Each endpoint running SQL Server transport spins up a fixed number of threads (controlled by
MaximumConcurrencyLevel property of
TransportConfig section) for each input queue. Each thread runs in a loop, polling the database for messages awaiting processing.
By default, there are 5 input queues created for every endpoint (apart from the main one, there are two for handling timeouts, one for the retries and another one for callbacks). As a consequence, if
MaximumConcurrencyLevel is set to 10, there are 41 threads running and constantly polling the database. The callback queue has a separate concurrency settings which default to 1 polling thread.