Concurrency when using GORM in Grails

Question

Let's say I have a counter function which updates a counter using raw SQL:

 public void updateCounter() {
   executeSql("UPDATE counter SET count_value = count_value + 1 WHERE id = 1;");
 }

The database would make sure that two concurrent calls to the counter are handled as expected - that all calls would update the counter with one increment and no updates would get lost.

Rather than executing this by issuing a raw SQL command I'd like to use GORM. The naïve way to do it would be something along the lines of:

 public void updateCounter() {
   Counter c = Counter.get(1)
   c.countValue += 1
   c.save()
 }

In this case I'd assume that an update could be lost if two threads call the updateCounter() method at the same moment. What is the correct "Grails/GORM-way" to handle this concurrency issue?

Answer 1

You could use either 'pessimistic' or 'optimistic' locking strategies, which both supported by Hibernate and therefore by GORM. The default GORM strategy is 'optimistic' (which utilizes the version column/property of the persistent domain entity, created by default). It could be used like this:

...
try {
 Counter c = Counter.get(1)
 c.countValue += 1
 c.save(flush:true)
}
catch(org.springframework.dao.OptimisticLockingFailureException e) {
// deal with concurrent modification here
}
...

If you prefer the 'pessimistic' locking strategy instead (which will block all other concurrent reads, btw), you could do it using explicit 'lock' GORM meta-method, like so:

...
Counter c = Counter.lock(1) //lock the entire row for update
c.countValue += 1
c.save(flush:true) //GORM will autorelease the lock once the TX is committed
...

Hope this helps.