Service Broker Part 1: Service Broker Basics

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Part 1: Service Broker Basics

Microsoft first introduced Service Broker as part of the relational engine of SQL Server 2005. Service Broker is an asynchronous messaging framework with which you can implement scalable, distributed, high available, reliable, and secure database applications based on SQL Server. Here are some examples:

  • With asynchronous database triggers you are able to defer long running tasks inside a trigger to a later time. This will improve the performance of your database applications.
  • With distributed message-based database applications you are able to decouple related tasks within your database applications and process these tasks independently from each other at different times. This will help you with the scale out of your database applications.

As we start looking at the details of Service Broker, I want to give you a quick overview of the Service Broker objects with which you will be interacting, when implementing a basic Service Broker application.

Basic Terminology

Much of the action in Service Broker revolves around sending messages between services. A Service Broker service is an endpoint to which messages can be sent, and from which they can be retrieved. Each endpoint can implement some functionality from your database application that is executed asynchronously in the background. Messages are always exchanged between at least two services. A Service Broker service has the following characteristics:

  • Services are always defined within the scope of a database
  • The service contains application logic (code) and the associated messages (state)
  • A contract describes in which directions messages can be exchanged with the service
  • Messages are exchanged through reliable sessions that are called conversations
  • Each service is associated with a queue that stores messages sent to the service

Conversations are always started between two Service Broker services. The service which starts the conversation is called the initiator service, and the other service is called the target service. As you can see from this description, a Service Broker service is a native database object that has also links to other Service Broker objects.

Service Broker Objects

A Service Broker service consists of at least the following four objects that are also created as native database objects:

  • Message types
  • Contracts
  • Queues
  • Service program

Let’s have a more detailed look at these additional objects.

Message Types

The message type describes the content of the messages that are exchanged between Service Broker services. Each message type is associated with a unique name. When you are sending messages across databases, you have to define the identical message types in each participating database. Each message type can be associated with a validation method that Service Broker applies to messages of that type. Service Broker currently supports the following four validation options:

  • Well-formed XML
  • XML validated against a registered XML schema collection
  • No validation (e.g. for binary data)
  • Empty (the message body must be empty)

Service Broker performs the validation as soon as a message is received at a service. If an error occurs, an error message is returned back to the sender.

Contracts

After you have defined the necessary message types, you must group them together into contracts. A contract defines in which direction message types are exchanged between Service Broker services. A contract is therefore an agreement that states which message types a service can send and which message types a service can accept. Service Broker supports the following three sending directions:

  • SEND BY INITIATOR: The message is sent by the initiator service
  • SEND BY TARGET: The message is sent by the target service
  • SEND BY ANY: The message can be send by both the initiator and the target services

Contracts also have to be defined in all participating databases, when you are sending messages across different databases.

Queues

After you have defined your message types and your contracts, you have to create the queues for storing incoming messages in Service Broker. Queues must be defined for both the initiator and for the target service, even when you are implementing one-way messaging with Service Broker. The reason for this is that Service Broker also sends some messages internally (e.g. when an error occurred on the target service); therefore you always also need a queue for the initiator service. Each received message is represented as row in a queue. A queue is internally implemented as a hidden table which uses a special locking mechanism. Each record in that queue is associated with different columns, which contains things like the message body itself, the associated message type, and other information such as the receipt date.

Service Programs

The last object, a service program, is a stored procedure that gets executed when new messages arrive. With Service Broker these stored procedures can be also started automatically as soon as new messages arrive on a queue. This feature is referred to as activation. Service Broker distinguishes between internal and external activation. Internal activation means that a stored procedure deployed within the database gets executed. External activation means that an external program (maybe a .NET application) gets started as soon as a new message arrives on a queue. By now you have a basic understanding about the various objects that are involved in a basic Service Broker application. The following figure shows these objects and how they relate to each other.

Figure 1: The various Service Broker objects and their relationship

Your First Service Broker Application

After laying out the foundation of Service Broker, you will be now ready to implement your first Service Broker application. Your first Service Broker application is the famous Hello World application. The application will simply send a request message that contains your name in the message body from the initiator service to the target service. The target service receives this message, processes it, and finally sends back a response message with the famous “Hello World” and your provided name. Easy isn’t it? Let’s get started!

As discussed, almost everything in Service Broker is scoped to a database. Therefore you need to create a new database, as in Listing 1, which will host all the Service Broker objects you will be creating in the subsequent steps.

USE master
GO
IF EXISTS ( SELECT  *
            FROM    sys.databases
            WHERE   name = 'HelloWorldServiceBroker' ) 
    BEGIN
        PRINT 'Dropping database ''HelloWorldServiceBroker''' ;
        DROP DATABASE HelloWorldServiceBroker ;
    END
GO
CREATE DATABASE HelloWorldServiceBroker
GO
USE HelloWorldServiceBroker
GO

Listing 1: Creating a new database for the first Service Broker application

After creating your database for Service Broker, the next step is to define the necessary message types and the contract on which your Service Broker application is based. As I have said earlier, your first Service Broker application will consist of a request message and a response message. The direction of those two messages will be defined through one contract that your Service Broker application uses. Service Broker offers you the CREATE MESSAGE TYPE and the CREATE CONTRACT T-SQL statement for creating message types and contracts. Listing 2 shows the creation of these Message Type and Contract objects.

CREATE MESSAGE TYPE
[http://ssb.csharp.at/RequestMessage]
VALIDATION = WELL_FORMED_XML
GO
CREATE MESSAGE TYPE
[http://ssb.csharp.at/ResponseMessage]
VALIDATION = WELL_FORMED_XML
GO
CREATE CONTRACT [http://ssb.csharp.at/HelloWorldContract]
(
[http://ssb.csharp.at/RequestMessage] SENT BY INITIATOR,
[http://ssb.csharp.at/ResponseMessage] SENT BY TARGET
)
GO

Listing 2: Creating the message types and the contract

As you can see in Listing 2, each message type and each contract is given a unique name by adding a unique string to them. As a best practice you can use the URL of your website or company to make it . easy to distinguish your message types and contracts from others and avoid name collision when moving your Service Broker application to a distributed environment. However, you can use any valid SQL Server object name to name your message types and contracts.

After creating your message types and contracts, the final step is to create your queues and your actual Service Broker services that are interacting with each other, as shown in Listing 3. For these tasks Service Broker offers you the CREATE QUEUE and the CREATE SERVICE T-SQL statements.

CREATE QUEUE InitiatorQueue
WITH STATUS = ON
GO
CREATE QUEUE TargetQueue
WITH STATUS = ON
GO
CREATE SERVICE InitiatorService
ON QUEUE InitiatorQueue 
(
[http://ssb.csharp.at/HelloWorldContract]
)
GO
CREATE SERVICE TargetService
ON QUEUE TargetQueue
(
[http://ssb.csharp.at/HelloWorldContract]
)
GO

Listing 3: Creating the services and their associated queues

As you can see, when creating a service, you have to reference previously created queue and contracts, that that Service Broker service implements. In your case, the one and only defined contract – the http://ssb.csharp.at/HelloWorldContract contract is implemented by the InitiatorService and the TargetService service.

After creating all of the necessary Service Broker objects you are finally able to start a conversation between both services and exchange messages. Service Broker offers you the BEGIN DIALOG CONVERSATION T-SQL statement for starting a new conversation between two Service Broker services. The conversation itself is identified through a UNIQUEIDENTIFIER that is returned as a result back to you. You need to store this identifier somewhere (such as in a local variable), because you will need it afterwards when you want to send messages across the open conversation with the SEND ON CONVERSATION T-SQL statement. The following listing shows the necessary steps to open a conversation and send a message through it.

BEGIN TRANSACTION ;
DECLARE @ch UNIQUEIDENTIFIER
DECLARE @msg NVARCHAR(MAX) ;
BEGIN DIALOG CONVERSATION @ch
      FROM SERVICE [InitiatorService]
      TO SERVICE 'TargetService'
      ON CONTRACT [http://ssb.csharp.at/HelloWorldContract]
      WITH ENCRYPTION = OFF ;
SET @msg = '<HelloWorldRequest>
               Klaus Aschenbrenner
      </HelloWorldRequest>' ;
SEND ON CONVERSATION @ch 
   MESSAGE TYPE [http://ssb.csharp.at/RequestMessage] 
   (@msg) ;
COMMIT TRANSACTION
GO

Listing 4: Opening a new Service Broker conversation and sending a message

As you can see in Listing 4, you need to specify the initiator service, the target service, and also the contract when you are establishing a new conversation between two Service Broker services. Optionally you can also define if encryption should be used or not. For simplification we are currently not encrypting anything, because we are just sending messages within a database. The most important concept to understand here is the fact that the TO SERVICE is specified as a string literal. In our simple Service Broker application this seems a little bit confusing, but just think a second and assume that the target service is deployed somewhere on a different physical SQL Server instance. In that case you might not be able to reference this remote deployed Service Broker service. Therefore you have to specify it here as a string literal.

Maybe you are wondering now how Service Broker can know where the actual target service is physically deployed? For that missing piece of information you have to create a route within your database. A route just describes where a specific Service Broker service is physically deployed. In our case the target service is deployed within the same database (within the same SQL Server instance), and therefore you don’t have to deploy any additional routing information.

When you have established the conversation between your two Service Broker services, and when you have sent your first message through it, you can finally check the TargetQueue queue of the target service, if the sent message was received successfully. For that case you just have to execute a simple SELECT against that queue:

SELECT message_type_name , CAST(message_body AS XML)FROM TargetQueueGO

This SELECT statement returns one record back to you, as shown in Figure 2, which is the actual message you have successfully sent from the initiator service to the target service.

Figure 2: The received message on the target service

If you want to physically receive and process the sent message, you have to use the RECEIVE statement against the queue object. With the RECEIVE statement you are also able to extract information from the received message into local variables, from which you do the actual message processing. In this example you are using the XML functionality provided by SQL Server to extract the name from the received message. With that name in your hand it’s very easy to construct a new response message and send it back to the initiator service through the SEND ON CONVERSATION T-SQL statement. Listing 5 shows the code you have to execute for the target service. This code can be also put into a stored procedure, which will then act as a service program for your Service Broker service.

DECLARE @ch UNIQUEIDENTIFIER
DECLARE @messagetypename NVARCHAR(256)
DECLARE @messagebody XML
DECLARE @responsemessage XML
BEGIN TRANSACTION ;
RECEIVE TOP(1)
@ch = conversation_handle,
@messagetypename = message_type_name,
@messagebody = CAST(message_body AS XML)
FROM TargetQueue
PRINT 'Conversation handle: ' + CAST(@ch AS NVARCHAR(MAX))
PRINT 'Message type: ' + @messagetypename
PRINT 'Message body: ' + CAST(@messagebody AS NVARCHAR(MAX))
IF ( @messagetypename = 'http://ssb.csharp.at/RequestMessage' ) 
    BEGIN
-- Construct the response message
        SET @responsemessage = '<HelloWorldResponse>Hello World, '
            + @messagebody.value('/HelloWorldRequest[1]', 'NVARCHAR(MAX)')
            + '</HelloWorldResponse>' ;
-- Send the response message back to the initiating service
        SEND ON CONVERSATION @ch MESSAGE TYPE [http://ssb.csharp.at/ResponseMessage] (@responsemessage) ;
-- End the conversation on the target's side
        END CONVERSATION @ch ;
    END
COMMIT TRANSACTION
GO

Listing 5: Receiving a sent Service Broker message on the target service

The great thing here about Service Broker is the fact that you can do message and data processing within the context of a local database transaction. When you think of other message technologies on the Microsoft platform (like MSMQ - Microsoft Message Queuing), you have always to create distributed transactions, because you are working with at least two different resource managers. In addition, distributed transactions have a lot of overhead, which leads directly to poorly performing database applications. Here you can see very well why an embedded messaging framework within the relational engine of SQL Server makes a lot of sense.

As you can see in Listing 5, you are receiving the message from the queue, constructing the response message, and finally sending the response message back to the initiator service. But what is the END CONVERSATION T-SQL statement? Every opened conversation in Service Broker must be also closed on both sides – at the target service and also at the initiator service. As soon as you execute an END CONVERSATION on one side of the conversation, Service Broker sends an end dialog message to the other side. This means that in this example, you have sent two messages back to the initiator service, the response message, and also an end dialog message. You can verify it very easily by querying the queue of the initiator service:

SELECT message_type_name , CAST(message_body AS XML)FROM InitiatorQueueGO

The results are shown in Figure 3.

Figure 3: The received messages at the initiator service

As you can see in Figure 3, the queue contains two messages – the response message and also an end dialog message that is further described through the http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog message type. This message type is internally used by Service Broker. Listing 6 contains the code that is necessary to process the received messages. Because you just receive one message at time you have to execute this batch two times, so that each message gets processed individually.

DECLARE @ch UNIQUEIDENTIFIER
DECLARE @messagetypename NVARCHAR(256)
DECLARE @messagebody XML
BEGIN TRANSACTION ;
RECEIVE TOP (1)
@ch = conversation_handle,
@messagetypename = message_type_name,
@messagebody = CAST(message_body AS XML)
FROM InitiatorQueue
IF ( @messagetypename = 'http://ssb.csharp.at/ResponseMessage' ) 
    BEGIN
        PRINT 'Conversation handle: ' + CAST(@ch AS NVARCHAR(MAX))
        PRINT 'Message type: ' + @messagetypename
        PRINT 'Message body: ' + CAST(@messagebody AS NVARCHAR(MAX))
    END
IF ( @messagetypename = 'http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog' ) 
    BEGIN
-- End the conversation on the initiator's side
        END CONVERSATION @ch ;
    END
COMMIT TRANSACTION
GO

Listing 6: Receiving a sent Service Broker message on the initiator service

After executing this code twice, you have processed the response message, and finally also closed the conversation on the initiator service side. Therefore the whole conversation was cleaned up and removed from memory.

Summary

Congratulations, you have now successfully implemented your first Service Broker application! In part 2 of this series we will have a more detailed look into Service Broker, where I will clarify why, and in which circumstances, the usage of Service Broker makes sense. Stay tuned!

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