[GitHub] [jena] afs commented on issue #646: JENA-1785: A newly created node can remain invisible after commit

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afs commented on issue #646: JENA-1785: A newly created node can remain invisible after commit
URL: https://github.com/apache/jena/pull/646#issuecomment-562860832
 
 
   Firstly - thank you very much for working on this and the discussion. The more eyes on concurrency and transaction issues the better.
   
   This approach looks like a good one.
   I think it can be simplified and remove some lock use. 
   
   1/
   The principle that when there is a writer, only the writer can update the not-present cache is good. When there isn't a writer, any reader can update the not-present cache and there is no need to track the data version that I can see. This is because, when there is no writer, the readers can see the whole node table. If they find a node/nodeid but it isn't in the RDF data, all that happens is that use in a triple pattern fails. The not-present cache just speeds that up; that cache don't have to be very large either.
   
   So the rule is either its the active writer, or any reader and no writer active.
   
   ```
    private boolean inTopLevelTxn() {
           Transaction t = txn.get();
           if ( t == null )
               return false;
           // Either t is the write transaction or is a reader and no W txn is active. 
           if ( t.isWriteTxn() ) 
               return true;
           return !hasActiveWriteTransaction.get(); 
       }
   ```
   
   2/
   
   That means `hasActiveWriteTransaction` can be a `AtomicBoolean`, is managed in `updateStart`/`updateCommit`/`updateAbort`. This removes the need for synchronized because only one value is needed.
   
   I've made these changes for discussion on a scratch branch:
   
   https://github.com/apache/jena/compare/master...afs:jena1785_tdb2_misscache
   
   Two commits - the first is PR646, the second is the suggestions above - commit message "AFS-PR646" and currently commit https://github.com/apache/jena/commit/d7063a3ad5aa2751a57b8e3356b2fee122f45595
   
   3/
   There are changes in the test suite to build a test dataset with smaller cache sizes. Then the caches can be fully cycled in a reasonable time.

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