Integrating MongoDB

MongoDB can be integrated by using the supplied MongoDB event listeners from the barbelhisto-persistence-mongo package. Download the MongoDB listeners on Maven Central.

Listener registration

To register the listeners to BarbelHisto:

MongoClient mongoClient = SimpleMongoListenerClient.create("mongodb://localhost:12345").getMongoClient();
// update listener
SimpleMongoUpdateListener updateListener = SimpleMongoUpdateListener.create(mongoClient, "testDb", "testCol", Client.class, BarbelHistoContext.getDefaultGson());
// pre-fetch listener
SimpleMongoLazyLoadingListener loadingListener = SimpleMongoLazyLoadingListener.create(mongoClient, "testDb", "testCol", Client.class, BarbelHistoContext.getDefaultGson());
// BarbelHisto instance
BarbelHisto<Client> mongoBackedHisto = BarbelHistoBuilder.barbel().withSynchronousEventListener(updateListener)
               .withSynchronousEventListener(loadingListener).build();

The BarbelHisto instance will be pre-fetched with data as it is required on retrieve or save operations performed by the client. Also, the update listener will save new versions automatically to the defined MongoCollection.

Applications can define an instance of BarbelHisto as singleton, which increases performance. If you run in singleton context, use the singletonContext flag on the lazy loading listener.

SimpleMongoLazyLoadingListener loadingListener = 
            SimpleMongoLazyLoadingListener.create(client.getMongoClient(), 
            "testSuiteDb", 
            "testCol", 
            managedType,
            BarbelHistoContext.getDefaultGson(), 
            true);

Further performance imporvements can be achieved when adding the update listeners to the asynchronous service bus (instead of the synchronous bus) when creating the BarbelHistoinstance. However, this requires sensible attention to error handling and thread configuration, as usual in asynchronous processing setups.

The simple listener implementations provide support for all BarbelQueries. If you define custom queries against BarbelHisto include the BarbelQueries.DOCUMENT_ID as a filter criterion. Otherwise BarbelHisto may try to pre-fetch the complete data from the MongoCollection.

Pessimistic locking

If you need multiple BarbelHisto instances access the document collection you can use the MongoPessimisticLockingListener to perform a pessimistic locking. This way it is impossible that two clients concurrently update a journal persisted in MongoDB. You can register the pessimistic locking listener to the synchronous service bus:

SimpleMongoListenerClient client = SimpleMongoListenerClient.create("mongodb://localhost:12345");
SimpleMongoUpdateListener updateListener = SimpleMongoUpdateListener.create(client.getMongoClient(), "testDb", "testCol", managedType, BarbelHistoContext.getDefaultGson());
SimpleMongoLazyLoadingListener loadingListener = SimpleMongoLazyLoadingListener.create(client.getMongoClient(), "testDb", "testCol", managedType, BarbelHistoContext.getDefaultGson());
MongoPessimisticLockingListener lockingListener = MongoPessimisticLockingListener.create(client.getMongoClient(), "lockDb", "docLocks");
BarbelHisto<DefaultPojo> mongoBackedHisto = BarbelHistoBuilder.barbel().withSynchronousEventListener(updateListener)
                            .withSynchronousEventListener(loadingListener).withSynchronousEventListener(lockingListener);

In this setup it is possible to access the version data from mutliple instances of BarbelHisto at the same time.