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https://issues.apache.org/jira/browse/HIVE-16164?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15926850#comment-15926850
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Mohit Sabharwal commented on HIVE-16164:
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I think using EnvironmentContext for writing internal state so that it
can be passed around seems unclean to me. The motivation behind
EnvironmentContext was to allow users to specify arbitrary key value
pairs, like partition names for add_partitions call (HIVE-2994). IOW,
the properties are user provided, arbitrary in nature and so
should be considered strictly read-only.
Why don't we simply add an id field in ListnerEvent ? This field can be
null for any event that does not have a id associated with it (i.e.
for events that are not persisted in db). This will also do away
with unnecessarily creating a whole thrift EnvironmentContext
object when no property was passed by the user.
The MetaStoreListenerNotifier looks like a nice cleanup.
> Provide mechanism for passing HMS notification ID between transactional and
> non-transactional listeners.
> --------------------------------------------------------------------------------------------------------
>
> Key: HIVE-16164
> URL: https://issues.apache.org/jira/browse/HIVE-16164
> Project: Hive
> Issue Type: Improvement
> Components: Metastore
> Reporter: Sergio Peña
> Assignee: Sergio Peña
> Attachments: HIVE-16164.1.patch, HIVE-16164.2.patch
>
>
> The HMS DB notification listener currently stores an event ID on the HMS
> backend DB so that external applications (such as backup apps) can request
> incremental notifications based on the last event ID requested.
> The HMS DB notification and backup applications are asynchronous. However,
> there are sometimes that applications may be required to be in sync with the
> latest HMS event in order to process an action. These applications will
> provide a listener implementation that is called by the HMS after an HMS
> transaction happened.
> The problem is that the listener running after the transaction (or during the
> non-transactional context) may need the DB event ID in order to sync all
> events happened previous to that event ID, but this ID is never passed to the
> non-transactional listeners.
> We can pass this event information through the EnvironmentContext found on
> each ListenerEvent implementations (such as CreateTableEvent), and send the
> EnvironmentContext to the non-transactional listeners to get the event ID.
> The DbNotificactionListener already knows the event ID after calling the
> ObjectStore.addNotificationEvent(). We just need to set this event ID to the
> EnvironmentContext from each of the event notifications and make sure that
> this EnvironmentContext is sent to the non-transactional listeners.
> Here's the code example when creating a table on {{create_table_core}}:
> {noformat}
> ms.createTable(tbl);
> if (transactionalListeners.size() > 0) {
> CreateTableEvent createTableEvent = new CreateTableEvent(tbl, true, this);
> createTableEvent.setEnvironmentContext(envContext);
> for (MetaStoreEventListener transactionalListener :
> transactionalListeners) {
> transactionalListener.onCreateTable(createTableEvent); // <-
> Here the notification ID is generated
> }
> }
> success = ms.commitTransaction();
> } finally {
> if (!success) {
> ms.rollbackTransaction();
> if (madeDir) {
> wh.deleteDir(tblPath, true);
> }
> }
> for (MetaStoreEventListener listener : listeners) {
> CreateTableEvent createTableEvent =
> new CreateTableEvent(tbl, success, this);
> createTableEvent.setEnvironmentContext(envContext);
> listener.onCreateTable(createTableEvent); // <-
> Here we would like to consume notification ID
> }
> {noformat}
> We could use a specific key name that will be used on the EnvironmentContext,
> such as DB_NOTIFICATION_EVENT_ID.
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