Regardless of how carefully you plan, it's inevitable that the software you build will encounter a situation or chain of events that you did not anticipate, and experience some type of failure. (Trust me on this. I work in software testing and spend most of my waking hours either causing software failures, or debugging them.) Since it is impossible to avoid every failure, the ability to recover from failures is a crucial part of every software system design.
A classic type of failure involves moving money between bank accounts. It's a 2-phase set of actions. First, money is withdrawn from one account and, second, the money is deposited into a second account. This sounds simple, right? Well, suppose that the the system encounters a failure after it has withdrawn the money from the first account, but before it has had a chance to deposit into the second account? What happens to the money? How can you avoid losing this money? The answer is that you enclose these two actions into a transaction.
Transactions and ACID Properties
What exactly is a "transaction?" Like a lot of things in software, it's a buzzword that is overused. But beyond that, a transaction is a logical grouping of actions into a single larger action, where this larger action is performed, or not performed, based on an all-or-nothing evaluation of the results of the individual actions. In the case of the transfer of money example we just mentioned, wrapping the two transfers into a transaction would ensure that if either transfer failed, your money would not be lost, and both accounts would be restored to their original condition.
Transactions are frequently described in terms of having an "ACID" set of properties:
- Atomicity - When we talk about wanting a transaction to be "atomic" what we mean is that we want the transaction, which will be made up of a set of individual actions, to be treated as a single atomic unit. If any of these actions fails, such as one of the bank account transfers in our example, then the entire transaction is canceled. (The term used for this is "rolled back.") A transaction is an all-or-nothing construct. All the actions enclosed by the transaction must be completed successfully before the transaction itself is completed (or "committed.")
- Consistency - Transactions don't leave the data that it works on in a partially finished state. If a transaction rolls back, the data or database in question is returned to the (consistent) state that it was in before the transaction started.
- Isolation - In the context of a transaction, "isolation" refers to how any conditions or state created by a transaction are not visible to any other transaction. For example, if our bank account transfer was executed in a transaction, then while it was executing, any other transactions or operations would not be able to see any of results of the transaction until the entire transaction had completed.
- Durability - One of the dangers of the bank account transfer example, if it were to be attempted outside of a transaction, is that if something went wrong, data (or even worse, money!) would be lost. Because a transaction guarantees the consistency of the data, that data is durable. If the transaction rolls back, the original data is restored.
OK, this is all interesting, but it sounds like something better suited to databases than Service Oriented Architecture. How does a transactional model apply to the JBossESB in the SOA Platform where everything is either a message or a service?
Here's how: some transports supported by the Platform (InVM and JMS) support a transactional delivery of messages. The actual delivery of that message will not happen until the transaction is committed. How can you cause a rollback of a transaction in an action pipeline to occur? By configuring the application and its services to include the transaction and then by raising a RuntimeException in the action pipeline. The best way to explain and illustrate how this is with one of the SOA Platform's "quickstart" example programs. Let's take a look.
One of the great features of the SOA Platform is its extensive, and always growing, set of "quickstart" programs. These programs illustrate various features supported by the Platform and serve as a great resource for writing your own applications. For our example, we'll look at the aptly named "jms_transacted" quickstart.
This quickstart illustrates using the JMS transport with the JBossESB in the SOA Platform to handle transactions. The quickstart also shows how message redelivery can be performed with the JMS transport. Before we examine the quickstart's code, configuration files, and actual output, it's important that we make note of the quickstart's use of JCA (Java Connector Architecture) . JCA provides a standardized way to connect to JMS providers. For the JBossESB in the SOA Platform, jms-jca-providers provide transaction support for the action pipeline by enclosing actions in a JTA  transaction. These transactions ensure that messages are handled within an enclosing transaction. If something goes wrong in the transaction, the messages are put back into the JMS queues to be reprocessed. We'll see this in action when we run the quickstart.
The sequence of actions that the quickstart performs is:
- Like many of the SOA Platform's quickstarts, things begin by having a JMS message routed to a service to start the action pipeline.
- The first time that the pipeline is entered, a row is inserted into the quickstart's HSQLDB database. Note that while the SOA Platform supports multiple databases such as MySQL, Oracle, PostgreSQL and others, in production environments, it includes a fully functional HSQLDB database for demonstration purposes.
- Then, the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction class is called to raise an exception. This action class is configured to raise the exception (5) times, so as to force a rolback of the transaction that encloses the writes to the database. Each time that the exception is raised, the JCA adapter rolls back the transaction. The exception is also propagated to the JCA adapter and written error messages to the log.
- After the configured limit of (5) rollbacks is reached, the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction class completes and the transaction is committed by the SOA Platform's JBossESB's org/jboss/soa/esb/common/JBossESBTransactionService class so that the net result is only one row written to the database.
The Quickstart - In Detail
The transaction in quickstart includes both JMS messaging and writing to a database. Let's start by examining the connection to the HSQLDB database that the quickstart uses. First, we need to create a datasource definition to connect to the database.
In file: quickstart-ds.xml
<?xml version="1.0" encoding="UTF-8"?>Some items to make note of here are:
- Line 3 - We're defining a local-tx-datasource as we will want a JCA connection with transaction support.
- Line 4 - We're use this JNDI name to reference the datasource.
- Lines 5-13 - DB driver class, connection, URL, username/password, etc. for the database connection
- Line 12 - MBean interface for running Hypersonic in the same VM with JBoss - note that the service name and the JNDI name as we'll refer to them later.
In file: jbossesb-service.xml
<?xml version="1.0" encoding="UTF-8"?>There are (3) interesting things in this file:
<attribute name="ExistsSql">select * from jms_transacted_table</attribute>
- Line 4 - This MBean creates the database on startup. How does it connect to the database? See line 6.
- Line 6 - Here's a reference to our datasource. Note the JNDI name that we observed in the datasource definition file. How does the DatabaseInitializer know what the database schema look like? See line 9.
- Line 9 - Here's a reference to the quickstart's src/hsqldb/create.sql file. This file create a very simple table in our database for the quickstart to use:
create table jms_transacted_tableWhat else does the quickstart's database need? The service that creates the database (JmsTransactedDatabaseInitializer) has to be deployed. This is handled in the deployment.xml file (as are the JBoss Messaging queues that the quickstart will use):
unique_id INTEGER GENERATED BY DEFAULT AS IDENTITY(START WITH 1) NOT NULL PRIMARY KEY,
data_column VARCHAR(255) NOT NULL
<jbossesb-deployment>And finally, the jbossesb-service.xml quickstart-ds.xml files have to be included in the quickstart's .esb application archive file. This is handled by the "additional.deploys" property in the quickstart's build.xml file and the ../conf/base-build.xml file that is used by all the quickstarts.
cat -n build.xml | grep quick
OK, that takes care of the database. Now, let's look at how we configure the quickstart to take advantage of the transaction support on the JBossESB in the SOA Platform. As always, the place to begin is the jboss-esb.xml file.
<property name="additional.deploys" value="jbossesb-service.xml quickstart-ds.xml" />
In file: jboss-esb.xml
<?xml version = "1.0" encoding = "UTF-8"?>
<jbossesb xmlns="http://anonsvn.labs.jboss.com/labs/jbossesb/trunk/product/etc/schemas/xml/jbossesb-1.0.1.xsd" parameterReloadSecs="5">
<jms-jca-provider name="JBossMessaging" connection-factory="XAConnectionFactory">
<!-- The maximum number of times a message is redelivered before it is sent to the DLQ -->
<property name="dLQMaxResent" value="5"/>
description="JMS Secured quickstart sample">
<action name="printMessage" class="org.jboss.soa.esb.actions.SystemPrintln">
<property name="message" value="JMS Transacted Quickstart entered. Message body"/>
<property name="printfull" value="false"/>
<action name="insertDBAction" class="org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction">
<property name="datasource-name" value="java:JmsTransactedDB"/>
<property name="db-insert-sql" value="insert into jms_transacted_table(data_column) values(?)"/>
Will throw an Exception for the configured number of rollbacks. This should trigger the transaction to be
rolledback and the message placed back onto the JMS queue.
<action name="throwExceptionAction" class="org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction">
<property name="rollbacks" value="5"/> <!-- if greater than dLQMaxResent then message goes to the JMS DLQ -->
<action name="printMessageDone" class="org.jboss.soa.esb.actions.SystemPrintln">
<property name="message" value="JMS Transacted Quickstart processed successfully. Message body"/>
<property name="printfull" value="false"/>
<!-- The next action is for Continuous Integration testing -->
<action name="testStore" class="org.jboss.soa.esb.actions.StoreMessageToFile"/>
- Line 2 - Before we go any further, we should look at the XSD (XML schema definition) for jboss-esb.xml files. The schema definition is available here: http://anonsvn.jboss.org/repos/labs/labs/jbossesb/trunk/product/etc/schemas/xml/jbossesb-1.0.1.xsd. If you're going to be working with the SOA Platform, it's a good idea to become familiar with this XSD as all the elements that you work with in jboss-esb.xml are defined there. The two main elements are of course, providers and services. Let's look at the providers first. Two types of providers are supported. Schedule providers define schedule driven listeners that pull messages from queues at (you guessed it) scheduled time periods. In contrast, bus providers such as the jms-jca-providers that we're using in this quickstart, are associated with listeners that have messages pushed to them. 
- Line 5 - Here's the start of our jms-jca-provider. Note that the XAConnectionFactory provides support for distributed transactions
- Lines 7-13 - This is a reference to the channel and JMS queue that will be used when a JMS message that is sent to a JMS gateway to initiate the quickstart. Let's stop for a second and talk about the forms in which data is moved onto and across the JBossESB in the SOA Platform. One of the primary functions performed by JBossESB is the routing of messages between services. (I said it before, and I'll say it again; remember that, on an ESB, everything is either a message or a service.) The messages that JBoss ESB handles conform to org.jboss.soa.esb.message.Message. Service endpoints that can process messages in this format are described as being "ESB-aware." This is all well and good for services and applications that were designed and written with this message format in mind, but what about other services, including legacy applications, that communicate through other non-ESB aware data formats? JBossESB handles connecting services that communicate in non-ESB aware message formats through its gateway listeners. These listeners route incoming messages from outside the ESB to ESB services. The ESB's set of gateway listeners (generally referred to as "gateways") support a variety of message formats such as files, FTP, and JMS. These gateways accept messages in a non-ESB aware format onto the ESB and then convert them (actually, they wrap the message payload) into ESB-aware messages before sending them to their service's action pipeline.
- Line 11 - Remember, we want the service to process messages within a transaction.
- Lines 14-20 - Gateways are intended to enable the JBossESB to communicate to external data sources. The gateways themselves don't move data across the ESB. Accordingly, for each Gateway channel that we define, we have to define a corresponding ESB-aware channel to be used to route messages within the ESB.
- Line 21 - The activation-config defines how we link to the JCA. In the case of this quickstart, the interesting property is: dLQMaxResent. As the comment indicates, the “DLQMaxResent” controls how many times the JCA adapter resends the message before it is sent to the dead letter queue. This is a JBossESB service that handles messages for transports when the messages cannot be delivered. (The JMS transport actually has its own dead letter queue as JBoss Messaging is shipped with a set of pre-configured destinations defined in destinations-service.xml including: <mbean code="org.jboss.jms.server.destination.Queue" name="jboss.messaging.destination:service=Queue,name=DLQ") In this quickstart, the JCA adapter will try to send the message 5 times.
A service is where thing happen. Each service definition includes a set of listeners and a sequential set of actions referred to as an "action pipeline." The listeners "listen" for incoming messages for the service and route those messages to the action pipeline.
After the service name and category are defined on lines 31 and 32, we define the service's listeners:
- Lines 35-37 - Here's the JMS gateway listener definition. Notice how it references the bus ID of the channel that we defined in earlier in the provider definition. Also, notice how it's "is-gateway" property is set to true to indicate that this listener is a gateway.
- Lines 38-39 - And here is the ESB-aware listener that corresponds to the gateway listener.
- Line 41 - This is the start of the definitions of the sequence of actions that we referred to as the action pipeline. The message exchange pattern (or "mep") indicates that the messages are not being exchanged in a request/response pattern, but rather are being sent in only one direction. (In contrast, web services exposed on the ESB would follow a synchronous request/response pattern.)
- Lines 43-46 - This action invokes the simplest of the JBossESB's many useful out-of-the-box actions (http://soa.dzone.com/articles/works-great-right-out-box) to write some messages to the server log.
- Lines 48-51 - Here's where things start to get interesting. This custom action writes a row to our database table. Remember the "java:JmsTransactedDBdatasource" that we defined? Here's where gets used.
- Line 57 - Remember how we talked about how to cause a rollback of a transaction in the action pipeline to occur? We configured the .esb to include a transaction, and now we raise a RuntimeException in the action pipeline. The action defined here raises an exception of a type that we define in the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction class. The ActionProcessingPipeline will catch the exception and pass it up the stack to its caller, the JMS/JCA adapter. The Adapter then causes the transaction to rollback and the message is redelivered.
- Line 58 - When we defined the jms-jca-provider's activation-config, we specified a dLQMaxResent value of 5 to indicate that if the transaction was rolled back more than 5 times, the message processed in the transaction should be sent to the JMS dead letter queue. In line 58, we also set the value of the rollbacks property equal to 5, to ensure that the transaction can complete after 5 rollbacks, and the message is not sent to the dead letter queue service. As the comment helpfully explains, setting the rollbacks property to be greater than dLQMaxResent will send the message to the dead letter queue service.
- Lines 61-64 This action is only reached after the rollbacks have been performed and the transaction is able to complete. The action writes the message body to the server log.
After the SOA-P server is started, the quickstart, is deployed with this command: ant deploy
When the quickstart is deployed, the following is to the server.log:
2010-04-07 09:26:22,117 INFO [org.jboss.resource.connectionmanager.ConnectionFactoryBindingService] (HDScanner) Bound ConnectionManager 'jboss.jca:service=DataSourceBinding,name=JmsTransactedDB' to JNDI name 'java:JmsTransactedDB'Note how the JmsTransactedDB database is initialized by org.jboss.internal.soa.esb.dependencies.DatabaseInitializer class the when the quickstart is deployed.
2010-04-07 09:26:22,157 INFO [org.jboss.internal.soa.esb.dependencies.DatabaseInitializer] (HDScanner) Initializing java:/JmsTransactedDB from listed sql files
2010-04-07 09:26:22,326 INFO [org.jboss.soa.esb.listeners.deployers.mc.EsbDeployment] (HDScanner) Starting ESB Deployment 'Quickstart_JMS_Transacted.esb'
Next, we run the quickstart with this command: ant runtest
When the ant runtest target is executed, the following chain reaction is started:
The org.jboss.soa.esb.samples.quickstart.jmstransacted.test.SendJMSMessage class sends a (non-ESB aware) JMS message to the quickstart_jms_transacted_Request_gw queue. The message is routed through that queue's corresponding ESB-aware queue (quickstart_jms_transacted_Request_esb) to the SimpleListener service and the action pipeline is initiated.
The first time that the action pipeline is started, the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction class inserts a row into the database through the HSQLDB datasource. This results in the following messages being written to the server.log:
Then, the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction runtime exception is thrown.
Since this exception is thrown from inside the action pipeline, the JMS/JCA adapter rolls back the enclosing transaction. The exception is propagated to the JMS/JCA adapter as the exception is passed up the stack from the action pipeline to its the caller (which is the adapter). The exception is caught by the JBossESB's org.jboss.soa.esb.listeners.message.ActionProcessingPipeline, and is propagated up the stack to the JMS/JCA adapter (which is the action pipeline's caller). The adapter causes the transaction to rollback. This results in the row being removed from the database and in the original message being redelivered. Remember what we said about a transaction being an "all or nothing" event?
The transaction/rollback sequence then repeats until the configured number of rollbacks are performed. You can see a counter (maintained by the org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction class) of the number of times the action is attempted.
The printMessage action prints the message to the log:
2010-04-07 09:26:35,340 INFO [STDOUT] (WorkManager(2)-9) JMS Transacted Quickstart entered. Message body:And the insertDBAction action writes the row to the database - note the counter value of :
2010-04-07 09:26:35,340 INFO [STDOUT] (WorkManager(2)-9) [Hello Transacted JMS World]].
2010-04-07 09:26:35,415 INFO [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction] (WorkManager(2)-9) Successfully inserted [Hello Transacted JMS World] counter] into jms_transacted_tableBut wait! Then our exception is raised:
2010-04-07 09:26:35,488 ERROR [org.jboss.resource.adapter.jms.inflow.JmsServerSession] (WorkManager(2)-9) Unexpected error delivering message delegator->JBossMessage:PERSISTENT, deliveryId=0The JMS/JCA adapter rolls back the transaction, and the message is delivered again, and another row is written to the database. The counter is incremented to  this time and the exception is raised again.
java.lang.IllegalStateException: [Throwing Exception to trigger a transaction rollback]
2010-04-07 09:26:36,435 INFO [STDOUT] (WorkManager(2)-10) JMS Transacted Quickstart entered. Message body:When the rollback counter number is reached, the action pipeline completes and the transaction commits so that the row is finally written to the database. The following messages are written to the servler.log:
2010-04-07 09:26:36,435 INFO [STDOUT] (WorkManager(2)-10) [Hello Transacted JMS World]].
2010-04-07 09:26:36,438 INFO [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction] (WorkManager(2)-10) Successfully inserted [Hello Transacted JMS World] counter] into jms_transacted_table
2010-04-07 09:26:36,442 ERROR [org.jboss.resource.adapter.jms.inflow.JmsServerSession] (WorkManager(2)-10) Unexpected error delivering message delegator->JBossMessage:PERSISTENT, deliveryId=1
java.lang.IllegalStateException: [Throwing Exception to trigger a transaction rollback]
This sequence repeats (5) times until the counter is passed and the row is actually written to the database:
2010-04-07 09:26:40,489 INFO [STDOUT] (WorkManager(2)-14) JMS Transacted Quickstart entered. Message body:Now, as I mentioned in the beginning of this post, I work as a software QE engineer. One of the basic tenents of software QE is to never trust anything. Let's take a closer look at what happens when the quickstart runs.
2010-04-07 09:26:40,489 INFO [STDOUT] (WorkManager(2)-14) [Hello Transacted JMS World]].
2010-04-07 09:26:40,490 INFO [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.DBInsertAction] (WorkManager(2)-14) Successfully inserted [Hello Transacted JMS World] counter] into jms_transacted_table
2010-04-07 09:26:40,490 INFO [STDOUT] (WorkManager(2)-14) JMS Transacted Quickstart processed successfully. Message body:
2010-04-07 09:26:40,490 INFO [STDOUT] (WorkManager(2)-14) [Hello Transacted JMS World]].
Server logs are goldmines for gathering debugging information. Let's restart the server, but this time set the server logging level to DEBUG:
./run.sh -Djboss.server.log.threshold=DEBUGAnd then rerun the quickstart. The server.log will be much more verbose this time, but if you look closely, you can see the rollback counter in action:
grep nr-of-rollbacks server.logBefore we move on, let's look at one more thing and verify that only one row is actually written to the database. Now, the quickstart is very tidy and always cleans up after itself by removing rows from the database. Let's disable this cleanup by removing this target from the quickstart's build.xml file:
2010-04-07 23:14:19,776 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-11) rollbackCounter , nr-of-rollbacks 
2010-04-07 23:14:20,804 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-12) rollbackCounter , nr-of-rollbacks 
2010-04-07 23:14:21,819 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-13) rollbackCounter , nr-of-rollbacks 
2010-04-07 23:14:22,833 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-14) rollbackCounter , nr-of-rollbacks 
2010-04-07 23:14:23,848 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-15) rollbackCounter , nr-of-rollbacks 
2010-04-07 23:14:24,858 DEBUG [org.jboss.soa.esb.samples.quickstart.jmstransacted.test.ThrowExceptionAction] (WorkManager(2)-16) rollbackCounter , nr-of-rollbacks 
cat build.xml | grep truncate
And, let's start the server again, but with one additional command line option: sh ./run.sh -Djava.awt.headless=false
Why "headless=false?" After we run the quickstart again, the row that was written to the database should still be there. We'll look at it with the HSQL DB Manager service. This service requires the server to not be started in its default "headless" mode, as the service opens up a Java UI application. We invoke the DB Manager from the JMX console - see screenshot - the service tag is: database=jbossesb,service=Hypersonic.
Once the DM Manager application UI launches, we just connect to the "JmsTransactedDB" database - see screenshot - and execute a query to return all rows in the "jms_transacted_table" - see screenshot.
And, there is our one record!
Before we move on, it's worthwhile to note what we did NOT have to do to execute a transaction with the SOA Platform.
We did not have to write transaction specific code. The JBossESB in the Platform did the dirty work for us. All we had to do is to define the .esb application's configuration in its jboss-esb.xml file to include the transaction-relevant properties and reference a JMS/JCA adapter and the Platform did the rest. This is what makes middleware so useful. You could write the code to handle transactions all on your own, but wouldn't you rather concentrate on solving your own business problems instead of build infrastucture plumbing? 
The value that using transactions adds to an application is pretty obvious (just keep your bank account in mind). The value added by middleware in implementing transactions should also be obvious. You could write your own code to handle atomicity, consistency, isolation and durability, but, this would not be an easy task, and it would also take away a lot of time from your being able to concentrate on solving your own business logic problems. The SOA Platform's support for transactions over the JBossESB enables you to provide a higher level of reliability and durablity to your service based applications.
 Little, Mark, Maron, Jon, Pavlik, Greg. Transaction Processing: Design and Implementation, Upper Saddle RIver, new Jersey: Prentice Hall/Hewlett-Packard Professional Books, 2004.
As always, I want to thank the JBoss SOA Platform team and community (especially Kevin Conner for his timely review input for this blog post).