Low Overhead CapabilityDisabledError Check

[Greg]

With the upcoming maintenance, I want to add a check at the start of
the handler to see if I should display a maintenance notice. This will
run on every request, so it would be good to use as lightweight a
check as possible. The documentation says CapabilityDisabledError is
thrown by db.Model.put(), db.Model.delete(), and
db.run_in_transaction().

My question is this: can I use run_in_transaction() with a dummy
function - testing without the overhead of actually doing a put()?

I realise that in this case memcache is also being disabled, so I can
test by doing a memcache.set() instead, but it would be nice to
develop a function that can be called to test both, for cases when
maintenance only affects one or the other. Also could a Googler
comment on how syncronised the disabling of datastore and memcache
are? Milliseconds, seconds, or minutes?

[Ian Lewis]

Greg,

Please star this issue.
http://code.google.com/p/googleappengine/issues/detail?id=1116

One person noted in the comments that you could cache a system wide value when the first put request fails so that you don't have to do a test put() on every request to your application.

It would be interesting if run_in_transaction throws this exception even with dummy functions but I would guess that it doesn't throw the error unless it actually tries to a write.

Ian

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[Nick Johnson (Google)]

Hi Greg,

You might want to check out the Capabilities API: http://code.google.com/p/googleappengine/source/browse/trunk/python/google/appengine/api/capabilities/__init__.py

It will let you determine if a particular service is expected to be unavailable.

-Nick Johnson