Using advanced queueing v9.0.3.1

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EDB Postgres Advanced Server advanced queueing provides message queueing and message processing for the EDB Postgres Advanced Server database. User-defined messages are stored in a queue. A collection of queues is stored in a queue table. Create a queue table before creating a queue that depends on it.

On the server side, procedures in the DBMS_AQADM package create and manage message queues and queue tables. Use the DBMS_AQ package to add messages to or remove messages from a queue or register or unregister a PL/SQL callback procedure. For more information about DBMS_AQ and DBMS_AQADM, see DBMS_AQ.

On the client side, the application uses the EDB.NET driver to enqueue and dequeue messages.

Enqueueing or dequeueing a message

For more information about using EDB Postgres Advanced Server's advanced queueing functionality, see Built-in packages.

Server-side setup

To use advanced queueing functionality on your .NET application, you must first create a user-defined type, queue table, and queue, and then start the queue on the database server. Invoke EDB-PSQL and connect to the EDB Postgres Advanced Server host database. Use the following SPL commands at the command line.

Creating a user-defined type

To specify a RAW data type, create a user-defined type. This example shows creating a user-defined type named myxml:

CREATE TYPE myxml AS (value XML);

Creating the queue table

A queue table can hold multiple queues with the same payload type. This example shows creating a table named MSG_QUEUE_TABLE:

EXEC DBMS_AQADM.CREATE_QUEUE_TABLE
      (queue_table => 'MSG_QUEUE_TABLE',
       queue_payload_type => 'myxml',
       comment => 'Message queue table');
END;

Creating the queue

This example shows creating a queue named MSG_QUEUE in the table MSG_QUEUE_TABLE:

BEGIN
DBMS_AQADM.CREATE_QUEUE ( queue_name => 'MSG_QUEUE', queue_table => 'MSG_QUEUE_TABLE', comment => 'This queue contains pending messages.');
END;

Starting the queue

Once the queue is created, invoke the following SPL code at the command line to start a queue in the EDB database:

BEGIN
DBMS_AQADM.START_QUEUE
(queue_name => 'MSG_QUEUE');
END;

Client-side example

Once you've created a user-defined type, the queue table, and the queue, start the queue. Then, you can enqueue or dequeue a message using EDB .Net drivers.

Enqueue a message

To enqueue a message on your .NET application, you must:

Import the EnterpriseDB.EDBClient namespace.
Pass the name of the queue and create the instance of the EDBAQQueue.
Create an EDBAQMessage message set its payload.
Call the EDBAQQueue.Enqueue method.
The EDBAQMessage.MessageID property will be populated with a string uniquely identifying your message.

The following code shows how to use EDBAQQueue.Enqueue method. A custom message payload is created and then enqueued.

Note

As an example, we are using the ambient Connection via EDBAQQueue.Connection to begin a transaction, so that if anything goes wrong the queue won't be polluted.

.NET
.NET Framework

using EnterpriseDB.EDBClient;

namespace EnterpriseDB;

internal static class Program
{
    // Sample message payload
    class MyXML
    {
        public string Value { get; set; }
    }

    public static async Task Main(string[] args)
    {
        // not for production, move connection string to app settings
        string connectionString = "Server=127.0.0.1;Port=5444;User Id=enterprisedb;Password=edb;Database=edb";

        // Note registration of MyXml type
        var dataSourceBuilder = new EDBDataSourceBuilder(connectionString);
        dataSourceBuilder
            .MapComposite<MyXML>("enterprisedb.myxml");

        await using var dataSource = dataSourceBuilder.Build();
        await using var connection = await dataSource.OpenConnectionAsync();
        using var queue = CreateQueue("MSG_QUEUE", connection);

        // Enqueue 5 messages
        int messagesToSend = 5;
        for (int i = 0; i < messagesToSend; i++)
        {
            var payload = new MyXML()
            {
                Value = $"(<Message><MessageText>Test message: {i}</MessageText></Message>)"
            };

            if (TryEnqueueMessage(queue, payload, out var _))
            {
                // MessageId is populated with a unique identifier
                Console.WriteLine($"Message {i} ({message.MessageId}) enqueued");
            }
            else
            {
                Console.WriteLine($"Message {i} enqueue failed");
            }
        }

    }

    // Creates and returns a queue ready for use in our sample
    private static EDBAQQueue CreateQueue(string queueName, EDBConnection connection)
    {
        var queue = new EDBAQQueue(queueName, connection);
        queue.MessageType = EDBAQMessageType.Udt;
        queue.EnqueueOptions.Visibility = EDBAQVisibility.ON_COMMIT;
        queue.UdtTypeName = "myxml";

        return queue;
    }

    // Enqueues the payload
    // If the enqueuing was successfull, message variable receives the queue message and the function returns true
    // otherwise message is null and the function returns false
    private static bool TryEnqueueMessage<T>(EDBAQQueue queue, T payload, out EDBAQMessage message)
    {
        using EDBTransaction transaction = queue.Connection.BeginTransaction();

        try
        {
            message = new EDBAQMessage() { Payload = payload };
            queue.Enqueue(message);
            transaction.Commit();

            return true;
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error while enqueing message: {ex.Message}");
            transaction?.Rollback();

            message = null;
            return false;
        }
    }
}

using System;
using System.Threading.Tasks;
using EnterpriseDB.EDBClient;

namespace EnterpriseDB
{
    internal static class Program
    {
        // Sample message payload
        class MyXML
        {
            public string Value { get; set; }
        }

        public static async Task Main(string[] args)
        {
            // not for production, move connection string to app settings
            string connectionString = "Server=127.0.0.1;Port=5444;User Id=enterprisedb;Password=edb;Database=edb";

            // Note registration of MyXml type
            var dataSourceBuilder = new EDBDataSourceBuilder(connectionString);
            dataSourceBuilder
                .MapComposite<MyXML>("enterprisedb.myxml");

            using (var dataSource = dataSourceBuilder.Build())
            using (var connection = await dataSource.OpenConnectionAsync())
            using (var queue = CreateQueue("MSG_QUEUE", connection))
            {
                // Enqueue 5 messages
                int messagesToSend = 5;
                for (int i = 0; i < messagesToSend; i++)
                {
                    var payload = new MyXML()
                    {
                        Value = $"(<Message><MessageText>Test message: {i}</MessageText></Message>)"
                    };

                    if (TryEnqueueMessage(queue, payload, out var _))
                    {
                        // MessageId is populated with a unique identifier
                        Console.WriteLine($"Message {i} ({message.MessageId}) enqueued");
                    }
                    else
                    {
                        Console.WriteLine($"Message {i} enqueue failed");
                    }
                }
            }
        }

        // Creates and returns a queue ready for use in our sample
        private static EDBAQQueue CreateQueue(string queueName, EDBConnection connection)
        {
            var queue = new EDBAQQueue(queueName, connection);
            queue.MessageType = EDBAQMessageType.Udt;
            queue.EnqueueOptions.Visibility = EDBAQVisibility.ON_COMMIT;
            queue.UdtTypeName = "myxml";

            return queue;
        }

        // Enqueues the payload
        // If the enqueuing was successfull, message variable receives the queue message and the function returns true
        // otherwise message is null and the function returns false
        private static bool TryEnqueueMessage<T>(EDBAQQueue queue, T payload, out EDBAQMessage message)
        {
            using (EDBTransaction transaction = queue.Connection.BeginTransaction())
            {
                try
                {
                    message = new EDBAQMessage() { Payload = payload };
                    queue.Enqueue(message);
                    transaction.Commit();
        
                    return true;
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Error while enqueing message: {ex.Message}");
                    transaction?.Rollback();
        
                    message = null;
                    return false;
                }
            }
        }
    }
}

Dequeue a message

To dequeue a message on your .NET application, you must:

Import the EnterpriseDB.EDBClient namespace.
Pass the name of the queue and create the instance of the EDBAQQueue.
Call the EDBAQQueue.Dequeue() method.

Note

The following code shows how to use the EDBAQQueue.Dequeue method. A queue is retrieved by its name, and a attempt is made to dequeue a message.

If a PostgresException with SqlState set to P0002 is raised, then the queue is empty or the wait time (set with queue.DequeueOptions.Wait) has expired, and the code gracefully returns a null message.

.NET
.NET Framework

using EnterpriseDB.EDBClient;

namespace EnterpriseDB;
internal static class Program
{
    // Sample message payload
    class MyXML
    {
        public string Value { get; set; }
    }

    public static async Task Main(string[] args)
    {
        // not for production, move connection string to app settings
        string connectionString = "Server=127.0.0.1;Port=5444;User Id=enterprisedb;Password=edb;Database=edb";

        // Note registration of MyXml type
        var dataSourceBuilder = new EDBDataSourceBuilder(connectionString);
        dataSourceBuilder
            .MapComposite<MyXML>("enterprisedb.myxml");

        await using var dataSource = dataSourceBuilder.Build();
        await using var connection = await dataSource.OpenConnectionAsync();
        using var queue = CreateQueue("MSG_QUEUE", connection);

        // Dequeue 5 messages
        int messagesToDequeue = 5;
        for (int i = 0; i < messagesToDequeue; i++)
        {
            if (TryDequeueMessage(queue, out var message))
            {
                Console.WriteLine($"Message {message.MessageId} dequeued");
                        
                if (message?.Payload is MyXML myXML)
                {
                    Console.WriteLine($"MyXML Message received: {myXML.Value}");
                }
                else
                {
                    Console.WriteLine($"Other message received");
                }
            }
            else
            {
                Console.WriteLine($"No message");
            }
        }
    }

    // Creates and returns a queue ready for use in our sample
    private static EDBAQQueue CreateQueue(string queueName, EDBConnection connection)
    {
        var queue = new EDBAQQueue(queueName, connection);
        queue.MessageType = EDBAQMessageType.Udt;
        queue.DequeueOptions.Navigation = EDBAQNavigationMode.FIRST_MESSAGE;
        queue.DequeueOptions.Visibility = EDBAQVisibility.ON_COMMIT;
        queue.DequeueOptions.Wait = 1; // wait for 1 seconds
        queue.UdtTypeName = "myxml";

        return queue;
    }

    // Dequeues a payload
    // If the dequeuing was successfull, message variable receives the queue message and the function returns true
    // otherwise message is null and the function returns false
    private static bool TryDequeueMessage(EDBAQQueue queue, out EDBAQMessage message)
    {
        using EDBTransaction transaction = queue.Connection.BeginTransaction();

        try
        {
            message = queue.Dequeue();
            transaction.Commit();

            return true;
        }
        catch (PostgresException pgException) when (pgException.SqlState == "P0002")
        {
            // Queue empty or time out
            transaction.Commit();

            message = null;
            return false;
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error while dequeuing message: {ex.Message}");
            transaction?.Rollback();

            message = null;
            return false;
        }
    }
}

using System;
using System.Threading.Tasks;
using EnterpriseDB.EDBClient;

namespace EnterpriseDB
{
    internal static class Program
    {
        // Sample message payload
        class MyXML
        {
            public string Value { get; set; }
        }

        public static async Task Main(string[] args)
        {
            // not for production, move connection string to app settings
            string connectionString = "Server=127.0.0.1;Port=5444;User Id=enterprisedb;Password=edb;Database=edb";

            // Note registration of MyXml type
            var dataSourceBuilder = new EDBDataSourceBuilder(connectionString);
            dataSourceBuilder
                .MapComposite<MyXML>("enterprisedb.myxml");

            using (var dataSource = dataSourceBuilder.Build())
            using (var connection = await dataSource.OpenConnectionAsync())
            using (var queue = CreateQueue("MSG_QUEUE", connection))
            {
                // Dequeue 5 messages
                int messagesToDequeue = 5;
                for (int i = 0; i < messagesToDequeue; i++)
                {
                    if (TryDequeueMessage(queue, out var message))
                    {
                        Console.WriteLine($"Message {message.MessageId} dequeued");
                        
                        if (message?.Payload is MyXML myXML)
                        {
                            Console.WriteLine($"MyXML Message received: {myXML.Value}");
                        }
                        else
                        {
                            Console.WriteLine($"Other message received");
                        }
                    }
                    else
                    {
                        Console.WriteLine($"No message");
                    }
                }
            }
        }

        // Creates and returns a queue ready for use in our sample
        private static EDBAQQueue CreateQueue(string queueName, EDBConnection connection)
        {
            var queue = new EDBAQQueue(queueName, connection);
            queue.MessageType = EDBAQMessageType.Udt;
            queue.DequeueOptions.Navigation = EDBAQNavigationMode.FIRST_MESSAGE;
            queue.DequeueOptions.Visibility = EDBAQVisibility.ON_COMMIT;
            queue.DequeueOptions.Wait = 1; // wait for 1 seconds
            queue.UdtTypeName = "myxml";

            return queue;
        }

        // Dequeues a payload
        // If the dequeuing was successfull, message variable receives the queue message and the function returns true
        // otherwise message is null and the function returns false
        private static bool TryDequeueMessage(EDBAQQueue queue, out EDBAQMessage message)
        {
            using (EDBTransaction transaction = queue.Connection.BeginTransaction())
            {
                try
                {
                    message = queue.Dequeue();
                    transaction.Commit();

                    return true;
                }
                catch (PostgresException pgException) when (pgException.SqlState == "P0002")
                {
                    // Queue empty or time out
                    transaction.Commit();

                    message = null;
                    return false;
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Error while dequeuing message: {ex.Message}");
                    transaction?.Rollback();

                    message = null;
                    return false;
                }
            }
        }
    }
}

EDBAQ classes

The following EDBAQ classes are used in this application.

EDBAQDequeueMode

The EDBAQDequeueMode class lists all the dequeuer modes available.

Value	Description
Browse	Reads the message without locking.
Locked	Reads and gets a write lock on the message.
Remove	Deletes the message after reading. This is the default value.
Remove_NoData	Confirms receipt of the message.

EDBAQDequeueOptions

The EDBAQDequeueOptions class lists the options available when dequeuing a message.

Property	Description
ConsumerName	The name of the consumer for which to dequeue the message.
DequeueMode	Set from `EDBAQDequeueMode`. It represents the locking behavior linked with the dequeue option.
Navigation	Set from `EDBAQNavigationMode`. It represents the position of the message to fetch.
Visibility	Set from `EDBAQVisibility`. It represents whether the new message is dequeued as part of the current transaction.
Wait	The wait time for a message as per the search criteria.
Msgid	The message identifier.
Correlation	The correlation identifier.
DeqCondition	The dequeuer condition. It's a Boolean expression.
Transformation	The transformation to apply before dequeuing the message.
DeliveryMode	The delivery mode of the dequeued message.

EDBAQEnqueueOptions

The EDBAQEnqueueOptions class lists the options available when enqueuing a message.

Property	Description
Visibility	Set from `EDBAQVisibility`. It represents whether the new message is enqueued as part of the current transaction.
RelativeMsgid	The relative message identifier.
SequenceDeviation	The sequence when to dequeue the message.
Transformation	The transformation to apply before enqueuing the message.
DeliveryMode	The delivery mode of the enqueued message.

EDBAQMessage

The EDBAQMessage class lists a message to enqueue/dequeue.

Property	Description
Payload	The actual message to queue.
MessageId	The ID of the queued message.

EDBAQMessageProperties

The EDBAQMessageProperties lists the message properties available.

Property	Description
Priority	The priority of the message.
Delay	The duration after which the message is available for dequeuing, in seconds.
Expiration	The duration for which the message is available for dequeuing, in seconds.
Correlation	The correlation identifier.
Attempts	The number of attempts taken to dequeue the message.
RecipientList	The recipients list that overthrows the default queue subscribers.
ExceptionQueue	The name of the queue to move the unprocessed messages to.
EnqueueTime	The time when the message was enqueued.
State	The state of the message while dequeued.
OriginalMsgid	The message identifier in the last queue.
TransactionGroup	The transaction group for the dequeued messages.
DeliveryMode	The delivery mode of the dequeued message.

EDBAQMessageState

The EDBAQMessageState class represents the state of the message during dequeue.

Value	Description
Expired	The message is moved to the exception queue.
Processed	The message is processed and kept.
Ready	The message is ready to be processed.
Waiting	The message is in waiting state. The delay isn't reached.

EDBAQMessageType

The EDBAQMessageType class represents the types for payload.

Value	Description
Raw	The raw message type. Note: Currently, this payload type isn't supported.
UDT	The user-defined type message.
XML	The XML type message. Note: Currently, this payload type isn't supported.

EDBAQNavigationMode

The EDBAQNavigationMode class represents the different types of navigation modes available.

Value	Description
First_Message	Returns the first available message that matches the search terms.
Next_Message	Returns the next available message that matches the search items.
Next_Transaction	Returns the first message of next transaction group.

EDBAQQueue

The EDBAQQueue class represents a SQL statement to execute DMBS_AQ functionality on a PostgreSQL database.

Property	Description
Connection	The connection to use.
Name	The name of the queue.
MessageType	The message type that's enqueued/dequeued from this queue, for example `EDBAQMessageType.Udt`.
UdtTypeName	The user-defined type name of the message type.
EnqueueOptions	The enqueue options to use.
DequeuOptions	The dequeue options to use.
MessageProperties	The message properties to use.

EDBAQVisibility

The EDBAQVisibility class represents the visibility options available.

Value	Description
Immediate	The enqueue/dequeue isn't part of the ongoing transaction.
On_Commit	The enqueue/dequeue is part of the current transaction.

Note

To review the default options for these parameters, see DBMS_AQ.
EDB advanced queueing functionality uses user-defined types for calling enqueue/dequeue operations. Server Compatibility Mode=NoTypeLoading can't be used with advanced queueing because NoTypeLoading doesn't load any user-defined types.

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Using advanced queueing v9.0.3.1

Enqueueing or dequeueing a message

Server-side setup

Creating a user-defined type

Creating the queue table

Creating the queue

Starting the queue

Client-side example

Enqueue a message

Note

Dequeue a message

Note

EDBAQ classes

EDBAQDequeueMode

EDBAQDequeueOptions

EDBAQEnqueueOptions

EDBAQMessage

EDBAQMessageProperties

EDBAQMessageState

EDBAQMessageType

EDBAQNavigationMode

EDBAQQueue

EDBAQVisibility

Note

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