Hello,


I have a very large dataset, and want to update certain entity kinds. I am 
exploring MapReduce library in GoogleAppEngine. I have followed the 
examples listed here.

https://github.com/GoogleCloudPlatform/appengine-mapreduce/tree/master/java/example/src/com/google/appengine/demos/mapreduce/entitycount

What I am basically doing is this, in my MapSpecification

MapSpecification<Entity, Entity, Void> spec = new MapSpecification.Builder<>(
                new DatastoreKeyInput(query,2),
                new UrlFlattenMapper(),
                new DatastoreOutput())
                .setJobName("Flatten URLs entities")
                .build();

and My Mapper basically performs the operations on the Entity and then 
Emits it, for the DatastoreOutput writer to write it back into the database.

My problem is, the Entities are getting updated fine. The endSlice is also 
being called in my MapperTask. But the Jobs is not completing. I keep 
getting these errors

[INFO] INFO: RetryHelper(28.07 ms, 1 attempts, 
java.util.concurrent.Executors$RunnableAdapter@7f0264e0): Attempt #1 failed 
[java.lang.RuntimeException: Can't serialize object: 
MapOnlyShardTask[context=IncrementalTaskContext[jobId=3c041e68-5041-458c-994b-290cd941f8bb,
 shardNumber=1, shardCount=2, lastWorkItem=Topics("jzdh"), workerCallCount=297, 
workerTimeMillis=42513], inputExhausted=true, isFirstSlice=false]], sleeping 
for 1028 ms[INFO] Apr 26, 2016 4:39:37 PM 
com.google.appengine.tools.cloudstorage.RetryHelper doRetry[INFO] INFO: 
RetryHelper(1.085 s, 2 attempts, 
java.util.concurrent.Executors$RunnableAdapter@7f0264e0): Attempt #2 failed 
[java.lang.RuntimeException: Can't serialize object: 
MapOnlyShardTask[context=IncrementalTaskContext[jobId=3c041e68-5041-458c-994b-290cd941f8bb,
 shardNumber=1, shardCount=2, lastWorkItem=Topics("jzdh"), workerCallCount=297, 
workerTimeMillis=42513], inputExhausted=true, isFirstSlice=false]], sleeping 
for 2435 ms[INFO] Apr 26, 2016 4:39:37 PM 
com.google.appengine.tools.cloudstorage.RetryHelper doRetry[INFO] INFO: 
RetryHelper(3.562 s, 3 attempts, 
java.util.concurrent.Executors$RunnableAdapter@6d7fcd47): Attempt #3 failed 
[java.lang.RuntimeException: Can't serialize object: 
MapOnlyShardTask[context=IncrementalTaskContext[jobId=3c041e68-5041-458c-994b-290cd941f8bb,
 shardNumber=0, shardCount=2, lastWorkItem=Topics("jz63"), workerCallCount=289, 
workerTimeMillis=41536], inputExhausted=true, isFirstSlice=false]], sleeping 
for 3421 ms[INFO] Apr 26, 2016 4:39:39 PM 
com.google.appengine.tools.cloudstorage.RetryHelper doRetry[INFO] INFO: 
RetryHelper(3.567 s, 3 attempts, 
java.util.concurrent.Executors$RunnableAdapter@7f0264e0): Attempt #3 failed 
[java.lang.RuntimeException: Can't serialize object: 
MapOnlyShardTask[context=IncrementalTaskContext[jobId=3c041e68-5041-458c-994b-290cd941f8bb,
 shardNumber=1, shardCount=2, lastWorkItem=Topics("jzdh"), workerCallCount=297, 
workerTimeMillis=42513], inputExhausted=true, isFirstSlice=false]], sleeping 
for 3340 ms[INFO] Apr 26, 2016 4:39:41 PM 
com.google.appengine.tools.cloudstorage.RetryHelper doRetry[INFO] INFO: 
RetryHelper(7.015 s, 4 attempts, 
java.util.concurrent.Executors$RunnableAdapter@6d7fcd47): Attempt #4 failed 
[java.lang.RuntimeException: Can't serialize object: 
MapOnlyShardTask[context=IncrementalTaskContext[jobId=3c041e68-5041-458c-994b-290cd941f8bb,
 shardNumber=0, shardCount=2, lastWorkItem=Topics("jz63"), workerCallCount=289, 
workerTimeMillis=41536], inputExhausted=true, isFirstSlice=false]], sleeping 
for 6941 ms[INFO] Apr 26, 2016 4:39:42 PM 
com.google.appengine.tools.cloudstorage.RetryHelper doRetry

I havent been able to get around this issue, any help or pointers on what I 
could be doing would be greatly appreciated.


Thanks,

Vamsi


PS:I have raised this question in StackOverflow, but wanted to ask here for 
better exposure.

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