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From "Feng Honghua (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-10584) Inconsistency between tableExists and listTables in implementation
Date Fri, 21 Feb 2014 09:54:25 GMT
Feng Honghua created HBASE-10584:
------------------------------------

             Summary: Inconsistency between tableExists and listTables in implementation
                 Key: HBASE-10584
                 URL: https://issues.apache.org/jira/browse/HBASE-10584
             Project: HBase
          Issue Type: Bug
          Components: Client, master
            Reporter: Feng Honghua
            Assignee: Feng Honghua


# HBaseAdmin.tableExists is implemented by scanning meta table
# HBaseAdmin.listTables(and HBaseAdmin.getTableDescriptor) is implemented by talking with
HMaster which responses by querying the FSTableDescriptors, and FSTableDescriptors return
all tables by scanning all the table descriptor files in FS(cache also plays here, so most
of time it can be satisfied by cache)...

Actually HBaseAdmin requests HMaster to check if a table exists internally when implementing
deleteTable(see below), then why does it use a different(scanning meta table) way to implementing
tableExists() for outside user to use for the same purpose?
{code}
  tableExists = false;
  GetTableDescriptorsResponse htds;
  MasterKeepAliveConnection master = connection.getKeepAliveMasterService();
  try {
    GetTableDescriptorsRequest req =
    RequestConverter.buildGetTableDescriptorsRequest(tableName);
    htds = master.getTableDescriptors(null, req);
  } catch (ServiceException se) {
    throw ProtobufUtil.getRemoteException(se);
  } finally {
    master.close();
  }
  tableExists = !htds.getTableSchemaList().isEmpty();
{code}

Since creating table descriptor files and inserting item to meta table occur in different
time without atomic semantic, this inconsistency in implementation can lead to confusing behaviors
when create-table or delete-table fails midway, (before according cleanup is done) table descriptor
file may exists while no item exists in meta table (for create-table where table descriptor
file is created before inserting item to meta table), this leads to listTables including that
table, while tableExists says no. Similar inconsistency if delete-table fails mid-way...

Confusing behavior can happen during the process even though eventually it succeed:
# During table creation, when a user calls listTables and then calls tableExists for this
table after the table descriptor is created but before item is inserted to meta table. He
will find the listTables includes a table but tableExists return false for that same table,
this behavior is confusing and should only acceptable during the table is being deleted...
# Similar behavior occurs during table deletion.

Seems the benefit of implementing tableExists this way is we can avoid talking with HMaster,
considering we talk with HMaster for listTables and getTableDescriptor, such benefit can't
offset the drawback from inconsistency.



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