charm.js

var execFile = require('child_process').execFile;
var EE = require('events').EventEmitter;
var util = require('util');
var vm = require('vm');

module.exports = Charm;

function Charm(options) {
  if (!(this instanceof Charm)) return new Charm(options);

  // delay a tick so things can attach, maybe have a separate init that users pass in instead of this being magic on first require
  var self = this;
  process.nextTick(function() {
    //argv[0] is node, argv[1] is script name, the rest will be args for the hook
    var hook = process.argv[1];
    //if we have defined a prototype method for this call it instead. otherwise re-emit
    if (self[hook])
      self[hook].apply(self, process.argv.slice(2));
    else
      self.emit.apply(self, process.argv.slice(1));
  });

  // assuming charm.js is in lib/charm.js the config file defaults to config/config.js
  this.CONFIG_FILE = options.config || path.join(__dirname, '..', 'config', 'config.js');
}
util.inherits(Charm, EE);

Charm.prototype.relation_get = function(key, cb) {
  this.exec('relation-get', [key], cb);
};

// should be a dict I guess?
Charm.prototype.relation_set = function(key, value, cb) {
  this.exec('relation-set', [key, value], cb);
};

Charm.prototype.log = function(msg, cb) {
  this.exec('juju-log', [msg], cb);
};

/* evil this exits like set -e does */
Charm.prototype.exec = function(cmd, args, cb) {
  execFile(cmd, args, function(err, stderr, stdout) {
    if (err) {
      self.log(util.format('failed to run: %s %j %j', cmd, args, err));
      process.exit(1);
      return;
    }
    cb(stdout ? stdout.trim() : '');
  });
};

Charm.prototype.configUpdate = function(section, value, cb) {
  var self = this;
  fs.readFile(this.CONFIG_FILE, 'utf8', function(err, value) {
    // todo err handling -- like file creation?
    var config = vm.runInNewContext('(function(){ return ' + value + ' })()');
    var s = config[section] || {};
    util._extend(s, value);
    config[section] = s;
  });
};

Charm.prototype['database-relation-changed'] = function() {
  var self = this;
  // use control flow mechanism to avoid callback hell
  // preferably vasync
  self.relation_get('user', function(user) {
    self.relation_get('db', function(db) {
      self.relation_get('pass', function(pass) {
        self.relation_get('private-address', function(addr) {
          self.emit('database-relation-changed', user, db, pass, addr);
        });
      });
    });
  });
};

config.js

{
  'database': {
  },
}

database-relation-changed

#!/usr/bin/env node

var charm = require('charm')();

charm.on('database-relation-changed', function(user, db, pass, addr) {
  charm.configUpdate('database', {
    user: user,
    db: db,
    pass: pass,
    addr: addr,
  }, function() {
    // config successfully updated
  });
});

start

#!/usr/bin/env node

var charm = require('charm')();

charm.on('start', function() {
  charm.exec('service', ['node-app', 'start'], function(ret) {
  });
});

You can fetch all the params for a relation in a single relation-get call. you should also test conditionally those values have been set, and exit otherwise, the other side may not have had a chance to set values on the relation yet when the hook executes. juju guarantees the execution of relation-changed again when the remote settings are updated.