code: public ActiveExecution lock(String id)
              throws Exception {
          LOGGER.debug("locking {}", id);
          if (!myLocks.contains(id)) {
              if (locking.acquire(id, null)) {
                  myLocks.add(id);
                  ActiveExecution ae = new ActiveExecution();
                  ae.setMigrationJobId(id);
                  ae.set_id(id);
                  ae.setStartTime(new Date());
                  return ae;
              }
          }
          return null;
      }
docs: Attempts to lock a migration job. If successful, return the migration job
  and the active execution