Yesterday Google released v1.5.5 of their App Engine SDK. There are loads of nice changes in this release including the easing of some previously annoying limits, the one I am especially happy about is the doubling of the frontend request deadline from 30s to 60s, this will make our life a lot easier!
More details can be found here:
http://googleappengine.blogspot.com/2011/10/app-engine-155-sdk-release.html
Cully&Sully’s Chef Factor is Back! The first prize is a 12 week cookery course in Ballymaloe worth €12,000, so if you fancy your hand at cooking up a storm check out all of the details along with the current entries here: http://www.cheffactor.ie/
Q: How can use cURL to emulate a form post with a file upload?
A: It took me a bit of time to figure this out but it is quite straight forward really, you just need to use (possibly multiple) --form (or -F) arguments like this:
curl -F appID=1234 -F name=Emulate -F appFile@em.zip http://www.abc.com/na
This will emulate a form post to
http://www.abc.com/na
with two named data items (appID and name) and one file upload, the file upload will be named ‘appFile’, the local file em.zip will be uploaded with the post, and that should do it!
If you’re using wordpress and all of a sudden you can’t approve comments, then try deactivating the ‘Cimy Swift SMTP’ module – It worked for us!
On Wednesday Google released v1.5.3 of their App Engine cloud platform SDK, the release notes are here. There are some interesting new additions to the API, however the one that really caught our eye is the addition of a Compare and Swap (CAS) primitive to their Memcache API.
A CAS primitive can be used to build all sorts of higher level synchronisation primitives like mutexes etc. and so the existence of such a primitive may be very handy for synchronisation of GAE taskqueue tasks! The SDK is currently quite lacking in such synchronisation tools so it will be interesting what can be done with this new addition.
If, when you try to debug or run a PyDev Google app engine project in eclipse after following these instructions, you may experience this error message:
Variable references empty selection: ${project_loc}
If you get this error you should be able to make it go away by first clicking on you project in the PyDev package explorer window before trying to debug or run your project. Eclipse & PyDev can be just a little quirky…
Q: I keep getting ‘Error 500′ when I try to upload / deploy to my code to the google app engine via ‘appcfg.py update’ or similar, what’s going on?
A: Most often this error is caused by problems with the Google cloud infrastructure and as such there is nothing you can do to fix the problem except wait and keep retrying every now and again, the problem normally goes away in an hour or 2. Do however check that your various quotas havent run out.
Don’t be suprised if when you view the System Status table, you see that it is all green – in our experience this status grid rarely represents the status of the cloud as experienced by developers and users. You could report the problem to Google here as suggested in the error message however if you were to report every service interruption you wouldn’t be left with very much time to do much else… 
We were engulfed by a blizzard of Google task queue ‘TransientErrors’ over the weekend, the ‘TransientError’ is one most ellusive of google app engine errors. This is what the official documentation has to say of it:
exception TransientError(Error)
There was a transient error while accessing the queue. Please try again later.
This very detailed description has left many scratching their heads wondering (a) Why they are getting the errors and (b) What should they do about them!
So far, the best description I could find of the error is here:
http://osdir.com/ml/GoogleAppEngine/2009-06/msg01337.html
A TransientError is an unexpected but transient failure. Typically it
is a deadlined or otherwise missed connection between two backends in
our system. We distinguish TransientError from InternalError to say
that transient errors failed but were expected to succeed (and
retrying will probably work), whereas InternalError is quite
unexpected and retries will have no effect.
This seems to suggest that you shouldn’t worry too much about them as they happen rarely and the automatic retry should work – no harm done…
In our experience however it seems that the error is far from transient in that it often occurs in batches with the retries experiencing the error too – it causes our app to grind to a halt, the whole thing tends to snowball. It is like something fairly serious goes wrong with the google cloud and things just stop working.
Does anybody know if there is any reliable way of handling these errors when they occur?
And more seriously, can an application be made reliable on the google cloud with nasty undocumented errors like this cropping up in batches every few months? I suppose software development for cloud environments is still really only in its infancy, developers and platform suppliers have a lot to learn, but unexplained stealth errors like the TransientError really doesn’t help the situation!
Yesterday I came across this gem of a Blog post on Nick Johnson’s Blog about how to go about efficiently storing Google App Engine datastore objects in Memcache on the cloud.
I had previously read that the Datastore models were large and carried around a lot of baggage (software baggage that is, not the stuff on wheels!) – this is one reason why you are discouraged from passing them as arguments to tasks on task-queues.
This blog shows you how to extract and store only the essential info from the objects for storage in memcache using th methids model_to_protobuf () and model_from_protobuf()
Anyway, enough chat – here’s the post:
http://blog.notdot.net/2009/9/Efficient-model-memcaching
Q: How do I get a python object’s class name?
A: Use the object’s __class__ attribute and then get its __name__ attribute.
Another python introspection gem, to get an object’s class name just access its __class__ attribute, for example you can define a method to return the object’s name as follows:
