Writing a hook

We'll start with an example of a config-changed hook and break down the code piece by piece

#!/usr/bin/env perl
use charm;

use charm is the entrypoint to exposing charm routines useful for deploying the service. This provides facilities such as installing packages, printing logs, getting relation information, and configuring service level options.

log "Start of charm authoring for config-changed";

The log facility uses juju-log as the utility for logging what's happening in your charm.

my $port = execute(['config-get', 'port']);

config-get routine will pull config options defined in config.yaml.

# close existing bitlbee port

log "Opening port for bitlbee";

( my $output = qq{BITLBEE_PORT=$port
BITLBEE_OPTS="-F"
BITLBEE_DISABLED=0
BITLBEE_UPGRADE_DONT_RESTART=0
} );

path('/etc/default/bitlbee')->spew_utf8($output);

path is exposed from Path::Tiny so anything that applies to that module works the same here.

service_control('bitlbee', 'restart');

service_control is another helper for start/stopping services on the system where the charm is placed.

execute(['open-port', $port]);

open_port exposes a port accessible publicly, and its opposite close_port will remove that accessibility.