SQL Transport Design

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.CorrelationId header. This value is kept in a separate column to maintain wire-level compatibility with transport Version 2.

ReplyToAddress

The ReplyToAddress column contains the value of NServiceBus.ReplyToAddress header. This value is kept in a separate column to he maintain wire-level compatibility with transport Version 2 and lower.

Recoverable

The Recoverable column should always contain the value 1 to ensure wire-level compatibility with transport Version 2 and lower.

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:

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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:

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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.