[jira] [Created] (HBASE-8346) Prefetchiing .META. rows in case only when useCache is set to true

["Himanshu Vashishtha (JIRA)" <ji...@apache.org>]

Himanshu Vashishtha created HBASE-8346:
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             Summary: Prefetchiing .META. rows in case only when useCache is set to true
                 Key: HBASE-8346
                 URL: https://issues.apache.org/jira/browse/HBASE-8346
             Project: HBase
          Issue Type: Bug
          Components: Client
    Affects Versions: 0.95.0
            Reporter: Himanshu Vashishtha
            Priority: Minor


While doing a .META. lookup (HCM#locateRegionInMeta), we also prefetch some other region's info for that table. The usual call to the meta lookup has useCache variable set to true. 
Currently, it calls preFetch irrespective of the value useCache flag:
{code}
            if (Bytes.equals(parentTable, HConstants.META_TABLE_NAME) &&
                (getRegionCachePrefetch(tableName)))  {
              prefetchRegionCache(tableName, row);
            }
{code}
Later on, if useCache flag is set to false, it deletes the entry for that row from the cache with a forceDeleteCachedLocation() call. This always results in two calls to the .META. table in this case. The useCase variable is set to false in case we are retrying to find a region (regionserver failover).

It can be verified from the log statements of a client while having a regionserver failover. In the below example, the client was connected to a1217, when a1217 got killed. The region in question is moved to a1215. Client got this info from META scan, where as client cache this info from META, but then delete it from cache as it want the latest info. 
The result is even the meta provides the latest info, it is still deleted This causes even the latest info to be deleted.
{code}
13/04/15 09:49:12 DEBUG client.HConnectionManager$HConnectionImplementation: Cached location for t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is a1217.abc.com:40020
13/04/15 09:49:12 WARN client.ServerCallable: Received exception, tries=1, numRetries=30 message=Connection refused

13/04/15 09:49:12 DEBUG client.HConnectionManager$HConnectionImplementation: Removed all cached region locations that map to a1217.abc.com,40020,1365621947381
13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results = {NAME => 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.', STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED => 40382355b8c45e1338d620c018f8ff6c,}

13/04/15 09:49:13 DEBUG client.MetaScanner: Scanning .META. starting at row=t,user7225973201630273569,00000000000000 for max=10 rows using hconnection-0x7786df0f
13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results = {NAME => 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.', STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED => 40382355b8c45e1338d620c018f8ff6c,}

13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation: Cached location for t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is *a1215.abc.com:40020*
13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation: *Removed a1215.abc.com:40020* as a location of t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. for tableName=t from cache

13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results = {NAME => 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.', STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED => 40382355b8c45e1338d620c018f8ff6c,}
13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation: Cached location for t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is *a1215.abc.com:40020*
13/04/15 09:49:13 WARN client.ServerCallable: Received exception, tries=2, numRetries=30 message=org.apache.hadoop.hbase.exceptions.UnknownScannerException: Name: -6313340536390503703, already closed?
13/04/15 09:49:13 DEBUG client.ClientScanner: Advancing internal scanner to startKey at 'user760712450403198900'
{code}

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