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https://issues.apache.org/jira/browse/HIVE-16886?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16145772#comment-16145772
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anishek commented on HIVE-16886:
--------------------------------
[~akolb] the transaction for handling the dbnotification update is a nested
transaction and i didnt want to set the whole transaction to setSerializedRead,
as even though currently this is done as the last set of actions in HMS, it
might change in future and then every additional queries run in the transaction
will be serializedRead. Setting the same on the query was again confusing as
the result of change should not be visible to other transactions till commit
and hence even though we are setting is on the query object, it might just be
internally doing it on the current open transactions which brings us back to
the first problem.
Also since code to handle {{select.. for update}} was already there as part of
ACID, i just reused the same, as it gives more confidence that there are no
problems with this semantics rather than the setSerializedRead on Datanucleus.
> 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,
> HIVE-16886.2.patch, HIVE-16886.3.patch, HIVE-16886.4.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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