BizTalk Server 2006 provides great flexibility in addressing high availability because you can strategically dedicate logical hosts to run specific areas of functionality such as receiving and sending messages or processing orchestrations.

A BizTalk host is a logical container within a BizTalk Server group that can house BizTalk Server items such as adapter handlers, receive locations (including pipelines), and orchestrations. You typically group items that have similar scale properties into a particular host.

After you create a host (a logical container), you can add physical BizTalk Server computers (host instances) to the host. A host instance runs as an NT Service on the designated BizTalk Server. For each host, you can have only one instance of a particular BizTalk Server computer. However, you can have instances of a particular host on one or more computers, and you can have instances of different hosts on a particular computer.

Items that are contained in BizTalk hosts can perform the following functions:

• Receiving. These items do the initial processing of messages after they are picked up in a receive location. When a host contains a receiving item, such as a receive location or pipeline, it acts as a security boundary, and the message decoding and decrypting occurs in a pipeline within the host.
  
  
• Sending. These items do the final processing of messages before they are sent out to the send port. When a host contains a sending item, such as a send port or pipeline, the host acts as a security boundary, and the message signing and encryption occurs in a pipeline within the host.
  
  
• Processing. These items process messages based on the instructions in an orchestration.
  
  

One BizTalk Host can contain items that receive, send, and process messages. We recommend that you create different hosts for each function to create security boundaries and for easier management and scalability. In particular, we recommend that you use different hosts for processing and for receive/send operations, and that you separate trusted and non-trusted items.

For example, if you receive one message, run an orchestration, and send ten messages, you want to separate the receive and send functionality into two separate hosts because the send items will have ten times more traffic than the receive items. If you receive one message, run an orchestration, and send one message, you can think of these items as a unit of work and group them into one single host. Alternatively, you can separate them into three different hosts to increase performance and flexibility, although this also increases the management cost.

BizTalk hosts are one of two types, In-process or Isolated. In-process hosts run inside of the BizTalk Server runtime process and Isolated hosts do not run in the BizTalk Server runtime process. The following table lists the items that each of these host types may contain.

For more information about hosts and host instances, see "Managing BizTalk hosts and Host Instances" in BizTalk Server 2006 Help Managing BizTalk Hosts and Host Instances.

To provide high availability for BizTalk hosts, you must have two or more host instances for each host in your environment (on two or more computers). By having more than one host instance for each host you make sure that if one host instance becomes unavailable, the other computers that are running instances of the same host can resume the functions of the problematic or failed host instance, and that the overall system can continue performing with minimal disruption.