Azure Service Bus ordered processing of message

Question

If for a Azure Service Bus topic, there is single subscription with some filter. There is a microservice A which has created a SubscriptionClient for the Subscription with concurrency of 1 for reading messages. Also if there are 2 such replicas of this service A, and say, there are 3 messages in an unpartitioned service bus topic inserted to topic at t1, t2 and t3 time.

t1 < t2 < t3

1. Is there a possibility that t2 message can get delivered by service bus to Replica-2 before t1 gets delivered to Replica-1 ?

2. If not, what is scaling strategy for service bus topics while processing subscriptions and adding replicas of consuming microservice.

Note: Compared to kafka, it ensures that a message for 1 partition is delivered to only one replica and to one thread that is listening to that partition and thus ordered processing of message is guaranteed. But not sure w.r.t service bus topic like Azure Service bus, if multiple replicas are listening to same subscription with different subscriptionClients, can they receive/process out-of-order messages ?

Answer 1

If you want to enable ordered message processing with Azure Service Bus, you have to use Sessions.

You can use a message's SessionId as an equivalent of the partitionId you may use in Kafka. This way, you can still scale your consumers, limited by the number of distinct SessionId values at any given time.

Message sessions. Implement workflows that require message ordering or message deferral.

Sessions provide concurrent de-multiplexing of interleaved message streams while preserving and guaranteeing ordered delivery.

When multiple concurrent receivers pull from the queue, the messages belonging to a particular session are dispatched to the specific receiver that currently holds the lock for that session. With that operation, an interleaved message stream in one queue or subscription is cleanly de-multiplexed to different receivers and those receivers can also live on different client machines, since the lock management happens service-side, inside Service Bus.

Service Bus Partitions spread out the load across multiple nodes, and don't provide any ordering guarantees.

Partitioning means that the overall throughput of a partitioned entity is no longer limited by the performance of a single message broker or messaging store. In addition, a temporary outage of a messaging store does not render a partitioned queue or topic unavailable.