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.