What causes Memcache operation failed, giving up exception? - google-app-engine

Question

I have an app engine java project and am using objectify. I get a stack trace sporadically in the "stack driver error reporting" view of the app engine web console related to putting an item into memcache. This is the code:
try {
TestItem t = new TestItem(...);
ofy().save().entity(t).now();
} catch (Exception e) {
}
and this is the error I'll see sporadically:
com.googlecode.objectify.cache.MemcacheServiceRetryProxy invoke: Memcache operation failed, giving up
java.lang.reflect.InvocationTargetException
at com.google.appengine.runtime.Request.process-i4dx9s2kED3CVcPe(Request.java)
at sun.reflect.GeneratedMethodAccessor27.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:44)
at com.googlecode.objectify.cache.MemcacheServiceRetryProxy.invoke(MemcacheServiceRetryProxy.java:68)
at com.sun.proxy.$Proxy9.putAll(Unknown Source)
at com.googlecode.objectify.cache.KeyMemcacheService.putAll(KeyMemcacheService.java:91)
at com.googlecode.objectify.cache.EntityMemcache.empty(EntityMemcache.java:319)
at com.googlecode.objectify.cache.CachingAsyncDatastoreService$5.trigger(CachingAsyncDatastoreService.java:445)
at com.googlecode.objectify.cache.TriggerFuture.isDone(TriggerFuture.java:87)
at com.googlecode.objectify.cache.TriggerFuture.get(TriggerFuture.java:102)
at com.googlecode.objectify.impl.ResultAdapter.now(ResultAdapter.java:34)
at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:22)
at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:10)
at com.googlecode.objectify.util.ResultTranslator.nowUncached(ResultTranslator.java:21)
at com.googlecode.objectify.util.ResultCache.now(ResultCache.java:30)
at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:22)
at com.googlecode.objectify.util.ResultWrapper.translate(ResultWrapper.java:10)
at com.googlecode.objectify.util.ResultTranslator.nowUncached(ResultTranslator.java:21)
at com.googlecode.objectify.util.ResultCache.now(ResultCache.java:30)
at com.me.test.Test.putSomethinInMemcache(Test.java:13)
...
Caused by: com.google.appengine.api.memcache.MemcacheServiceException: Memcache putAll: Unknown exception setting 1 keys
at com.google.appengine.api.memcache.MemcacheServiceApiHelper$RpcResponseHandler.handleApiProxyException(MemcacheServiceApiHelper.java:69)
at com.google.appengine.api.memcache.AsyncMemcacheServiceImpl$RpcResponseHandlerForPut.handleApiProxyException(AsyncMemcacheServiceImpl.java:349)
at com.google.appengine.api.memcache.MemcacheServiceApiHelper$1.absorbParentException(MemcacheServiceApiHelper.java:111)
at com.google.appengine.api.utils.FutureWrapper.handleParentException(FutureWrapper.java:52)
at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:91)
at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:89)
at com.google.appengine.api.memcache.MemcacheServiceImpl.quietGet(MemcacheServiceImpl.java:26)
at com.google.appengine.api.memcache.MemcacheServiceImpl.putAll(MemcacheServiceImpl.java:115)
... 52 more
It doesn't appear to be caught in the try-statement. I just see it in that admin console mentioned earlier.
Does anyone know what this means, or how I can catch it? My main worry is that there could be an old copy of the object stuck in memcache after this operation fails.
Using objectify 5.1.10.
Thanks

Answer 1

This is a get() operation. If memcache is unavailable during a get() operation, Objectify just reads from the datastore. The error is logged and performance suffers somewhat but the app marches on.
It is technically possible to have errors during write operations (any save() clears the cache entry; reads will repopulate the cache). This could in theory leave stale info in the cache. There's nothing that can really be done about this - if you can't clear the cache entry, it's going to be stuck there. My advice is that if you have sensitive data but want it cached, put a reasonable timeout on the cache entry (#Cache(expirationSeconds=60) or whatnot).