BizTalk 2009 singleton orchestration receives messages in the wrong order
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I have a somewhat unusual problem - I have two singleton orchestrations, let's call them Singleton-A and Singleton-B.

Singleton-A assembles a message of type Message-A and sends it to the message box via a direct bound orchestration port. Because it's a singleton it must be running on only one host instance. So each Message-A gets sent to the messagebox in the correct order 1,2,3,4... etc. I can confirm this is true by using HAT. 

Singleton-B has a subscription to Message-A. Singleton-B has a logical receive port through which it receives the messages from Singleton-A. This receive port has ordered delivery set to true. Singleton-B of course is also single threaded. But when the messages sent by Singleton-A are consumed by Singleton-B they are consumed in the wrong order 1,2,4,3... etc. Again looking in HAT I can see this is true.

The order in which Singleton-B processes the messages seems completely non-deterministic. What is happening here? Is the subscription evaluation and/or host-queue en-queueing process multithreaded? Is it fair to assume that BizTalk will not support ordered delivery between singleton orchestrations?