Not yet, but reading the docs. Has 'once only delivery' which is good. Still trying to find if it does FIFO (i think its hard to achieve this in a distributed system).
Just like RabbitMQ, Apache Kafka and many other distributed systems, writes go through an elected leader, who is able to ensure ordering guarantees.
Specifically with Apache Pulsar, each topic has an owner broker (leader) who accepts writes and serves readers.
It should be noted that Apache Pulsar supports shared subscriptions which allow two or more consumers to compete for the same messages, like having two consumers on a RabbitMQ queue. Here FIFO order cannot be guaranteed for all kinds of reasons.
Basically, they have the same ordering guarantee as Kafka: FIFO is guaranteed for messages from the same producer, within the same partition. If you need producer-level FIFO, then you can only use 1 partition.
There's no ordering guarantee for messages coming from different producer.
In a distributed system, an ordering guarantee by producer (and optionally partition "key" which in the end is like an extension of the topic key) is pretty much all you need and all you can get.
If you have two producers then how would one decide which one sent the message first? Go by some timestamp? Clocks are unreliable for these purposes so it comes down to consesus. And letting the queue decide which one came first is equivalent. Once a message got acknowledged by the queue, ordering cannot change anymore.