Buffering Messages in the Message Producer

What the problem is solving?

Consider the following cases:

  • a new entity is created and then a message contains its ID is sent to the message queue, but the database transaction was rolled back
  • a new entity is created and then a message contains its ID is sent to the message queue, but before the database transaction is committed
  • the database transaction was committed successfully, but sending of related messages to the message queue was failed

Using the buffering we try to solve the first two cases. But be aware that this approach works well only if the master-slave replication is not used, what is the common case, at least for small and middle size databases. In case of master-slave replication is used the described issues are still possible because the transferring of changes from the master to the slave requires some time and if the consumer is connected to the slave database it may receive not up-to-date data. So, the message queue processors should be ready to handle this issue.

The buffering does not solve the third described case, but fortunately it is quite rare, the most common case when this can happens is probably that the message queue broker is not reachable.

The buffering works in the following way: before sending a message to the message queue, the database transaction is checked whether it is open (including nested) or not. If the transaction is not open, the message will be sent right away. But when an open transaction is detected, then the message will be stored to a buffer and it will be sent only after all the transactions (including nested) were committed. In case if the transaction is rolled back the buffer is cleared up without sending them.

Implementation Details

BufferedMessageProducer implements the buffering of messages and it is used as a decorator over other types of producers. It works in the following way:

  • when the buffering is enabled, the producer stores messages in the internal buffer; the messages from the buffer are send to the queue only when flushBuffer method is called; in case if clearBuffer method is called all messages are removed from the buffer without sending them to the queue
  • when the buffering is disabled, the producer sends messages directly to the queue via the decorated producer

By default the buffering is disabled.

  • DbalTransactionWatcher watches the default DBAL transaction in order to enable the buffering mode of BufferedMessageProducer when the root transaction starts (call enableBuffering method of the producer) and send all collected messages when the root transaction is committed (call flushBuffer and then disableBuffering methods of the producer) or remove all collected messages from the buffer without sending them when the root transaction is rolled back (call clearBuffer and then disableBuffering methods of the producer).

The watcher service is tagged by oro.doctrine.connection.transaction_watcher tag. OroPlatform handles this tag out of the box. But if you use the MessageQueue bundle without OroPlatform, you need to register the Oro\Component\DoctrineUtils\DependencyInjection\AddTransactionWatcherCompilerPass compiler pass and class loader for the transaction watcher aware connection proxy in your application, for example:

class AppBundle extends Bundle
{
    public function __construct(KernelInterface $kernel)
    {
        TransactionWatcherConfigurator::registerConnectionProxies($kernel->getCacheDir());
    }

    public function build(ContainerBuilder $container)
    {
        $container->addCompilerPass(
            new AddTransactionWatcherCompilerPass('oro.doctrine.connection.transaction_watcher')
        );
    }
}