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https://issues.apache.org/jira/browse/HIVE-18940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16400948#comment-16400948
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Vihang Karajgaonkar commented on HIVE-18940:
--------------------------------------------
I think may be I didn't make my suggestion very clear. I did not suggest using
auto-increment for commit id. auto-increment is just use to uniquely identify
an event. This useful because now we are adding tons of information in one
event. The relationship between a commit id and event id is one to many. There
could be many event ids associated with one commit id. The commit id itself is
a monotonically increasing global number generated just before a transaction
does an actual commit. Here is the pseudo code of my suggestion above:
Modify NOTIFICATION_LOG to add a new field CommitID
Modify NOTIFICATION_LOG table such that EVENT_ID is an auto-increment
Remove the use of NOTIFICATION_SEQUENCE (or perhaps we can reuse it to generate
commit id)
Now here is how the methods generating events might look like
{code:java}
public someMethodWhichGeneratesEvents() {
List<Event> allEventsInThisTransaction;
openTransaction(allEventsInThisTransaction);
//do some work which adds events to allEventsInThisTransaction
//openTransaction again; pass the allEventsInThisTransaction
openTransaction(allEventsInThisTransaction);
//dummy commit
commit(allEventsInThisTransaction);
//actual commit
commit(allEventsInThisTransaction);
}
{code}
The commits would look like this
{code:java}
public boolean commit(List<Event> eventsInThisTransaction) {
counter--;
//if this is an actual commit
if (counter == 0) {
//generate one monotonically increasing number from database
//same logic as used for generating event id currently from
NOTIFICATION_SEQUENCE table
//lock is held for a constant time irrespective of how many events this
transaction generates
commitId = getCommitID();
//add events
for (Event event : eventsInThisTransaction) {
event.setCommitId(commitID);
addNotificationEvent(event);
}
return pm.commit();
}
{code}
So if the commit fails, no events are generated. If the commit succeeds all the
events are generated with unique event ids and the same commit id. If there are
holes in commit id, it means that the transaction has failed and is guaranteed
never to be filled.
> Hive notifications serialize all write DDL operations
> -----------------------------------------------------
>
> Key: HIVE-18940
> URL: https://issues.apache.org/jira/browse/HIVE-18940
> Project: Hive
> Issue Type: Bug
> Components: Metastore
> Affects Versions: 3.0.0
> Reporter: Alexander Kolbasov
> Priority: Major
>
> The implementation of DbNotificationListener uses a single row to store
> current notification ID and uses {{SELECT FOR UPDATE}} to lock the row. This
> serializes all write DDL operations which isn't good.
> We should consider using database auto-increment for notification ID instead.
> Especially on mMySQL/innoDb it is supported natively with relatively
> light-weight locking.
> This creates potential issue for consumers though because such IDs may have
> holes. There are two types of holes - transient hole for a transaction which
> have not committed yet and will be committed shortly and permanent holes for
> transactions that fail. Consumers need to deal with it. It may be useful to
> add DB-generated timestamp as well to assist in recovery from holes.
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