to quickly generate a rock and a matching template to generate a charm. A rock is a special kind of OCI-compliant container image, while a charm is a software operator for cloud operations that use the Juju orchestration engine. The result is an Express app that can be easily deployed, configured, scaled, and integrated 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 an Express app.
Use that to create a rock with Rockcraft.
Use that to create a charm with Charmcraft.
Use that to test, deploy, configure, etc., your Express app on a local Kubernetes cloud with Juju.
Repeat the process, mimicking a real development process.

Important

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

Set things up¶

First, install Multipass.

Create the Express app¶

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

Install npm and express-generator to initialize the Express module:

sudo apt update -y && sudo apt install npm -y
sudo npm install -g express-generator@4
express app
cd app

Run the Express app locally¶

Install the necessary packages for the app so it can run:

npm install

With the packages installed, let’s run the Express app to verify that it works:

npm start

Test the Express app 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 -I localhost:3000

If the output has an HTTP/1.1 200 OK status message, then we know the newly-built app is working.

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

Pack the Express app into a rock¶

Now let’s create a container image for our Express app. We’ll use a rock, which is an OCI-compliant container image based on Ubuntu.

First, we’ll need a rockcraft.yaml project file. We’ll take advantage of a pre-defined extension in Rockcraft with the --profile flag that caters initial rock files for specific web app frameworks. Using the expressjs-framework profile, Rockcraft automates the creation of rockcraft.yaml and tailors the file for an Express app. From the ~/expressjs-hello-world directory, initialize the rock:

cd ..
rockcraft init --profile expressjs-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:

~/expressjs-hello-world/rockcraft.yaml¶

name: expressjs-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 ExpressJS app # 79 char long summary
description: |
    This is expressjs-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 expressjs-hello-world.

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

dpkg --print-architecture

Edit the platforms key in rockcraft.yaml if required.

Let’s pack the rock:

rockcraft pack

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

For more options when packing rocks

See the pack command reference.

Once Rockcraft has finished packing the Express rock, the terminal will respond with something similar to Packed expressjs-hello-world_0.1_<architecture>.rock. The file name reflects your system’s architecture. After the initial pack, subsequent rock packings are faster.

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

This command contains the following pieces:

--insecure-policy: adopts a permissive policy that removes the need for a dedicated policy file.
--dest-tls-verify=false: disables the need for HTTPS and verify certificates while interacting with the MicroK8s registry.
oci-archive: specifies the rock we created for our Express app.
docker: specifies the name of the image in the MicroK8s registry.

Create the charm¶

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

mkdir charm
cd charm

Similar to the rock, we’ll take advantage of a pre-defined extension in Charmcraft with the --profile flag that caters initial charm files for specific web app frameworks. Using the expressjs-framework profile, Charmcraft automates the creation of the files needed for our charm, including a charmcraft.yaml project file, requirements.txt and source code for the charm. The source code contains the logic required to operate the Express app.

Initialize a charm named expressjs-hello-world:

charmcraft init --profile expressjs-framework --name expressjs-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:

~/expressjs-hello-world/charm/charmcraft.yaml¶

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

name: expressjs-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 ExpressJS app.

...

Verify that the name is expressjs-hello-world. Ensure that platforms includes the architecture of your host. Edit the platforms key in the project file if required.

Let’s pack the charm:

charmcraft pack

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 expressjs-hello-world_ubuntu-24.04-<architecture>.charm. The file name reflects your system’s architecture. After the initial pack, subsequent charm packings are faster.

For more options when packing charms

See the pack command reference.

Deploy the Express app¶

A Juju model is needed to handle Kubernetes resources while deploying the Express app. The Juju model holds the app along with any supporting components. In this tutorial, our model will hold the Express app, ingress, and a PostgreSQL database.

Let’s create a new model:

juju add-model expressjs-hello-world

Constrain the Juju model to your architecture:

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

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

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

It will take a few minutes to deploy the Express app. 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.

Tip

To monitor your deployment, keep a juju status session active in a second terminal.

See more: Juju | juju status

The Express app should now be running. We can monitor the status of the deployment using juju status, which should be similar to the following output:

~$ juju status

Model                  Controller      Cloud/Region        Version  SLA          Timestamp

expressjs-hello-world dev-controller microk8s/localhost 3.6.5 unsupported 12:24:51+03:00

App                    Version  Status  Scale  Charm                  Channel  Rev  Address        Exposed  Message

expressjs-hello-world           active      1  expressjs-hello-world             0  10.152.183.38  no

Unit Workload Agent Address Ports Messageexpressjs-hello-world/0* active idle 10.1.157.75

Let’s expose the app using ingress. Deploy the nginx-ingress-integrator charm and integrate it with the Express app:

juju deploy nginx-ingress-integrator --channel=latest/stable --trust
juju integrate nginx-ingress-integrator expressjs-hello-world

The hostname of the app needs to be defined so that it is accessible via the ingress. We will also set the default route to be the root endpoint:

juju config nginx-ingress-integrator \
  service-hostname=expressjs-hello-world path-routes=/

Note

By default, the port for the express-framework extension should be 8080. If you want to change the default port, it can be done with the configuration option port that will be exposed as the PORT to the Express app.

Monitor juju status until everything has a status of active.

Use curl -I http://expressjs-hello-world --resolve expressjs-hello-world:80:127.0.0.1 to send a request via the ingress. It should show the HTTP/1.1 200 OK status message.

Note

The --resolve expressjs-hello-world:80:127.0.0.1 option to the curl command is a way of resolving the hostname of the request without setting a DNS record.

The development cycle¶

So far, we have worked though the entire cycle, from creating an app to deploying it. But now – as in every real-world case – we will go through the experience of iterating to develop the app, and deploy each iteration.

Configure the Express app¶

To demonstrate how to provide a configuration to the Express app, we will make the greeting configurable. We will expect this configuration option to be available in the Express app configuration under the keyword GREETING. Change to the ~/expressjs-hello-world/app/routes directory and replace the code in index.js with the following:

~/expressjs-hello-world/app/routes/index.js¶

var express = require('express');
var router = express.Router();

let greeting = process.env["APP_GREETING"]

if (!greeting){
  greeting = "Hello, world!";
}

/* GET home page. */
router.get('/', function(req, res, next) {
  res.send(greeting);
});

module.exports = router;

Update the rock¶

Since we’re changing the app we should update the version of the rock. Increment the version in rockcraft.yaml to 0.2 such that the top of the rockcraft.yaml file looks similar to the following:

~/expressjs-hello-world/rockcraft.yaml¶

name: expressjs-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 ExpressJS app # 79 char long summary
description: |
    This is expressjs-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 pack
rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
  oci-archive:expressjs-hello-world_0.2_$(dpkg --print-architecture).rock \
  docker://localhost:32000/expressjs-hello-world:0.2

Redeploy the charm¶

We’ll redeploy the new version with juju refresh.

Change back into the charm directory using cd charm.

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

~/expressjs-hello-world/charm/charmcraft.yaml¶

# configuration snippet for ExpressJS app

config:
  options:
    greeting:
      description: |
        The greeting to be returned by the Express 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 Express app:

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

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

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

Now let’s change the greeting:

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

After we wait for a moment for the app to be restarted, using curl http://expressjs-hello-world --resolve expressjs-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 app 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 expressjs-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 app. We will create a migrate.sh file containing this logic.

Back out to the ~/expressjs-hello-world/app directory. Create the migrate.sh file using a text editor and paste the following code into it:

~/expressjs-hello-world/app/migrate.sh¶

#!/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 app/migrate.sh

To connect the Express app to PostgreSQL, we will use the pg-promise library. 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. Create a new file called visitors.js in the ~/expressjs-hello-world/app/routes directory and paste the following code into it:

~/expressjs-hello-world/app/routes/visitors.js¶

var express = require("express");
var router = express.Router();
const pgp = require("pg-promise")(/* options */);
const PG_CONNECT_STR = process.env["POSTGRESQL_DB_CONNECT_STRING"];

console.log("PG_CONNECT_STR", PG_CONNECT_STR);
let db = null;

if (PG_CONNECT_STR) {
  db = pgp(PG_CONNECT_STR);
}
/* GET visitors count. */
router.get("/", async function (req, res, next) {
  console.log("visitors request");

  if (!db) {
    console.error("Database connection is not initialized");
    return res.status(500).send("Database connection error");
  }

  try {
    const result = await db.one("SELECT count(*) FROM visitors");
    const numVisitors = result.count;

    res.send(`Number of visitors: ${numVisitors}\n`);
  } catch (error) {
    console.error("An error occurred while executing query:", error);
    res.status(500).send("Error retrieving visitors count");
  }
});

module.exports = router;

Open index.js in a text editor and replace the code in it with the following to record the number of visitors:

~/expressjs-hello-world/app/routes/index.js¶

var express = require('express');
var router = express.Router();
const pgp = require("pg-promise")(/* options */);
const PG_CONNECT_STR = process.env["POSTGRESQL_DB_CONNECT_STRING"];

/* GET home page. */
router.get('/', async function(req, res, next) {
  console.log("new hello world request");

  if (!PG_CONNECT_STR) {
    console.error("Database connection string is not set");
    return res.status(500).send("Database connection error");
  }

  const db = pgp(PG_CONNECT_STR);

  try {
    const userAgent = req.get("User-Agent");
    const timestamp = new Date();

    await db.none(
      "INSERT INTO visitors (timestamp, user_agent) VALUES ($1, $2)",
      [timestamp, userAgent]
    );

    const greeting = process.env["APP_GREETING"] || "Hello, world!";
    res.send(greeting + "\n");
  } catch (error) {
    console.error("An error occurred:", error);
    res.status(500).send("An error occurred while processing your request");
  } finally {
    db.$pool.end(); // Close the database connection pool
  }
});

module.exports = router;

Change back to the ~/expressjs-hello-world/app directory, open app.js in a text editor and replace its content with the following code to add the new route:

~/expressjs-hello-world/app/app.js¶

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');
var visitorsRouter = require('./routes/visitors');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', indexRouter);
app.use('/users', usersRouter);
app.use('/visitors', visitorsRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

Add the new package in the Express project:

npm install pg-promise

Update the rock again¶

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

~/expressjs-hello-world/rockcraft.yaml¶

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:

~/expressjs-hello-world/rockcraft.yaml¶

name: expressjs-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 ExpressJS app # 79 char long summary
description: |
    This is expressjs-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 pack
rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
  oci-archive:expressjs-hello-world_0.3_$(dpkg --print-architecture).rock \
  docker://localhost:32000/expressjs-hello-world:0.3

Update the charm again¶

Change back into the charm directory using cd charm.

The Express 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:

~/expressjs-hello-world/charm/charmcraft.yaml¶

# requires snippet for Express app with a database

requires:
  postgresql:
    interface: postgresql_client
    optional: false

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

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

Now let’s deploy PostgreSQL and integrate it with the Express app:

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

Wait for juju status to show that the App is active again. During this time, the Express app may enter a blocked state as it waits to become integrated with the PostgreSQL database. Due to the optional: false key in the endpoint definition, the Express app will not start until the database is ready.

Once the Express app is active, running curl http://expressjs-hello-world --resolve expressjs-hello-world:80:127.0.0.1 should still return the Hi! greeting.

To check the local visitors, use curl http://expressjs-hello-world/visitors --resolve expressjs-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:

~$ curl http://expressjs-hello-world --resolve expressjs-hello-world:80:127.0.0.1

Hi!

~$ curl http://expressjs-hello-world/visitors --resolve expressjs-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 Express app
Deploying the app locally
Packaging the app using Rockcraft
Building the app with Ops code using Charmcraft
Deplyoing the app using Juju
Exposing the app using an ingress
Configuring the app
Integrating the app with a database

If you’d like to quickly tear things down, start by exiting the Multipass VM:

exit

And then you can proceed with its deletion:

multipass delete charm-dev
multipass purge

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

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

You can also clean up your Multipass instance by exiting and deleting it using the same commands as above.

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