PostgreSQL Transport Design

Each queue in PostgreSQL Transport is implemented as table inside a database. Depending on the endpoint configuration, each endpoint could use multiple queues, and therefore tables, e.g. for callbacks.

Structure

The queue table consists of the following columns

ID

The Id is a uuid generated by the sending code. It is not used by the transport itself.

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.

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 PostgreSqlTransport("connectionString")
{
    CreateMessageBodyComputedColumn = true
};

Seq

The Seq column is used to define the FIFO order of the queue. It is auto-incremented by PostgreSQL(serial). The receive message SQL query returns a message with the lowest value of Seq that is not locked by any other concurrent receive operation.

Behavior

The following section describes the runtime behavior of PostgreSQL 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 Seq not locked by any 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.

PostgreSQL transport operates in two modes: peek and receive. It starts in the peek mode and checks via max(seq) - min(seq) 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.

Under certain conditions, the initial estimate of the 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 will fail (i.e. delete will return no results). When this happens, the transport immediately switches back to the peek mode.

The default peek interval, if no peek has yet been run or the previous peek returned 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 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 PostgreSqlTransport("connectionString")
{
    QueuePeeker =
    {
        Delay = TimeSpan.FromSeconds(5)
    }
};

Peek batch size configuration

Use the following code:

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

Read more information about tuning endpoint message processing.