How to do a database lock in AppEngine (GAE)? - google-app-engine

Question

In GAE, I've got a table full of "one offs" -- things like "last-used sequence number" and the like that don't really fall into other tables. It's a simple String-key with String-value pair.
I've got some code to grab a named integer and increment it, like so:
#PersistenceCapable(detachable="true")
public class OneOff
{
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;
#Persistent
private String dataKey;
#Persistent
private String value;
public OneOff(String kk, String vv)
{
this.dataKey = kk;
this.value = vv;
}
public static OneOff persistOneOff(String kk, String vv)
{
OneOff oneoff= new OneOff(kk, vv);
PersistenceManager pm = PMF.get().getPersistenceManager();
try
{
pm.makePersistent(oneoff);
}
finally
{
pm.close();
}
return oneoff;
}
// snip...
#SuppressWarnings("unchecked")
synchronized
public static int getIntValueForKeyAndIncrement(String kk, int deFltValue)
{
int result = 0;
OneOff oneOff = null;
PersistenceManager pm = PMF.get().getPersistenceManager();
Query query = pm.newQuery(OneOff.class);
query.setFilter("dataKey == kkParam");
query.declareParameters("String kkParam");
List<OneOff> oneOffs = (List<OneOff>) query.execute(kk);
int count = oneOffs.size();
if (count == 1)
{
oneOff = oneOffs.get(0);
result = Integer.parseInt(oneOff.value);
}
else if (count == 0)
{
oneOff = new OneOff(kk, "default");
result = deFltValue;
}
else
{
// Log WTF error.
}
// update object in DB.
oneOff.value = "" + (result+1);
try
{
pm.makePersistent(oneOff);
}
finally
{
pm.close();
}
return result;
}
// etc...
However, when I make these calls:
int val1 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val2 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val3 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
Sometimes I get the desired increment and sometimes I get the same value. It appears that my DB access is running asynchronously, when I'd like to lock the DB for this particular transaction.
I thought that
synchronized
public static
was supposed to do that for me, but apparently not (probably due to multiple instances running!)
At any rate -- how do I do the thing that I want? (I want to lock my DB while I get & update this value, to make the whole thing concurrency-safe.)
Thanks!
== EDIT ==
I have accepted Robert's as the correct answer, since transactions were, indeed, what I wanted. However, for completeness, I have added my updated code below. I think it's correct, although I'm not sure about the if(oneOff==null) clause (the try-catch bit.)
public static int getIntValueForKeyAndIncrement(String kk, int defltValue)
{
int result = 0;
Entity oneOff = null;
int retries = 3;
// Using Datastore Transactions
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
while (true)
{
com.google.appengine.api.datastore.Transaction txn = datastore.beginTransaction();
try
{
Key oneOffKey = KeyFactory.createKey("OneOff", kk);
oneOff = datastore.get (oneOffKey);
result = Integer.parseInt((String) oneOff.getProperty("value"));
oneOff.setProperty("value", "" + (result+1));
datastore.put(oneOff);
txn.commit();
break;
}
catch (EntityNotFoundException ex)
{
result = defltValue;
}
catch (ConcurrentModificationException ex)
{
if (--retries < 0)
{
throw ex;
}
}
if (oneOff == null)
{
try
{
Key oneOffKey = KeyFactory.createKey("OneOff", kk);
oneOff = new Entity(oneOffKey);
oneOff.setProperty("value", "" + (defltValue+1));
datastore.put(txn, oneOff);
datastore.put(oneOff);
txn.commit();
break;
}
finally
{
if (txn.isActive())
{
txn.rollback();
}
}
}
else
{
if (txn.isActive())
{
txn.rollback();
}
}
}
return result;
}

Answer 1

You should be updating your values inside a transaction. App Engine's transactions will prevent two updates from overwriting each other as long as your read and write are within a single transaction. Be sure to pay attention to the discussion about entity groups.