Reactive with Bash or Other Languages

Reactive handlers can be written in any language, provided they conform to the ExternalHandler protocol. In short, they must accept a --test and --invoke argument and do the appropriate thing when called with each.

There are helpers for writing handlers in bash, which allow you to write handlers using a decorator-like syntax similar to Python handlers. For example:

#!/bin/bash
source charms.reactive.sh

@when 'db.database.available' 'admin-pass'
function render_config() {
    db_conn=$(relation_call --flag 'db.database.available' connection_string)
    admin_pass=$(config-get 'admin-pass')
    charms.reactive render_template 'app-config.j2' '/etc/app.conf'
}

@when_not 'db.database.available'
function no_db() {
    status-set waiting 'Waiting on database'
}

@when_not 'admin-pass'
function no_db() {
    status-set blocked 'Missing admin password'
}

@when_file_changed '/etc/app.conf'
function restart_service() {
    service myapp restart
}

reactive_handler_main