Details
-
Improvement
-
Status: Resolved
-
Major
-
Resolution: Won't Fix
-
None
-
None
-
None
-
None
Description
Currently, metastore objects are identified uniquely by their names (eg. catName, dbName and tblName for a table is unique). Once a table or partition is created it could be altered in many ways. There is no good way currently to identify the version of the object once it is altered. For example, suppose there are two clients (Hive and Impala) using the same metastore. Once some alter operations are performed by a client, another client which wants to do a alter operation has no good way to know if the object which it has is the same as the one stored in metastore. Metastore updates the transient_lastDdlTime every time there is a DDL operation on the object. However, this value cannot be relied for all the clients since after HIVE-1768 metastore updates the value only when it is not set in the parameters. It is possible that a client which alters the object state, does not remove the transient_lastDdlTime and metastore will not update it. Secondly, if there is a clock skew between multiple HMS instances when HMS-HA is configured, time values cannot be relied on to find out the sequence of alter operations on a given object.
This JIRA propose to use JDO versioning support by Datanucleus http://www.datanucleus.org/products/accessplatform_4_2/jdo/versioning.html to generate a incrementing sequence number every time a object is altered. The value of this object can be set as one of the values in the parameters. The advantage of using Datanucleus the versioning can be done across HMS instances as part of the database transaction and it should work for all the supported databases.
In theory such a version can be used to detect if the client is presenting a object which is "stale" when issuing a alter request. Metastore can choose to reject such a alter request since the client may be caching a old version of the object and any alter operation on such stale object can potentially overwrite previous operations. However, this is can be done in a separate JIRA.