Does Pulsar partitioned topics support global ordering when consuming from different exclusive subscriptions?

Question

In the Apache Pulsar documentation we have:

Given we have a producer that is producing to only one partitioned topic and two exclusive subscriptions (an exclusive subscription has guaranteed ordering in the topic) for such topic! Will messages be consumed in order in each subscription?

Answer 1

It depends on your routing mode.

If you use SinglePartition mode and don't provide a key in your messages, all messages from your single producer will go to the same partition and will be in order. But that's probably not what you want since you lose the advantage of using a partitioned topic.

If you use SinglePartition or RoundRobinPartition (default) mode and specify a key in your messages, all the messages with the same key will be placed in the same partition and be in order.

Otherwise, there are no ordering guarantees across partitions. Messages are in order only inside a given partition. From the doc : "A partitioned topic is actually implemented as N internal topics, where N is the number of partitions". So a subscription on a partitioned topic is the same as a multi-topic subscription. This post has some tests for multi-topic subscriptions https://jack-vanlightly.com/blog/2019/9/4/a-look-at-multi-topic-subscriptions-with-apache-pulsar that shows that you can't rely on global ordering on partitioned/multi topics.

Your two subscriptions are independent and will receive all messages with the ordering guarantees described above. Note that if per-key ordering would work for your use case, you might want to use a Key_Shared subscription