Written on August 21, 2015

Grails, GPars and Hibernate

The GPars library is a great way to add concurrency supporting constructs to your Groovy code. It equips Groovy projects with powerful concurrency concepts like parallel collections, map reduce operations, actors and dataflow variables.

Adding the GPars dependency

GPars can also be added to Grails applications (in our case, we have a Grails 2.2.5 application, so this article was not tested with any versions up or below). Simply add GPars as a dependency in the BuildConfig.groovy:

dependecies {
  compile 'org.codehaus.gpars:gpars:some_version_number' // we use 1.1.0 for our code
}

That is actually enough to have the GPars features enabled, once included, you can access the parallel collection methods in the targeted collection classes, like:

GParsPool.withPool {
  [1, 2, 3, 4].findParallel { it == 3 }
}

You can see in the above example that the GPars convention is to use the original method name with a Parallel appended. The GParsPool.withPool method has to be called in order to initialise the GPars thread pool and equip collection classes with the parallel methods. An overview of all the available methods can be found in GParsExecutorsPoolEnhancer.

GPars and Hibernate

Our requirement was to speed up a Quartz job iterating over all our customers - which are all Hibernate entities. As the customers could be easily grouped into different sets, the precondition for jork-join processing were met. But the question was how to enable Hibernate read-only processing in the eachParallel GPars method as we wanted to do something like:

differentCustomerGroups.eachParallel { CustomerGroup customerGroup ->
  // process all customers of the given customer group
  // IN THE CURRENT HIBERNATE SESSION
}

As every Hibernate session is bound to the current thread, it was necessary to create a new Session for the current thread created by GPars, attach it and close it once processing was done.

The key to enabling this (in Grails 2.2.5 at least), was to use the persistenceInterceptor bean that can be injected into any Grails artefact:

def persistenceInterceptor

It implements the PersistenceContextInterceptor interface which can be used to initialise and destroy the current persistence context. In the case of its Hibernate implementation, HibernatePersistenceContextInterceptor, the persistence context is the current Hibernate session. Thus, the persistence context interceptor bean can be used to initialise and destroy the session in our closure. The eachParallel uses the bean like that (this pattern can also be found in other places in Grails btw):

differentCustomerGroups.eachParallel { CustomerGroup customerGroup ->
  // init the persistence context
  persistenceInterceptor.init()

  try {
    int offset = 0
    def customers = Customer.executeQuery("select c from customer c where c.customerGroup = ?", customerGroup, [readOnly: true, max: 100, offset: offset])

    // loop over customers till all are processed ...

    // flush the context
    persistenceInterceptor.flush()
  } finally {
    // destroy the context and release resources
		persistenceInterceptor.destroy()
  }
}

This is effectively enough to create and destroy a new Hibernate session to be used by the GPars code. Note, that we also had to disable the automatic Hibernate session creation done by Quartz, specifying the def sessionRequired = false property in the job class:

class CustomerJob {
  def concurrent = false
  def sessionRequired = false // do not create a Hibernate session on job startup

  def execute() {
    // ...
  }
}

Another thing to note is maybe the readOnly option that was given to the executeQuery method. It disables snap-shotting of entities, which in our case was possible (since the customer instances were not modified themselves).

Conclusion

GPars is a library that adds concurrency constructs to your class and also provides concurrency concepts like actors, agents and dataflow variables. This article shows how to handle the Hibernate session in code that is concurrently executed by GPars. The persistence context interceptor is a class provided by Grails that enables setting up and destroying the persistence context in arbitrary places. Note that this article is based on Grails 2.2.5.

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