private Optional<String> getChangelogSystem() {
    return Optional.ofNullable(get(CHANGELOG_SYSTEM, get(JobConfig.JOB_DEFAULT_SYSTEM)));
  }
Gets the System to use for changelogs. Uses the following precedence.

1. If job.changelog.system is defined, that value is used.
2. If job.default.system is defined, that value is used.
3. empty optional

Note: Changelogs can be defined using
stores.storeName.changelog=systemName.streamName  or
stores.storeName.changelog=streamName

If the former syntax is used, that system name will still be honored. For the latter syntax, this method is used.

@return the name of the system to use by default for all changelogs, if defined.
samza-core/src/main/java/org/apache/samza/config/StorageConfig.java