Concurrency handling

Component: SQL Server Transport
NuGet Package NServiceBus.SqlServer (2.x)
Target NServiceBus Version: 5.x

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.

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