Major Description
I'm not sure if this is the only cause of the exception "org.datanucleus.exceptions.NucleusObjectNotFoundException: No such database row)" from the metastore, but one cause seems to be related to a drop command failing, and being retried by the client.
Based on focusing on a single thread in the metastore with DEBUG level logging, I was seeing the objects that were intended to be dropped remaining in the PersistenceManager cache even after a rollback. The steps seemed to be as follows:
1) First attempt to drop the table, the table is pulled into the PersistenceManager cache for the purposes of dropping
2) The drop fails, e.g. due to a lock wait timeout on the SQL backend, this causes a rollback of the transaction
3) The drop is retried using a different thread on the metastore Thrift server or a different server and succeeds
4) Back on the original thread of the original Thrift server someone tries to perform some write operation which produces a commit. This causes those detached objects related to the dropped table to attempt to reattach, causing JDO to query the SQL backend for those objects which it can't find. This causes the exception.
I was able to reproduce this regularly using the following sequence of commands:
Hive client 1 (Hive1): connected to a metastore Thrift server running a single thread, I hard coded a RuntimeException into the code to drop a table in the ObjectStore, specifically right before the commit in preDropStorageDescriptor, to induce a rollback. I also turned off all retries at all layers of the metastore.
Hive client 2 (Hive2): connected to a separate metastore Thrift server running with standard configs and code
1: On Hive1, CREATE TABLE t1 (c STRING);
2: On Hive1, DROP TABLE t1; // This failed due to the hard coded exception
3: On Hive2, DROP TABLE t1; // Succeeds
4: On Hive1, CREATE DATABASE d1; // This database already existed, I'm not sure why this was necessary, but it didn't work without it, it seemed to have an affect on the order objects were committed in the next step
5: On Hive1, CREATE DATABASE d2; // This database didn't exist, it would fail with the NucleusObjectNotFoundException
The object that would cause the exception varied, I saw the MTable, the MSerDeInfo, and MTablePrivilege from the table that attempted to be dropped.