[storm] How to set isolation level in storm

[Jason Baker]

On Fri, Jun 19, 2009 at 1:26 PM, Gustavo Niemeyer<gustavo at niemeyer.net> wrote:
>> On Fri, Jun 19, 2009 at 10:33 AM, Tres Seaver<tseaver at palladion.com> wrote:
>>> A store is a per-thread wrapper around a database, right?  And the
>>> database is what is created from the db_uri?  So the properties are
>>> properties of the database, rather than the store.
>
> Indeed.
>
> (...)
>> I'm not 100% sure what will happen if you create two stores in the
>> same thread, using different database objects returned by
>> create_database, though...?
>
> Nothing surprising will happen.  What may be surprising is that
> depending on what is done, the *database* may cause the thread to
> deadlock (or at least wait for a timeout) because it's waiting for the
> "other connection" to finish and, if both connections are on the same
> thread, the other connection will of course not finish.  That's all
> independent of Storm, though.
>
> --
> Gustavo Niemeyer
> http://niemeyer.net
>
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> storm mailing list
> storm at lists.canonical.com
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>

If anyone's interested, here's something I cooked up to make testing
easier with Oracle (as DDL statements just don't seem to execute in
SERIALIZABLE mode):

class OracleConnection(Connection):
   ...
   def as_read_committed(self):
       return _isolation_context(self)

class _isolation_context(object):
   def __init__ (self, connection):
       self.connection = connection
   def __enter__(self):
       self.connection.commit()
       self.connection.execute('ALTER SESSION SET isolation_level =
read committed')

   def __exit__(self, exc_type, exc_value, traceback):
       if exc_value:
           self.connection.rollback()
       else:
           self.connection.commit()
       self.connection.execute('ALTER SESSION SET isolation_level =
serializable')
       self.connection.commit()

This way, I can do something like this:

   with connection.as_read_committted():
       connection.execute(...)

Without worrying about an exception being thrown and causing the
entire connection to get stuck in the wrong mode.  In light of this
thread, I'm sure there's a better way to do this, but someone might
find the general concept useful if they need to change the isolation
level temporarily.