There is significant difference between dealing with infrequent crash event and dealing with frequent shutdown by GAE. The difference is huge when you want to have some intermediate data in memory for performance reason. When there is a system crash, you just start over, which is tolerable in most cases. Restarting app by GAE is a total different story because it makes storing data in memory no longer a valid approach. For my google app, I see it is restarted by GAE even after a few minutes. My usual singleton object become useless because the data objects it holds are recreated every few minutes. This is why I think GAE's frequent restart behavior is a constraint forcing me (probably other developers) to change design pattern, which may be good or bad.
In this case, the change is not good, I'm afraid. Let's look at the counter example. Normally it's trivial to keep a counter of some sort on the server. But in GAE, it's non-trivial. You can't get a total count from datastore easily and storing a counter in memory is not reliable. SO, you have to do some creative work-around as proposed in GAE documentation. Am I making sense? In any case, I'm hoping someone has an easy/reliable way to keep tracking a counter in memory within GAE. I'll appreciate any suggestion. -aj On Wed, Feb 17, 2010 at 1:56 PM, Stephan Hartmann <hartm...@metamesh.de>wrote: > Hi AJ, > > Your consideration is not specific to GAE. You always have the potential > risk that a server could crash and then all your unstored data changes will > get lost. > So for critical data you should use a write-through cache. However, in a > distributed environment like GAE (but not specific to GAE, every standard > conform servlet container will support this) you have to take special care > to keep the caches of all nodes in sync, or just use a distributed cache > like Memcache in GAE. > > Regards, > Stephan > > 2010/2/17 AJ Chen <ajc...@web2express.org> > >> yes, the new console is a good addition. however, because the app can be >> shutdown/restarted by GAE at any time, you would still need to put the >> initialization code in context listener (or similar place) so that it will >> be called automatically when the app is restarted. >> >> >> I just realize a potential major issue in GAE environment, which may >> require some paradigm shift in server programming (at least to me). Usually, >> one the server side, you may have a singleton class to keep some data >> objects closeby as well as updating the data at run time. The data may >> change so fast that they are conveniently kept in memory for some time >> before put into permanent storage. This is safe because the web server does >> not kill the app at will. Now that GAE may kill the app and restart it at >> any time, keeping data in memory becomes a big potential problem because the >> data will be gone after the app starting. This means you would have to save >> the new data into datastore immediately. If you have lots of intermediate >> data or temporary data, you have to save them to datastore immediately as >> well. This always-using-datastore situation created by GAE may slow down >> some applications in addition to a lot more coding for storage. >> >> Without knowing what exactly happens when GAE automatically shutdowns the >> app, my worry may be wrong. Does anyone see the similar issue? Any >> suggestion for safely keeping data in memory? >> >> -aj >> >> >> On Wed, Feb 17, 2010 at 10:00 AM, Vlad Skarzhevskyy < >> skarzhevs...@gmail.com> wrote: >> >>> There is an Custom Admin Console pages in new SDK 1.3.1. >>> I think the best place to preload the data to your application is >>> using servlet/page exposed in this Console. >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "Google App Engine for Java" group. >>> To post to this group, send email to >>> google-appengine-j...@googlegroups.com. >>> To unsubscribe from this group, send email to >>> google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> >>> . >>> For more options, visit this group at >>> http://groups.google.com/group/google-appengine-java?hl=en. >>> >>> >> >> >> -- >> AJ Chen, PhD >> Chair, Semantic Web SIG, sdforum.org >> http://web2express.org >> @web2express on twitter >> Palo Alto, CA, USA >> 650-283-4091 >> *Monitoring social media in real time* >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Google App Engine for Java" group. >> To post to this group, send email to >> google-appengine-j...@googlegroups.com. >> To unsubscribe from this group, send email to >> google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> >> . >> For more options, visit this group at >> http://groups.google.com/group/google-appengine-java?hl=en. >> > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To post to this group, send email to > google-appengine-j...@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > -- AJ Chen, PhD Chair, Semantic Web SIG, sdforum.org http://web2express.org @web2express on twitter Palo Alto, CA, USA 650-283-4091 *Monitoring social media in real time* -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. 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