protected void setLoggingRequestHelperFactory(LoggingRequestHelperFactory loggingRequestHelperFactory) {
    this.loggingRequestHelperFactory = loggingRequestHelperFactory;
  }
This method attempts to obtain the log file names for the specified component
  on the specified host.  A cache lookup is first attempted. If the cache does not contain
  this data, an asynchronous task is launched in order to make the REST request to
  the LogSearch server to obtain this data.

Once the data is available in the cache, subsequent calls for a given Host/Component
  combination should return non-null.

@param component the component name
@param host the host name
@param cluster the cluster name

@return a Set<String> that includes the log file names associated with this Host/Component
        combination, or null if that object does not exist in the cache.
ambari-server/src/main/java/org/apache/ambari/server/controller/logging/LogSearchDataRetrievalService.java