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 NServiceBus.SqlServer transport Version 1.

ReplyToAddress

The ReplyToAddress column contains the value of the NServiceBus.ReplyToAddress header. This value is kept in a separate column to 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.CorrelationId and NServiceBus.ReplyToAddress) 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 with the actual values provided in the headers (NServiceBus.CorrelationId and NServiceBus.ReplyToAddress). The value Recoverable should always be true/1.

Expires

The Expires column contains the optional date and time when the message will 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 transport = new SqlServerTransport("connectionString")
{
    CreateMessageBodyComputedColumn = true
};

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 that 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.

SQL Server transport operates in two modes: peek and receive. It starts in the peek mode and checks, using a select count query the number of pending messages. If the number is greater then zero, it switches to the receive mode and starts spawning receive tasks that use the delete command to receive messages.

The maximum number of concurrent receive tasks never exceeds the value set by LimitMessageProcessingConcurrencyTo (the number of tasks does not translate to the number of running threads which is controlled by the TPL scheduling mechanisms).

When all tasks are done the transport switches back to the peek mode.

In certain conditions, the initial estimate of a number of pending messages might be wrong e.g. when there is more than one instance of a scaled-out endpoint consuming messages from the same queue. In this case, one of the received tasks is going to fail (delete returns no results). When this happens, the transport immediately switches back to the peek mode.

The default peek interval, if there is has been no messages in the queue, is 1 second. The recommended range for this setting is between 100 milliseconds to 10 seconds. If a value higher than the maximum recommended settings is used, a warning message will be logged. While a value less than 100 milliseconds will put too much unnecessary stress on the database, a value larger than 10 seconds should also be used with caution as it may result in messages backing up in the queue.

Queue peek settings

Peek delay configuration

Use the following code:

var transport = new SqlServerTransport("connectionString")
{
    QueuePeeker =
    {
        Delay = TimeSpan.FromSeconds(5)
    }
};

Peek batch size configuration

Use the following code:

var transport = new SqlServerTransport("connectionString")
{
    QueuePeeker =
    {
        MaxRecordsToPeek = 50
    }
};

Read more information about tuning endpoint message processing.