public String getSystem(String streamId) {
    String system = get(String.format(SYSTEM_FOR_STREAM_ID, streamId));
    return (system != null) ? system : new JobConfig(this).getDefaultSystem().orElse(null);
  }
Gets the System associated with the specified streamId.
It first looks for the property
streams.{streamId}.system
<p>
If no value was provided, it uses
job.default.system

@param streamId the identifier for the stream in the config.
@return the system name associated with the stream or null.
samza-core/src/main/java/org/apache/samza/config/StreamConfig.java