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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