code: public Future<?> submitIndexBuild(final SecondaryIndexBuilder builder)
      {
          Runnable runnable = new Runnable()
          {
              public void run()
              {
                  metrics.beginCompaction(builder);
                  try
                  {
                      builder.build();
                  }
                  finally
                  {
                      metrics.finishCompaction(builder);
                  }
              }
          };
          if (executor.isShutdown())
          {
              logger.info("Compaction executor has shut down, not submitting index build");
              return null;
          }
  
          return executor.submit(runnable);
      }
docs: Is not scheduled, because it is performing disjoint work from sstable compaction.