In hybrid integration scenarios, it is common that we need to send messages to BizTalk Server through Azure Service Bus queues or relays. However, the Service Bus has a restriction on the message size depending on the licensing tier. For example, if the Service Bus is of the standard tier, then the maximum size a message to the Service Bus can be is 256 kilobytes. If the Service Bus is on the premium plan, then the maximum size of the message is 1 MB. However, the premium plan costs a lot compared to the standard edition.

In order to resolve this issue, we can make use of Azure APIM to route messages depending on their size.

Azure APIM routes all messages that are larger than 256 kilobytes to a Logic App that stores the message in to a blob location, puts the blob location information into a message, and places it into a Service Bus queue from which BizTalk picks the messages. When the message arrives at BizTalk, it knows how to access the message from the blob location.

If the message size is less than 256 kilobytes, Azure APIM routes it to an Azure Function that puts the message into the Service Bus queue that BizTalk picks the messages from:

With an inbound policy, we can make use of control flow, rewrite-uri, and set-backend-service policies to achieve the scenario just described: