SQL Server Service Broker allows for setting up two types of activation, Internal Activation or External Activation. To handle increased message traffic, in internal activation you specify a stored procedure (multiple instances might be created depending on your setting) to be called and this way you scale up your distributed application, whereas in case of external activation Service Broker sends notification (QUEUE_ACTIVATION event) to an external application/program outside SQL Server to read the message from the queue and process it. This way you actually scale out your distributed application. External activation allows putting heavy weight processing logic outside SQL Server in a separate process than SQL Server which gives better performance and scalability or might run under different credential than the SQL Server service account.
In my last article I talked about setting up internal activation, writing a stored procedure which will be called upon on activation. In this article, I will be talking about external activation in detail. For more details click here.