Invoke the Sync Server.
function invoke(dataset_id, params, callback) {
  debug('invoke');

  if (arguments.length < 3) throw new Error('invoke requires 3 arguments');

  // Verify that fn param has been passed
  if (!params || !params.fn) {
    var err = new Error('no fn parameter provided in params "' + util.inspect(params) + '"');
    debugError('[%s] warn %s %j', dataset_id, err, params);
    return callback(err, null);
  }

  var fn = params.fn;

  // Verify that fn param is valid
  if (invokeFunctions.indexOf(fn) < 0) {
    return callback(new Error('invalid fn parameter provided in params "' + fn + '"'), null);
  }

  // We can only continue if sync has connected to its dependencies i.e. mongo & redis
  if (!syncConnected) {
    return callback(new Error('Sync not connected'));
  }

  var fnHandler =  module.exports[fn] || server[fn] || server.api[fn];

  server.api.start(function() {
    return fnHandler(dataset_id, params, callback);
  });
}