In SQL Server Transport each queue is represented as table inside a database. Depending on the endpoint configuration, each endpoint might use multiple queues/tables e.g. for callbacks.
Structure
The queue table consists of the following columns
ID
The Id is a Guid/uniqueidentifier generated by the sending code. It is not used by SQL Server transport itself.
CorrelationId
The CorrelationId column contains the value of NServiceBus. header. This value is kept in a separate column to maintain wire-level compatibility with NServiceBus.SqlServer transport Version 1.
ReplyToAddress
The ReplyToAddress column contains the value of NServiceBus. header. This value is kept in a separate column to he maintain wire-level compatibility with NServiceBus.SqlServer transport Version 1.
Recoverable
The Recoverable column should always contain the value 1 to ensure wire-level compatibility with NServiceBus.SqlServer transport Version 1.
Backwards Compatibility
The CorrelationId, ReplyToAddress and Recoverable columns are required for backwards compatibility with version 1 of the NServiceBus.SqlServer transport.
When receiving messages sent by endpoints that use later versions, the values of correlation ID and reply-to address should be read from the headers (NServiceBus. and NServiceBus.) instead. The value Recoverable can be ignored as it is always true/1.
When sending messages to endpoints that use later versions, the values of correlation ID and reply-to address columns could be set to NULL and the actual values are provided in the headers (NServiceBus. and NServiceBus.). The value Recoverable should always be true/1.
Expires
The Expires column contains the optional date and time when the message is going to expire. An expired message is dropped by the transport. Depending on version, expired messages might be actively purged from the queue. For details see discarding expired messages.
There is a non-clustered index on the [Expires] column. This index speeds up the purging of expired messages from the queue table. If the SQL Server transport discovers that a required index is missing, it logs an appropriate warning. The following SQL statement can be used to create the missing index:
create nonclustered index [Index_Expires]
on [schema].[queuename]
(
[Expires] asc
)
include
(
[Id],
[RowVersion]
)
Headers
The Headers column contains a JSON representation of message headers.
Body
The Body column contains the serialized message body.
BodyString
The BodyString column contains the message body formatted in a human-readable format. It must be explicitly enabled using transport configuration options:
var transportConfig = endpointConfiguration.UseTransport<SqlServerTransport>();
transportConfig.CreateMessageBodyComputedColumn();
RowVersion
The RowVersion column is used to define the FIFO order of the queue. It is auto-incremented by SQL Server (identity(1,1)). The receive message T-SQL query returns a message with the lowest value of RowVersion that is not locked by any other concurrent receive operation.
The clustered index of the queue table is based on the RowVersion column to ensure the new messages are always added at the end of the table.
Behavior
The following section describes the runtime behavior of SQL Server transport when sending and receiving messages.
Sending
Messages are sent by executing an insert command against the queue table.
Receiving
Messages are received by executing a delete command against the queue table. The delete is limited to a row with the lowest RowVersion not locked by other concurrent delete. This ensures that multiple threads within an endpoint instance and multiple instances of the same scaled-out endpoint can operate at full speed without conflicts.