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https://issues.apache.org/jira/browse/HIVE-16738?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16021190#comment-16021190
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Mohit Sabharwal commented on HIVE-16738:
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[~anishek], not sure I understand. Note that ObjectStore/RawStore is a thread
local (see HiveMetaStore.java).
> Notification ID generation in DBNotification might not be unique across HS2
> instances.
> --------------------------------------------------------------------------------------
>
> Key: HIVE-16738
> URL: https://issues.apache.org/jira/browse/HIVE-16738
> Project: Hive
> Issue Type: Bug
> Components: HiveServer2
> Affects Versions: 3.0.0
> Reporter: anishek
> Assignee: anishek
> Fix For: 3.0.0
>
>
> Going to explain the problem in scope of "replication" feature for hive 2
> that is being built, as it is easier to explain:
> To allow replication to work we need to set
> "hive.metastore.transactional.event.listeners" to DBNotificationListener.
> For use cases where there are multiple HiveServer2 Instances running
> {code}
> private void process(NotificationEvent event, ListenerEvent listenerEvent)
> throws MetaException {
> event.setMessageFormat(msgFactory.getMessageFormat());
> synchronized (NOTIFICATION_TBL_LOCK) {
> LOG.debug("DbNotificationListener: Processing : {}:{}",
> event.getEventId(),
> event.getMessage());
> HMSHandler.getMSForConf(hiveConf).addNotificationEvent(event);
> }
> // Set the DB_NOTIFICATION_EVENT_ID for future reference by other
> listeners.
> if (event.isSetEventId()) {
> listenerEvent.putParameter(
> MetaStoreEventListenerConstants.DB_NOTIFICATION_EVENT_ID_KEY_NAME,
> Long.toString(event.getEventId()));
> }
> }
> {code}
> the above code in DBNotificationListner having the object lock wont be
> guarantee enough to make sure that all events get a unique id. The
> transaction isolation level at the db "read-comitted" or "repeatable-read"
> would also not guarantee the same, unless a lock is at the db level
> preferably on table {{NOTIFICATION_SEQUENCE}} which only has one row.
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