The Message Box Layer
The heart of the publish/subscribe engine in Microsoft BizTalk Server is the MessageBox database. The MessageBox is made up of two components: one or more Microsoft SQL Server databases and the Messaging Agent. The SQL Server database provides the persistence store for many things including messages, message parts, message properties, subscriptions, orchestration state, tracking data, host queues for routing, and others. The BizTalk Server group may have one or more MessageBox databases into which it publishes messages and from which subscribers to those messages extract messages.
The database provides some of the logic related to routing messages and fulfilling subscriptions. The Message Agent, however, is the component that encapsulates and abstracts the database component and is the interface used by BizTalk Server to interact with the MessageBox. The Message Agent is a Component Object Model (COM) component that provides interfaces for publishing messages, subscribing to messages, retrieving messages, and so on. This interface is the only mechanism used by other BizTalk Server components, including the adapter framework and orchestrations, to interact with the MessageBox.
Using the tests mapped to this layer you can monitor the health of the BizTalk server MessageBox and the efficiency of the SQL Server agent jobs.
Figure 1 : The tests mapped to the Message Box layer