This page refers to the legacy Azure Service Bus transport that uses the WindowsAzure.ServiceBus NuGet package. This package is currently on extended support until May 2022, after which it will be out of support according to the support policy. All users should migrate to the Azure Service Bus transport transport.

Messaging topology is a specific arrangement of the messaging entities, such as queues, topics, subscriptions, and rules.

Azure Service Bus transport operates on a topology created on the broker. Topology handles exchanging messages between endpoints, by creating and configuring Azure Service Bus entities.

Versions 7 and above

The following topologies are available:

  1. EndpointOrientedTopology
  2. ForwardingTopology

The EndpointOrientedTopology is backward compatible with the Azure Service Bus transport Version 6 and below.

The ForwardingTopology was introduced to take advantage of the broker nature of the Azure Service Bus and to leverage its native capabilities. It is the recommended option for new projects.

Both topologies create a single input queue per endpoint and implement Publish-Subscribe mechanism. However, there is a significant difference in the way that mechanism is implemented.

No default topology is set by the Azure Service Bus transport. Topology has to be explicitly configured using configuration API.

Endpoint-oriented topology

In the EndpointOrientedTopology each publishing endpoint creates a topic called <publishing_endpoint_name>.events. The subscribing endpoints subscribe to the topic, by creating a subscription for a particular event type called <subscriber_endpoint_name>.<event_type_name>. Note that each subscription has a single rule, used to filter a specific event type.

When a publisher raises an event, it is published to the publisher topic. Then the subscription entities filter events using the associated rules. Finally, the subscriber endpoint pulls events for processing.

The example below demonstrates a publisher called Publisher and a subscriber called Subscriber. Publisher can raise two events, EventA and EventB. The Subscriber subscribes to both events and two subscription entities are created, one per event type. Each subscription entity filters out events using a rule associated with the subscription.

The EndpointOrientedTopology topology has several drawbacks:

  1. In order to subscribe to an event, the subscriber must know the publishing endpoint's name, causing coupling between publisher and subscriber. Refer to the Publisher names configuration article for more details.
  2. Multiple subscription entities per subscriber cause polymorphic events to be delivered multiple times to the subscribing endpoint. A workaround has to be implemented in order to handle polymorphic events correctly.
  3. When a single subscriber is offline for an extended period of time and events are not consumed, it can cause event overflow. Since a single topic per publisher is used for all the subscribers, when event overflow is happening it will affect all subscribers and not just the one that is offline.


Forwarding topology

The ForwardingTopology is designed to take advantage of several native broker features offered by the Azure Service Bus. Unlike EndpointOrientedTopology, it doesn't work with a single topic per publisher. All publishers use a single topic bundle.

Subscriptions are created under topic bundle with one subscription entity per subscribing endpoint. Each subscription contains multiple rules; there's one rule per event type that the subscribing endpoint is interested in. This enables a complete decoupling between publishers and subscribers. All messages received by subscription are forwarded to the input queue of the subscriber.

This topology solves the polymorphic events and the event overflow problems that the EndpointOrientedTopology has.


Quotas and limitations

The ForwardingTopology supports up to 2,000 endpoints with up to 2,000 events in total. Since multiple rules per subscriptions are used, topics cannot be partitioned.

Topologies comparison

Decoupled Publishers / Subscribersnoyes
Polymorphic events supportnoyes
Event overflow protectionnoyes
Subscriber auto-scaling based on queue sizenoyes
Reduced number of connections to the brokernoyes

Versions 6 and below

The Azure Service Bus transport has always supported a single default topology out-of-the-box. It is equivalent to the EndpointOrientedTopology introduced in Version 7.

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