public String getCoordinatorSystemName() {
    return Optional.ofNullable(getCoordinatorSystemNameOrNull())
        .orElseThrow(() -> new ConfigException(
            "Missing job.coordinator.system configuration. Cannot proceed with job execution."));
  }
job.config.loader.factory specifies {@link ConfigLoaderFactory} to get {@link ConfigLoader}
samza-core/src/main/java/org/apache/samza/config/JobConfig.java