Ant Kutschera

About Ant Kutschera

Ant is a freelance Java architect and developer. He has been writing a blog and white papers since 2004 and writes about anything he finds interesting, related to Java or software. Most recently he has been working on enterprise systems involving Eclipse RCP, Google GWT, Hibernate, Spring and J2ME. He believes very strongly in being involved in all parts of the software life cycle.

Play 2.0 framework and XA transactions

XA transactions are useful and out of the box, Play 2.0 today does not have support for them. Here I show how to add that support:

First off, some examples when XA is useful:

- JPA uses two physical connections if you use entities from two different persistence.xml – those two connections might need to be committed in one transaction, so XA is your only option
- Committing a change in a database and at the same time committing a message to JMS. E.g. you want to guarantee that an email is sent after you successfully commit an order to the database, asynchronously. There are other ways, but JMS provides a transactional way to do this with little overhead in having to think about failure.
- Writing to a physically different database because of any of several political reasons (legacy system, different department responsible for different database server / different budgets).
- See http://docs.codehaus.org/display/BTM/FAQ#FAQ-WhywouldIneedatransactionmanager

So the way I see it, XA is something Play needs to ‘support’.

Adding support is very easy. I have created a play plugin which is based on Bitronix. Resources are configured in the Bitronix JNDI tree (why on earth does Play use a config file rather than JNDI?! anyway…) You start the transaction like this, ‘withXaTransaction’:

    def someControllerMethod = Action {
                    withXaTransaction { ctx =>
                        TicketRepository.

addValidation(user.get, bookingRef, ctx)
                        ValidationRepository.addValidation(bookingRef, user.get, ctx)
                    }
                    val tickets = TicketRepository.getByEventUid(eventUid)
                    Ok(views.html.ticketsInEvent(eventUid, getTickets(eventUid), user, eventValidationForm))
    }

The ctx object is an XAContext (my own class) which lets you look up resources like a datasource, or set rollback in case of a failure. So the validation repo does this, using ScalaQuery (I used ‘withSession’ rather than ‘withTransaction!’):

 def addValidation(bookingRef: String, validator: User, ctx: XAContext) = {
        val ds = ctx.lookupDS("jdbc/maxant/scalabook_admin")
        Database.forDataSource(ds) withSession { implicit db: Session =>
            Validations.insert(Validation(bookingRef, validator.email, new java.sql.Timestamp(now)))
        }
    }    

And the ticket repo does the following with JMS:

 def addValidation(user: User, bookingRef: String, ctx: XAContext) = {

        val xml = 
            
                {bookingRef}
                {user.email}
            

        val qcf = ctx.lookupCF("jms/maxant/scalabook/ticketvalidations")
        val qc = qcf.createConnection("ticketValidation","password")
        val qs = qc.createSession(false, Session.AUTO_ACKNOWLEDGE)
        val q = qs.createQueue("ticketValidationQueue") //val q = ctx.lookup(QUEUE).asInstanceOf[Queue]
        val sender = qs.createProducer(q)
        val m = qs.createTextMessage(xml.toString)
        sender.send(m)
        sender.close
        qs.close
        qc.close
    }    

I’ve tested it with writing to MySQL and sending a JMS message to JBoss (HornetQ) and it seems to work well (except getting hornetQ to play with Bitronix was a bitch – see here: https://community.jboss.org/thread/206180?tstart=0).

The scala code for the XA support is:

package ch.maxant.scalabook.play20.plugins.xasupport

import play.api.mvc.RequestHeader
import play.api.mvc.Results
import play.api.mvc.Request
import play.api.mvc.AnyContent
import play.api.mvc.Result
import play.api.mvc.Action
import play.api.mvc.Security
import play.api._
import play.api.mvc._
import play.api.data._
import play.api.data.Forms._
import ch.maxant.scalabook.persistence.UserRepository
import bitronix.tm.TransactionManagerServices
import java.util.Hashtable
import javax.naming.Context._
import javax.naming.InitialContext
import javax.sql.DataSource
import bitronix.tm.BitronixTransaction
import java.io.File
import org.scalaquery.session.Database
import org.scalaquery.SQueryException
import scala.collection.mutable.ListBuffer
import java.sql.Connection
import java.sql.SQLException
import org.scalaquery.session.Session
import bitronix.tm.BitronixTransactionManager
import javax.jms.ConnectionFactory

class XAContext {

    private val env = new Hashtable[String, String]()
    env.put(INITIAL_CONTEXT_FACTORY, "bitronix.tm.jndi.BitronixInitialContextFactory")
    private val namingCtx = new InitialContext(env);

    var rollbackOnly = false
    
    def lookup(name: String) = {
        namingCtx.lookup(name)
    }
    def lookupDS(name: String) = {
        lookup(name).asInstanceOf[DataSource]
    }
    def lookupCF(name: String) = {
        lookup(name).asInstanceOf[ConnectionFactory]
    }
}

trait XASupport { self: Controller =>

    private lazy val tm = play.api.Play.current.plugin[XASupportPlugin] match {
      case Some(plugin) => plugin.tm
      case None => throw new Exception("There is no XASupport plugin registered. Make sure it is enabled. See play documentation. (Hint: add it to play.plugins)")
    }

    /**
     * Use this flow control to make resources used inside `f` commit with the XA protocol.
     * Conditions: get resources like drivers or connection factories out of the context passed to f.
     * Connections are opened and closed as normal, for example by the withSession flow control offered 
     * by ScalaQuery / SLICK.
     */
    def withXaTransaction[T](f: XAContext => T): T = {
        tm.begin

        //get a ref to the transaction, in case when we want to commit we are no longer on the same thread and TLS has lost the TX.
        //we have no idea what happens inside f!  they might spawn new threads or send work to akka asyncly
        val t = tm.getCurrentTransaction
        Logger("XASupport").info("Started XA transaction " + t.getGtrid())
        val ctx = new XAContext()
        var completed = false
        try{
            val result = f(ctx)
            completed = true
            if(!ctx.rollbackOnly){
                Logger("XASupport").info("committing " + t.getGtrid() + "...")
                t.commit
                Logger("XASupport").info("committed " + t.getGtrid())
            }
            result
        }finally{
            if(!completed || ctx.rollbackOnly){
                //in case of exception, or in case of set rollbackOnly = true
                Logger("XASupport").warn("rolling back (completed=" + completed + "/ctx.rollbackOnly=" + ctx.rollbackOnly)
                t.rollback
            }
        }
    }
}

class XASupportPlugin(app: play.Application) extends Plugin {

    protected[plugins] var tm: BitronixTransactionManager = null
    
    override def onStart {
        //TODO how about getting config out of jar!
        val file = new File(".", "app/bitronix-default-config.properties").getAbsolutePath
        Logger("XASupport").info("Using Bitronix config at " + file)
        val prop = System.getProperty("bitronix.tm.configuration", file) //default
        System.setProperty("bitronix.tm.configuration", prop) //override with default, if not set

        //start the TM
        tm = TransactionManagerServices.getTransactionManager
        
        Logger("XASupport").info("Started TM with resource config " + TransactionManagerServices.getConfiguration.getResourceConfigurationFilename)
    }

    override def onStop {
        //on graceful shutdown, we want to shutdown the TM too
        Logger("XASupport").info("Shutting down TM")
        tm.shutdown
        Logger("XASupport").info("TM shut down")
    }

}

Use the code as you like, I’m giving it away for free :-) Just don’t complain if it don’t work ;-)

It would be nice to see this plugin extended and turned into something a little more production ready. Even nicer would be for Play to support a transaction manager natively, including fetching resources out of JNDI.

Happy coding and don’t forget to share!

Reference: Play 2.0 framework and XA transactions from our JCG partner Ant Kutschera at the The Kitchen in the Zoo blog.

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One Response to "Play 2.0 framework and XA transactions"

  1. Nicolas Forney says:

    Great article, thank you for sharing it.
    Have you also done a Java implementation for it? I’m trying to find the best way to handle multi persistence Unit within a play 2 app developed in Java and XA transaction seems to be a solution.

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