A reliable counter needs to be stored in the datastore and sharded -
this has always been the only place to put data that your app depends
on. Static variables are only useful for caching a small amount of
data for speed - never to be depended on. Also keep in mind that your
app is not just initialised after being quiet for some time. I have
found that new instances are constantly being spun up and down.
Perhaps one per concurrent request even?
You are absolutely right that many things are much harder to code on
App Engine. But I expect that extra time to pay off in the long run
with increased reliability, scalability and having Google Engineers
maintain the servers rather than me.
On 18 Feb 2010, at 10:52, AJ Chen wrote:
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.
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