code: public void backupAck(long id, Address from, int topologyId) {
        BaseAckTarget ackTarget = collectorMap.get(id);
        if (ackTarget instanceof SingleKeyCollector) {
           ((SingleKeyCollector) ackTarget).backupAck(topologyId, from);
        } else if (ackTarget instanceof MultiTargetCollectorImpl) {
           ((MultiTargetCollectorImpl) ackTarget).backupAck(topologyId, from);
        }
     }
docs: Acknowledges a write operation completion in the backup owner.
  
  @param id         the id from {@link CommandInvocationId#getId()}.
  @param from       the backup owner.
  @param topologyId the topology id.