Multi-thread fetch using transaction on separate database goes in LMDBException: invalid argument

[lfoppiano]

I have two databases attached to the same Env: relations and lookup, I need to first fetch the values from the lookup (key -> List of ids) and then use the ids to fetch from relations another list of IDs.

I wrote the transaction in the external try-with-resources because I want to keep the same consistency for the two get, however I'm fetching data from two different databases.

Here the piece of code:

   @Override
    public List<Identifier> fetchRelations(final String organisationOriginalID) {
        System.out.println("fetch relations organisation...");
        List<Identifier> relations = new ArrayList<>();
        List<Identifier> uniqueIds = new ArrayList<>();

        final Identifier identifier = new Identifier(organisationOriginalID, OrganisationAttributeNames.LABEL);
        final byte[] serialisedIdentifier = bytes(identifier.toString());

        try (Transaction tx = env.createReadTransaction()) {
            try (Database lookupDB = env.openDatabase(LMDB_DATABASE_LOOKUPS)) {

                byte[] value = lookupDB.get(tx, serialisedIdentifier);
                if (value != null) {

                    List<Identifier> ids = deserialize(value);
                    uniqueIds.addAll(ids);
                }

            if (!CollectionUtils.isEmpty(uniqueIds)) {
                Database relationDB = env.openDatabase(LMDB_DATABASE_RELATIONS);

                uniqueIds.forEach(id -> {

                    byte[] value = relationDB.get(tx, bytes(id.toString()));
                    if (value != null) {
                        relations.addAll(deserialize(value));
                    }

                });
            }
        }

        return relations;
    }

When I run it singe or multi-thread, the behaviour is a bit strange. The database is not written from other processes.

I tried several approaches, but I could not really understand whether I'm doing something wrong (not closing resources) or there is any other problems.

1. when I use read Transaction, namely byte[] value = lookupDB.get(tx, serialisedIdentifier); I get the following exception (pointing at the line where the get is called):

Exception in thread "main" org.fusesource.lmdbjni.LMDBException: Invalid argument
    at org.fusesource.lmdbjni.Util.checkErrorCode(Util.java:44)
    at org.fusesource.lmdbjni.Database.get(Database.java:202)
    at org.fusesource.lmdbjni.Database.get(Database.java:193)
    at org.fusesource.lmdbjni.Database.get(Database.java:186)
    at org.entitycooking.sample.hal.operations.OrganisationLMDBOperations.fetchRelations(OrganisationLMDBOperations.java:91)
    at org.entitycooking.integration.mapping.EntityMapper.fetchGraph(EntityMapper.java:52)
    at org.entitycooking.algorithm.training.EntityMatcherTrainer.extractFeaturesFromExample(EntityMatcherTrainer.java:165)
    at org.entitycooking.algorithm.training.EntityMatcherTrainer.lambda$extractFeatures$4(EntityMatcherTrainer.java:153)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
    at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

2. If I remove the transaction at all, then the application run for a bit longer time, until it crash badly:

   java(35152,0x700005847000) malloc: *** error for object 0x7feb8bebdd40: double free
   *** set a breakpoint in malloc_error_break to debug

Any idea, suggestion?

Thank you in advance

[lfoppiano]

For the issue 1, I think the problem comes when I open the transaction before opening the database (in both single and multi-thread). Does this make sense?

[krisskross]

Always open Env and Database as the application starts and keep them open. You maintain consistency by using the same transaction for both databases.

If you need multi-threading and sharing same transaction you need to set the NO_TLS option.

Never close Env or Database before an open transaction, or you will get a SIGSEGV.

[lfoppiano]

Thanks! The transaction is not shared among threads, it's created/destroyed within the method, so it's good like that.

Are this information somewhere in the documentation? If not I can help you updating it ;-)

[krisskross]

The lmdb documentation is valid for lmdbjni aswell.

[Johny-Kirth]

INFO: org.fusesource.lmdbjni.LMDBException: Invalid argument Am getting this exception while setting maxreaders in LMDBJNI...

Env env = new Env(); env.open(LMDB_DIR); env.setMaxReaders(150); db = env.openDatabase("sub");

Env is created and initialized in the given path.. But I cannot set maxreaders in LMDBJNI as exception arises. If I skip setting maxreaders, then also am getting exception

org.fusesource.lmdbjni.LMDBException:MDB_READERS_FULL : Environment maxreaders limit reached at org.fusesource.lmdbjni.Util.checkErrorCode at org.fusesource.lmdbjni.Env.createTransaction at org.fusesource.lmdbjni.Env.createReadTransaction at org.fusesource.lmdbjni.Database.get(Database.java:170)

Kindly resolve this issue as this is banging my head for more than a week... Since am new to java programming... Thanks in advance

[lfoppiano]

@Johny-Kirth it's perhaps recommended to use https://github.com/lmdbjava/lmdbjava instead.

Which exception do you get when setting the max readers?

[Johny-Kirth]

@Johny-Kirth it's perhaps recommended to use https://github.com/lmdbjava/lmdbjava instead.

Which exception do you get when setting the max readers?

INFO: org.fusesource.lmdbjni.LMDBException: Invalid argument Sep 23, 2019 7:31:29 PM com.adventnet.wms.servercommon.logging.ConsoleLogger write INFO: at org.fusesource.lmdbjni.Util.checkErrorCode(Util.java:44) Sep 23, 2019 7:31:29 PM com.adventnet.wms.servercommon.logging.ConsoleLogger write INFO: at org.fusesource.lmdbjni.Env.setMaxReaders(Env.java:347)