public function registerJob($identifier, $callback)
    {
        if (array_key_exists($identifier, $this->identifierToId)) {
            $idNum = $this->identifierToId[$identifier];
        } else {
            $idNum = count($this->identifierToId) + 1;
            $this->idToIdentifier[$idNum] = $identifier;
        }
        $this->jobs[$identifier] = call_user_func(
            $callback,
            $idNum
        );
        $this->identifierToId[$identifier] = $idNum;
    }