Add support for Pub/Sub message ordering

[meltsufin]

PubSubAdmin.createSubscription() will need to be expanded to allow enabling ordering, and whenever we create a PubsubMessage, we need to be able to set the ordering key for it.
Also, the Pub/Sub Cloud Stream Binder will need to be updated to support the option.
See: https://cloud.google.com/pubsub/docs/ordering
See: https://cloud.google.com/pubsub/docs/publisher#using_ordering_keys

Q: Does it makes sense to expose the keys as partitions in a Kafka-binder-like API?

[meltsufin]

@ttomsu What do you think of focusing on this one after DLQ?

[DerMorb]

The PubSubMessageConverter.toPubSubMessage(Object payload, Map<String, String> headers) method might also need a way to set the ordering key or we need another way to provide the ordering key.
Right now the payload is ending up in the data and the headers in the attributes of a PubSubMessage but the ordering key is a separate field.

Or do we need to write our own message converter somehow like this?

public class OrderMessagePubSubMessageConverter extends JacksonPubSubMessageConverter {

    public OrderMessagePubSubMessageConverter(ObjectMapper objectMapper) {
        super(objectMapper);
    }

    @Override
    public PubsubMessage toPubSubMessage(Object payload, Map<String, String> headers) {
        PubsubMessage pubsubMessage = super.toPubSubMessage(payload, headers);
        return PubsubMessage.newBuilder()
                            .mergeFrom(pubsubMessage)
                            .setOrderingKey(???)
                            .build();
    }
}

[meltsufin]

@DerMorb I think we just need to pick a name for the header by adding it to GcpPubSubHeaders and then extend all implementations of PubSubMessageConverter to marshal that header back and forth between headers and PubsubMessage.orderingKey. It's kind of what you're suggesting, but more general.

[meltsufin]

Now that #85 is in, what's left?

1. In Spring Cloud Stream Binder for Pub/Sub, we should add the ability to automatically create subscriptions that support message ordering. This will involve adding enableMessageOrdering property to PubSubConsumerProperties and updating PubSubChannelProvisioner.createSubscription.
2. In Spring Integration Outbound Channel Adapter AKA PubSubMessageHandler we can add a property to specify a default ordering key.
3. Once (2) is done, we can expose this ability to the Binder by adding the orderingKey property to the PubSubProducerProperties and using it in the PubSubMessageChannelBinder.createProducerMessageHandler.

[vijaysdey88]

Is this feature implemented in latest release?

[meltsufin]

No, only the latest SNAPSHOT release has the change.

[pivotaleventing]

Hello,

Do you have a plan when this will be published?

[elefeint]

Publisher endpoints for message ordering have been available since version 2.0.3