import org.apache.cassandra.metrics.ClientRequestMetrics; //导入依赖的package包/类
public StorageProxy(APIClient client) {
    super(MBEAN_NAME, client, new ClientRequestMetrics("Read", "storage_proxy/metrics/read"),
            new ClientRequestMetrics("RangeSlice", "/storage_proxy/metrics/range"),
            new ClientRequestMetrics("Write", "storage_proxy/metrics/write"),
            new CASClientRequestMetrics("CASWrite", "storage_proxy/metrics/cas_write"),
            new CASClientRequestMetrics("CASRead", "storage_proxy/metrics/cas_read"));
}
 

import org.apache.cassandra.metrics.ClientRequestMetrics; //导入依赖的package包/类
/**
 * Performs the actual traveling of a graph. This could be complex, let's do it step by step
 */

public static List<Row> travel(TravelCommand command, ConsistencyLevel consistency_level)
throws UnavailableException, IsBootstrappingException, ReadTimeoutException, InvalidRequestException
{
	
	if (StorageService.instance.isBootstrapMode())
    {
        readMetrics.unavailables.mark();
        ClientRequestMetrics.readUnavailables.inc();
        throw new IsBootstrappingException();
    }

	long start = System.nanoTime();
	List<Row> rows = null;
	
	try{
		logger.info("@daidong debug: " + "In travel command execution");
        rows = executeTravel(command, consistency_level);
	} finally
    {
    	//this could be very important for profiling, but currently, i have not time to figure out how it works.
        long latency = System.nanoTime() - start;
        //readMetrics.addNano(latency);
        // TODO avoid giving every command the same latency number.  Can fix this in CASSADRA-5329
    }
	return rows;
}