I am looking for solution for DXP 7.0
I have used liferay sample guestbook entry entity for POC.
I want to save entity to search index but not store in database.
So in the EntrylocalServiceImpl ,I have removed the line

entryPersistence.update(entry);

I have replace it with following code:

Indexer<Entry> indexer = IndexerRegistryUtil.nullSafeGetIndexer(Entry.class);
indexer.reindex(entry);

but it doesnt trigger index and the doReindex method of the EntryIndexer class is not getting invoked.
Any alternative to achieve this?

Nishith, did you test the sample? Is search and indexing working before you change anything?

@Russel
Yes. If I add the 

entryPersistence.update(entry);

The entry gets indexed. Once I remove that line. It does not index.

Okay, your indexing code looks fine to me, and search and indexing is working as expected using the usual mechanism. I think one easy thing you should try is removing the @Indexable annotation from the method. I don't know all that this does, but it's probably kicking off an indexing action (delete or reindex) whenever a persistence call is made. I don't know why it would get in the way of your manual indexing code, but it might be a feature of the annotation (sanitize any other indexing calls that might interfere, or something). I'm on a limb with these guesses, but removing teh annotation is probably worth a shot, if you haven't already.

@Russel
I have already tried removing the annotation. It DOES NOT help.