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https://issues.apache.org/jira/browse/HIVE-16886?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16121179#comment-16121179
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anishek commented on HIVE-16886:
--------------------------------

for the supported database NL_ID will be generated either using the 
*auto_increment* or *identity* column, the problem though is column mappings of 
type *datastore-identity* cannot be mapped by datanucleus to the java 
representation of the notification log member variable, which means replication 
will not work since the event id is serialized to a file.  hence we cant use 
NL_ID.

> HMS log notifications may have duplicated event IDs if multiple HMS are 
> running concurrently
> --------------------------------------------------------------------------------------------
>
>                 Key: HIVE-16886
>                 URL: https://issues.apache.org/jira/browse/HIVE-16886
>             Project: Hive
>          Issue Type: Bug
>          Components: Hive, Metastore
>            Reporter: Sergio Peña
>
> When running multiple Hive Metastore servers and DB notifications are 
> enabled, I could see that notifications can be persisted with a duplicated 
> event ID. 
> This does not happen when running multiple threads in a single HMS node due 
> to the locking acquired on the DbNotificationsLog class, but multiple HMS 
> could cause conflicts.
> The issue is in the ObjectStore#addNotificationEvent() method. The event ID 
> fetched from the datastore is used for the new notification, incremented in 
> the server itself, then persisted or updated back to the datastore. If 2 
> servers read the same ID, then these 2 servers write a new notification with 
> the same ID.
> The event ID is not unique nor a primary key.
> Here's a test case using the TestObjectStore class that confirms this issue:
> {noformat}
> @Test
>   public void testConcurrentAddNotifications() throws ExecutionException, 
> InterruptedException {
>     final int NUM_THREADS = 2;
>     CountDownLatch countIn = new CountDownLatch(NUM_THREADS);
>     CountDownLatch countOut = new CountDownLatch(1);
>     HiveConf conf = new HiveConf();
>     conf.setVar(HiveConf.ConfVars.METASTORE_EXPRESSION_PROXY_CLASS, 
> MockPartitionExpressionProxy.class.getName());
>     ExecutorService executorService = 
> Executors.newFixedThreadPool(NUM_THREADS);
>     FutureTask<Void> tasks[] = new FutureTask[NUM_THREADS];
>     for (int i=0; i<NUM_THREADS; i++) {
>       final int n = i;
>       tasks[i] = new FutureTask<Void>(new Callable<Void>() {
>         @Override
>         public Void call() throws Exception {
>           ObjectStore store = new ObjectStore();
>           store.setConf(conf);
>           NotificationEvent dbEvent =
>               new NotificationEvent(0, 0, 
> EventMessage.EventType.CREATE_DATABASE.toString(), "CREATE DATABASE DB" + n);
>           System.out.println("ADDING NOTIFICATION");
>           countIn.countDown();
>           countOut.await();
>           store.addNotificationEvent(dbEvent);
>           System.out.println("FINISH NOTIFICATION");
>           return null;
>         }
>       });
>       executorService.execute(tasks[i]);
>     }
>     countIn.await();
>     countOut.countDown();
>     for (int i = 0; i < NUM_THREADS; ++i) {
>       tasks[i].get();
>     }
>     NotificationEventResponse eventResponse = 
> objectStore.getNextNotification(new NotificationEventRequest());
>     Assert.assertEquals(2, eventResponse.getEventsSize());
>     Assert.assertEquals(1, eventResponse.getEvents().get(0).getEventId());
>     // This fails because the next notification has an event ID = 1
>     Assert.assertEquals(2, eventResponse.getEvents().get(1).getEventId());
>   }
> {noformat}
> The last assertion fails expecting an event ID 1 instead of 2. 



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