docs: port Flask tutorial from Juju.is (canonical#1976)

The current Flask tutorial:
https://juju.is/docs/sdk/write-your-first-kubernetes-charm-for-a-flask-app

This PR will move the tutorial off of Discourse and Charm SDK and onto
the Charmcraft RTD.

---------

Co-authored-by: Michael DuBelko <michael.dubelko@gmail.com>
Co-authored-by: Alex Lowe <alex.lowe@canonical.com>

docs/reuse/tutorial/setup_stable.rst

@@ -0,0 +1,70 @@
+First, `install Multipass <https://multipass.run/docs/install-multipass>`_.
+
+Use Multipass to launch an Ubuntu VM with the name ``charm-dev``
+from the 24.04 blueprint:
+
+.. code-block:: bash
+
+    multipass launch --cpus 4 --disk 50G --memory 4G --name charm-dev 24.04
+
+Once the VM is up, open a shell into it:
+
+.. code-block:: bash
+
+    multipass shell charm-dev
+
+In order to create the rock, you'll need to install Rockcraft:
+
+.. code-block:: bash
+
+    sudo snap install rockcraft --classic
+
+``LXD`` will be required for building the rock.
+Make sure it is installed and initialised:
+
+.. code-block:: bash
+
+    sudo snap install lxd
+    lxd init --auto
+
+In order to create the charm, you'll need to install Charmcraft:
+
+.. code-block:: bash
+
+    sudo snap install charmcraft --channel latest/stable --classic
+
+.. warning::
+
+    This tutorial requires version ``3.0.0`` or later of Charmcraft. Check the
+    version of Charmcraft using ``charmcraft --version`` If you have an older
+    version of Charmcraft installed, use
+    ``sudo snap refresh charmcraft --channel latest/edge`` to get the latest
+    edge version of Charmcraft.
+
+MicroK8s is required to deploy the Flask application on Kubernetes. Install MicroK8s:
+
+.. code-block:: bash
+
+    sudo snap install microk8s --channel 1.31-strict/stable
+    sudo adduser $USER snap_microk8s
+    newgrp snap_microk8s
+
+Wait for MicroK8s to be ready using ``sudo microk8s status --wait-ready``.
+Several MicroK8s add-ons are required for deployment:
+
+.. code-block:: bash
+
+    sudo microk8s enable hostpath-storage
+    # Required to host the OCI image of the Flask application
+    sudo microk8s enable registry
+    # Required to expose the Flask application
+    sudo microk8s enable ingress
+
+Juju is required to deploy the Flask application.
+Install Juju and bootstrap a development controller:
+
+.. code-block:: bash
+
+    sudo snap install juju --channel 3.5/stable
+    mkdir -p ~/.local/share
+    juju bootstrap microk8s dev-controller

docs/tutorial/code/flask/app.py

@@ -0,0 +1,14 @@
+# initial hello world Flask app
+
+import flask
+
+app = flask.Flask(__name__)
+
+
+@app.route("/")
+def index():
+    return "Hello, world!\n"
+
+
+if __name__ == "__main__":
+    app.run()

docs/tutorial/code/flask/greeting_app.py

@@ -0,0 +1,16 @@
+# Flask app with a greeting configuration
+
+import flask
+
+app = flask.Flask(__name__)
+app.config.from_prefixed_env()
+
+
+@app.route("/")
+def index():
+    greeting = app.config.get("GREETING", "Hello, world!")
+    return f"{greeting}\n"
+
+
+if __name__ == "__main__":
+    app.run()

docs/tutorial/code/flask/greeting_charmcraft.yaml

@@ -0,0 +1,9 @@
+# configuration snippet for Flask application with a configuration
+
+config:
+  options:
+    greeting:
+      description: |
+        The greeting to be returned by the Flask application.
+      default: "Hello, world!"
+      type: string

docs/tutorial/code/flask/requirements.txt

@@ -0,0 +1 @@
+Flask

docs/tutorial/code/flask/task.yaml

@@ -0,0 +1,220 @@
+###########################################
+# IMPORTANT
+# Comments matter!
+# The docs use the wrapping comments as
+# markers for including said instructions
+# as snippets in the docs.
+###########################################
+summary: Getting started with Flask tutorial
+
+kill-timeout: 180m
+
+environment:
+
+execute: |
+  # Move everything to $HOME so that Juju deployment works
+  mv *.yaml *.py *.txt $HOME
+  cd $HOME
+
+  # Don't use the staging store for this test
+  unset CHARMCRAFT_STORE_API_URL
+  unset CHARMCRAFT_UPLOAD_URL
+  unset CHARMCRAFT_REGISTRY_URL
+
+  # Add setup instructions
+  # (Ran into issues in prepare section)
+  # (don't install charmcraft)
+  snap install rockcraft --classic
+  snap install lxd
+  lxd init --auto
+  snap install microk8s --channel=1.31-strict/stable
+  snap refresh juju --channel=3.5/stable --amend
+
+  # Juju config setup
+  lxc network set lxdbr0 ipv6.address none
+  mkdir -p ~/.local/share
+
+  # MicroK8s config setup
+  microk8s status --wait-ready
+  microk8s enable hostpath-storage
+  microk8s enable registry
+  microk8s enable ingress
+
+  # Bootstrap controller
+  juju bootstrap microk8s dev-controller
+
+  # [docs:create-venv]
+  sudo apt-get update && sudo apt-get install python3-venv -y
+  python3 -m venv .venv
+  source .venv/bin/activate
+  pip install -r requirements.txt
+  # [docs:create-venv-end]
+
+  flask run -p 8000 &
+  retry -n 5 --wait 2 curl --fail localhost:8000
+
+  # [docs:curl-flask]
+  curl localhost:8000
+  # [docs:curl-flask-end]
+
+  kill $!
+
+  # [docs:create-rockcraft-yaml]
+  rockcraft init --profile flask-framework
+  # [docs:create-rockcraft-yaml-end]
+
+  sed -i "s/name: .*/name: flask-hello-world/g" rockcraft.yaml
+  sed -i "s/amd64/$(dpkg --print-architecture)/g" rockcraft.yaml
+
+  # [docs:pack]
+  rockcraft pack
+  # [docs:pack-end]
+
+  # [docs:ls-rock]
+  ls *.rock -l
+  # [docs:ls-rock-end]
+
+  # [docs:skopeo-copy]
+  rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
+    oci-archive:flask-hello-world_0.1_$(dpkg --print-architecture).rock \
+    docker://localhost:32000/flask-hello-world:0.1
+  # [docs:skopeo-copy-end]
+
+  # [docs:create-charm-dir]
+  mkdir charm
+  cd charm
+  # [docs:create-charm-dir-end]
+
+  # [docs:charm-init]
+  charmcraft init --profile flask-framework --name flask-hello-world
+  # [docs:charm-init-end]
+
+  # [docs:charm-pack]
+  charmcraft pack
+  # [docs:charm-pack-end]
+
+  # [docs:ls-charm]
+  ls *.charm -l
+  # [docs:ls-charm-end]
+
+  # [docs:add-juju-model]
+  juju add-model flask-hello-world
+  # [docs:add-juju-model-end]
+
+  juju set-model-constraints -m flask-hello-world arch=$(dpkg --print-architecture)
+
+  # [docs:deploy-juju-model]
+  juju deploy ./flask-hello-world_ubuntu-22.04-$(dpkg --print-architecture).charm \
+    flask-hello-world --resource \
+    flask-app-image=localhost:32000/flask-hello-world:0.1
+  # [docs:deploy-juju-model-end]
+
+  # [docs:deploy-nginx]
+  juju deploy nginx-ingress-integrator --channel=latest/edge --base ubuntu@20.04
+  juju integrate nginx-ingress-integrator flask-hello-world
+  # [docs:deploy-nginx-end]
+
+  # [docs:config-nginx]
+  juju config nginx-ingress-integrator \
+    service-hostname=flask-hello-world path-routes=/
+  # [docs:config-nginx-end]
+
+  # give Juju some time to deploy the apps
+  juju wait-for application flask-hello-world --query='status=="active"' --timeout 10m
+  juju wait-for application nginx-ingress-integrator --query='status=="active"' --timeout 10m
+
+  # [docs:curl-init-deployment]
+  curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1
+  # [docs:curl-init-deployment-end]
+
+  cd ..
+  cat greeting_app.py > app.py
+  sed -i "s/version: .*/version: 0.2/g" rockcraft.yaml
+  rockcraft pack
+
+  # [docs:docker-update]
+  rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
+    oci-archive:flask-hello-world_0.2_$(dpkg --print-architecture).rock \
+    docker://localhost:32000/flask-hello-world:0.2
+  # [docs:docker-update-end]
+
+  cat greeting_charmcraft.yaml >> ./charm/charmcraft.yaml
+  cd charm
+  charmcraft pack
+
+  # [docs:refresh-deployment]
+  juju refresh flask-hello-world \
+    --path=./flask-hello-world_ubuntu-22.04-$(dpkg --print-architecture).charm \
+    --resource flask-app-image=localhost:32000/flask-hello-world:0.2
+  # [docs:refresh-deployment-end]
+
+  # give Juju some time to refresh the app
+  juju wait-for application flask-hello-world --query='status=="active"' --timeout 10m
+
+  # curl and check that the response is Hello
+  curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 | grep Hello
+
+  # [docs:change-config]
+  juju config flask-hello-world greeting='Hi!'
+  # [docs:change-config-end]
+
+  # make sure that the application updates
+  juju wait-for application flask-hello-world --query='status=="maintenance"' --timeout 10m
+  juju wait-for application flask-hello-world --query='status=="active"' --timeout 10m
+
+  # curl and check that the response is now Hi
+  curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 | grep Hi
+
+  cd ..
+  cat visitors_migrate.py >> migrate.py
+  cat visitors_app.py > app.py
+  sed -i "s/version: .*/version: 0.3/g" rockcraft.yaml
+  echo "psycopg2-binary" >> requirements.txt
+  rockcraft pack
+
+  # [docs:docker-2nd-update]
+  rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false \
+    oci-archive:flask-hello-world_0.3_$(dpkg --print-architecture).rock \
+    docker://localhost:32000/flask-hello-world:0.3
+  # [docs:docker-2nd-update-end]
+
+  cat visitors_charmcraft.yaml >> ./charm/charmcraft.yaml
+  cd charm
+  charmcraft pack
+
+  # [docs:refresh-2nd-deployment]
+  juju refresh flask-hello-world \
+    --path=./flask-hello-world_ubuntu-22.04-$(dpkg --print-architecture).charm \
+    --resource flask-app-image=localhost:32000/flask-hello-world:0.3
+  # [docs:refresh-2nd-deployment-end]
+
+  # [docs:deploy-postgres]
+  juju deploy postgresql-k8s --channel=14/stable --trust
+  juju integrate flask-hello-world postgresql-k8s
+  # [docs:deploy-postgres-end]
+
+  # give Juju some time to deploy and refresh the apps
+  juju wait-for application postgresql-k8s --query='status=="active"' --timeout 10m
+  juju wait-for application flask-hello-world --query='status=="active"' --timeout 30m | juju status --relations
+
+  juju status --relations
+
+  curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 | grep Hi
+  curl http://flask-hello-world/visitors --resolve flask-hello-world:80:127.0.0.1 | grep 1
+  curl http://flask-hello-world --resolve flask-hello-world:80:127.0.0.1 | grep Hi
+  curl http://flask-hello-world/visitors --resolve flask-hello-world:80:127.0.0.1 | grep 2
+
+  # Back out to main directory for clean-up
+  cd ..
+
+  # [docs:clean-environment]
+  # exit and delete the virtual environment
+  deactivate
+  rm -rf charm .venv __pycache__
+  # delete all the files created during the tutorial
+  rm flask-hello-world_0.1_$(dpkg --print-architecture).rock flask-hello-world_0.2_$(dpkg --print-architecture).rock \
+    flask-hello-world_0.3_$(dpkg --print-architecture).rock rockcraft.yaml app.py \
+    requirements.txt migrate.py
+  # Remove the juju model
+  juju destroy-model flask-hello-world --destroy-storage --no-prompt --force
+  # [docs:clean-environment-end]

docs/tutorial/code/flask/visitors_app.py

@@ -0,0 +1,42 @@
+# Flask application that keeps track of visitors using a database
+
+import datetime
+import os
+
+import flask
+import psycopg2
+
+app = flask.Flask(__name__)
+app.config.from_prefixed_env()
+
+DATABASE_URI = os.environ["POSTGRESQL_DB_CONNECT_STRING"]
+
+
+@app.route("/")
+def index():
+    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
+        user_agent = flask.request.headers.get('User-Agent')
+        timestamp = datetime.datetime.now()
+
+        cur.execute(
+            "INSERT INTO visitors (timestamp, user_agent) VALUES (%s, %s)",
+            (timestamp, user_agent)
+        )
+        conn.commit()
+
+
+    greeting = app.config.get("GREETING", "Hello, world!")
+    return f"{greeting}\n"
+
+
+@app.route("/visitors")
+def visitors():
+    with psycopg2.connect(DATABASE_URI) as conn, conn.cursor() as cur:
+        cur.execute("SELECT COUNT(*) FROM visitors")
+        total_visitors = cur.fetchone()[0]
+
+    return f"{total_visitors}\n"
+
+
+if __name__ == "__main__":
+    app.run()

docs/tutorial/code/flask/visitors_charmcraft.yaml

@@ -0,0 +1,6 @@
+# requires snippet for Flask application with a database
+
+requires:
+  postgresql:
+    interface: postgresql_client
+    optional: false