Azure Service Bus is composed of scale units. A scale unit is defined as a deployment unit, consisting of different components that are required to run the service. Each Service Bus namespace (a container of different services provided by Service Bus) is mapped to the scale unit, and each scale unit consists of the following components:

• Gateway nodes: Gateway nodes are responsible for authenticating service requests.
• Messaging broker nodes: These forward messages to relevant entities after service authentication.
• Gateway store: The gateway store holds the data relevant to the entities, such as a topic subscription.
• Messaging store: There can be multiple messaging stores within a Service Bus namespace and scale unit. Each messaging store holds messages relevant to different queues, topics, and subscriptions. Except in the case of Service Bus Premium Messaging, messaging stores are implemented on top of SQL database instances.

The overall messaging process can be described with the following diagram taken from Microsoft's website:

In the next section, we will discuss more concepts related to Azure Service Bus, such as brokered messaging, along with message headers and properties. These are key components of implementing messaging frameworks with Azure Service Bus.