Scaled-Out Processing Hosts
A scaled-out processing host improves performance and provides high availability by isolating orchestration functionality onto two or more separate host computers. This isolation lets you add multiple computers to a processing host for redundancy. A processing host in Microsoft BizTalk Server runs one or more host instances that coordinate various business processes and creates an instance of programmatic objects for orchestrations.
The following figure shows a BizTalk Server deployment that provides high availability for the processing host by having two computers that are running instances of the processing host. Note that in this figure the receiving and sending hosts are not highly available.
In this configuration, the work for processing orchestrations is load balanced between two BizTalk Server computers that have instances of the processing host and run independently of each other. If one computer encounters errors or fails, BizTalk Server automatically uses the host instance on the other computer to process remaining orchestrations.
Maintaining Orchestration State
BizTalk Server maintains orchestration state centrally in Microsoft SQL Server, and not locally on each BizTalk Server computer. By persisting the state in the MessageBox database, BizTalk Server overcomes the limitation of relying on a single processing host instance to process the orchestration, and lets any processing host instance process the orchestration. If an error occurs while BizTalk Server processes an orchestration, another instance of the same processing host can complete the orchestration from the last persisted state.
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