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https://issues.apache.org/jira/browse/HIVE-16886?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16139346#comment-16139346
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anishek commented on HIVE-16886:
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Thinking more on this, might be we just do "select for update' for getting the
next event id from notification_sequence. this should work on most of the
problems we have but will make metastore operations slow.
also just wondering even in single standalone HMS we can have this issue, as
the NOTIFICATION_TBL_LOCK is within a nested transaction and multiple threads
can still read the same value across threads and update it separately.
> 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
> Assignee: anishek
> Attachments: datastore-identity-holes.diff, HIVE-16886.1.patch
>
>
> 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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