Contention happens using Task Queue

[Yun Li]

I am using Objectify. This error happens when updating a single datastore entity. There is NO TRANSACTION. My working flow as follows:

1 Entity entity1 = Objectify.read(id1)

2 Modify entity1's property and put "update(entity1)" in a taskqueue

3 When task executes: Objectify.save(entity1)

Then I get java.util.ConcurrentModificationException: too much contention on these datastore entities

It's so weird because the log shows that even two requests try to update entity1, their execution interval could be more than 10s. I am a new GAE user, anybody helps me ?

[Jeff Schnitzer]

You'll need to post some more code details - including the structure of the object you are trying to change. Also you don't mention whether this is an exception or just a log message. It's easy to accidentally create contention with the task queue, especially if you're in a transaction and enqueueing non-transactional tasks (which often start immediately).

Jeff

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[Yun Li]

Entity entity= EntityDao.get(entityID);

// get parameters, gpsLat, batteryLevel...etc.

        queue.add(TaskOptions.Builder.withUrl("/update").param(Parameters.EntityID, entityID).param("gpsLong", gpsLong)
                        .param("gpsLat", gpsLat)
                        .param("batt", batteryLevel)
                        .param("connection", connectedState)
                        .param("time", lastTimeUpdated)
        );

After some updating---> set the gpsLat, batt, connection, etc. 

EntityDao.put(updatedEntity);

which is:

ofy().save().entity(updatedEntity).now();

And my GAE log says:

java.util.ConcurrentModificationException: too much contention on these datastore entities. please try again. at com.google.appengine.api.datastore.DatastoreApiHelper.translateError(DatastoreApiHelper.java:57) at com.google.appengine.api.datastore.DatastoreApiHelper$1.convertException(DatastoreApiHelper.java:129) at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:96) at com.google.appengine.api.datastore.Batcher$ReorderingMultiFuture.get(Batcher.java:131) at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:88) at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:88) at com.googlecode.objectify.cache.TriggerFuture.get(TriggerFuture.java:102) at com.googlecode.objectify.impl.ResultAdapter.now(ResultAdapter.java:34) at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:22) at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:10) at com.googlecode.objectify.util.ResultTranslator.nowUncached(ResultTranslator.java:21) at com.googlecode.objectify.util.ResultCache.now(ResultCache.java:30) at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:22) at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:10) at com.googlecode.objectify.util.ResultTranslator.nowUncached(ResultTranslator.java:21) at com.googlecode.objectify.util.ResultCache.now(ResultCache.java:30) at com.phonehalo.server.dao.TrackerDao.put(TrackerDao.java:17) at com.phonehalo.server.servlet.TrackRupdate.doPost(TrackRupdate.java:65) at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at com.google.inject.servlet.ServletDefinition.doService(ServletDefinition.java:263) at com.google.inject.servlet.ServletDefinition.service(ServletDefinition.java:178) at com.google.inject.servlet.ManagedServletPipeline.service(ManagedServletPipeline.java:91) at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:62) at com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:168) at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58) at com.googlecode.objectify.cache.AsyncCacheFilter.doFilter(AsyncCacheFilter.java:59) at com.googlecode.objectify.ObjectifyFilter.doFilter(ObjectifyFilter.java:49) at com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163) at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58) at com.google.inject.servlet.ManagedFilterPipeline.dispatch(ManagedFilterPipeline.java:118) at com.google.inject.servlet.GuiceFilter.doFilter(GuiceFilter.java:113)

.....

[Yun Li]

And there is NO transaction at all. We don't use transaction. But I am not sure if objectify uses the transaction in put. 

On Thursday, December 3, 2015 at 9:59:00 PM UTC-8, Jeff Schnitzer wrote:

[Christian F. Howes]

is the object part of an entity group?  there is a limit on the number of writes to an entity group (with or without transactions)

[Alex Martelli]

On Wed, Dec 16, 2015 at 6:20 PM, Christian F. Howes <christi...@starmakerinteractive.com> wrote:

is the object part of an entity group?  there is a limit on the number of writes to an entity group (with or without transactions)

Yep, and, note that "being part of an entity group" just means "having an ancestor"...

Alex

 

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[Jeff Schnitzer]

Also a nearly guaranteed way to create contention is to modify an entity and then non-transactionally enqueue a task which modifies that entity. The task fires immediately and the two writes conflict.

The great thing about enqueueing transactional tasks is that the task is guaranteed to fire *after* the commit.

Jeff