Memcache writer exception

[Deepak Singh]

Hi All,

Most frequently i am getting following exception

1. java.lang.NullPointerException
   	at com.google.appengine.tools.appstats.MemcacheWriter.commit(MemcacheWriter.java:150)
   	at com.google.appengine.tools.appstats.AppstatsFilter.doFilter(AppstatsFilter.java:151)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   	at com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:102)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   	at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   	at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   	at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
   	at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
   	at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
   	at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
   	at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
   	at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:266)
   	at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
   	at org.mortbay.jetty.Server.handle(Server.java:326)
   	at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
   	at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
   	at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
   	at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
   	at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:146)
   	at com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:447)
   	at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:454)
   	at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:461)
   	at com.google.tracing.TraceContext.runInContext(TraceContext.java:703)
   	at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:338)
   	at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:330)
   	at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:458)
   	at com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:251)
   	at java.lang.Thread.run(Thread.java:679)
   

2. C2013-01-16 22:39:48.155

   Uncaught exception from servlet
   java.lang.NullPointerException
   	at com.google.appengine.tools.appstats.MemcacheWriter.commit(MemcacheWriter.java:150)
   	at com.google.appengine.tools.appstats.AppstatsFilter.doFilter(AppstatsFilter.java:151)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   	at com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:102)
   	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)

what precaution i should take to avoid this error ?

Deepak Singh

[Ryan Chazen]

Appstats persists the stats to memcache so that you don't use up pricey datastore writes on stats data. What's going on there is that the memcache write appears to be failing (?), and this is making it crash out.

You are probably triggering a bug in appstats to cause this - ParseBlobUploadFilter implies this is an upload data request? I don't know enough about AppEngine internals and the SDK is closed source, so who knows.
File an issue on the issue tracker so Google can fix it.

Solutions:
Turn off AppStats for production apps, use it for testing/debugging only. This is also good as using it for production means you have less room in memcache for your own objects, and the memcache is fairly small...
or
Exclude appstats (in web.xml) from running on that particular servlet that is causing the problem.

[Deepak Singh]

Got it.

How much data memcache can store ?

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Deepak Singh