juju-charm-config

name	juju-charm-config
description	Adding and validating charm configuration options. WHEN: add charm configuration options, declare config in charmcraft.yaml, validate config values in config-changed, apply config to a Pebble layer, apply config to a machine charm, write Scenario tests for config.
license	Apache-2.0
metadata	{"author":"Canonical/cantrip","version":"1.0.0","summary":"Adding and validating Juju charm configuration options; applying config to Pebble layers and machine workloads.","tags":["juju","ops","config","config-changed"]}

Adding Configuration Options

Charm configuration lets operators customise behaviour without modifying code. Config values are set with juju config and delivered to the charm via config-changed events.

Key Concepts

• Config is declared in charmcraft.yaml with types, defaults, and descriptions
• config-changed fires whenever any config value changes
• Config values persist across hook executions — read them from self.config any time
• Validate early — check config in config-changed and set BlockedStatus if invalid

Step 1: Declare Config Options

In charmcraft.yaml:

config:
  options:
    log-level:
      type: string
      default: info
      description: |
        Logging verbosity. One of: debug, info, warning, error.
    port:
      type: int
      default: 8080
      description: |
        Port the workload listens on.
    enable-tls:
      type: boolean
      default: false
      description: |
        Enable TLS for the workload.
    max-connections:
      type: int
      default: 100
      description: |
        Maximum number of concurrent connections.

Supported Types

Type	Juju Type	Python Type	Example
String	string	str	"info"
Integer	int	int	8080
Float	float	float	0.5
Boolean	boolean	bool	true

Step 2: Handle config-changed

import ops

VALID_LOG_LEVELS = {"debug", "info", "warning", "error"}


class MyCharm(ops.CharmBase):
    def __init__(self, framework: ops.Framework):
        super().__init__(framework)
        self.framework.observe(self.on.config_changed, self._on_config_changed)

    def _on_config_changed(self, event: ops.ConfigChangedEvent):
        log_level = self.config.get("log-level", "info")
        if log_level not in VALID_LOG_LEVELS:
            self.unit.status = ops.BlockedStatus(
                f"Invalid log-level: {log_level!r}. "
                f"Must be one of: {', '.join(sorted(VALID_LOG_LEVELS))}"
            )
            return

        port = self.config["port"]
        if not (1 <= port <= 65535):
            self.unit.status = ops.BlockedStatus(
                f"Invalid port: {port}. Must be between 1 and 65535."
            )
            return

        # Config is valid — apply it.
        self._apply_config()
        self.unit.status = ops.ActiveStatus()

    def _apply_config(self):
        """Apply current config to the workload."""
        # For K8s charms, update the Pebble layer.
        # For machine charms, update config files and restart services.
        ...

Step 3: Apply Config to Pebble (K8s Charms)

def _apply_config(self):
    container = self.unit.get_container("workload")
    if not container.can_connect():
        return

    layer = {
        "services": {
            "workload": {
                "override": "replace",
                "command": "/app/start",
                "environment": {
                    "LOG_LEVEL": self.config["log-level"],
                    "PORT": str(self.config["port"]),
                    "ENABLE_TLS": str(self.config["enable-tls"]).lower(),
                    "MAX_CONNECTIONS": str(self.config["max-connections"]),
                },
            },
        },
    }
    container.add_layer("workload", layer, combine=True)
    container.replan()

Step 4: Apply Config to Machine Charms

def _apply_config(self):
    import subprocess

    config_content = self._render_config_file()
    config_path = "/etc/myapp/config.yaml"

    # Write the config file.
    with open(config_path, "w") as f:
        f.write(config_content)

    # Restart the service.
    subprocess.check_call(["systemctl", "restart", "myapp"])

Step 5: Use Config in Other Hooks

Config is always available via self.config, not just in config-changed:

def _on_database_changed(self, event: ops.RelationChangedEvent):
    port = self.config["port"]
    # Use port when configuring the database connection.
    ...

Step 6: Test Config with Scenario

from ops import testing


def test_valid_config():
    state = testing.State(config={"log-level": "debug", "port": 9090})
    ctx = testing.Context(MyCharm)
    out = ctx.run(ctx.on.config_changed(), state)
    assert out.unit_status == testing.ActiveStatus()


def test_invalid_log_level():
    state = testing.State(config={"log-level": "verbose"})
    ctx = testing.Context(MyCharm)
    out = ctx.run(ctx.on.config_changed(), state)
    assert isinstance(out.unit_status, testing.BlockedStatus)
    assert "Invalid log-level" in out.unit_status.message


def test_invalid_port():
    state = testing.State(config={"port": 0})
    ctx = testing.Context(MyCharm)
    out = ctx.run(ctx.on.config_changed(), state)
    assert isinstance(out.unit_status, testing.BlockedStatus)

Step 7: Set Config as an Operator

# Set a single option.
juju config my-charm log-level=debug

# Set multiple options.
juju config my-charm log-level=debug port=9090

# Reset to default.
juju config my-charm --reset log-level

# View current config.
juju config my-charm

Design Principles

1. Validate all config values. Set BlockedStatus with a clear message if any value is invalid. Never silently ignore bad config.

2. Use sensible defaults. The charm should work out of the box with default config values. Operators should only need to change config for customisation.

3. Config names use hyphens (e.g., log-level, max-connections). This is the Juju convention. Access them in Python with self.config["log-level"].

4. Keep config options stable. Renaming or removing config options breaks existing deployments. Add new options freely; deprecate old ones gradually.

5. Document each option clearly. The description field in charmcraft.yaml is what operators see when running juju config my-charm --all. Write it for a human audience.

6. Apply config idempotently. config-changed fires on every juju config call, even if the value did not actually change. The handler should apply config without side effects if nothing changed.

Common Pitfalls

• Declaring an option but never reading it; or validating without BlockedStatus. charmlint covers both deterministically (CFG004 flags any option that does not appear as self.config[...] / self.config.get(...) in src/; CFG005 flags charms with config options but no BlockedStatus reference at all). Run charmlint to surface these — the skill body no longer recites them.
• Not handling config-changed at all — config values are still accessible via self.config, but the workload will not be updated until the charm explicitly applies them.
• Crashing on invalid config — always validate and set BlockedStatus instead of raising exceptions.
• Assuming config types are correct — Juju enforces the type declared in metadata, but it is good practice to validate ranges and allowed values.
• Forgetting to replan() after updating a Pebble layer — the service will not restart with the new config until replan() is called.