On Dec 1, 2007, at 11:10 PM, Simon Wittber wrote:
>
> I'd like to set up a sharding strategy where shards are chosen based
> on this code:
>
> SHARD_COUNT = 5
>
> databases = dict((i,create_engine('sqlite://shard_%s.db' % i)) for i
> in xrange(SHARD_COUNT))
>
> def shard_chooser(mapper, instance, clause=None):
> return instance.primary_key_id % SHARD_COUNT
>
> Ie, I have 5 shards, and I choose the shard based on primary_key % 5.
>
> To do this, I need to ensure primary keys are globally unique, so I
> use a function similar to the id_generator function in the shard
> example here:
>
> http://www.sqlalchemy.org/trac/browser/sqlalchemy/trunk/examples/sharding/attribute_shard.py
>
> and use that function in my Table definitions with the 'default' named
> argument.
>
> The problem with this approach, is that the shard_chooser function is
> called before the primary_key is created and set on the instance, and
> therefore raises a TypeError.
>
> return instance.user_id % SHARD_COUNT
> TypeError: unsupported operand type(s) for %: 'NoneType' and 'int'
>
> Is there any way around this issue, other than explicitly calling an
> id_generator function, and setting the primary key before the instance
> gets passed to the shard chooser?
I would call id_generator() *within* shard_chooser() if the pk is not
available, that way shard_chooser is guaranteed to have the info that
it needs. other options include MapperExtension.before_insert() but
I dont see that as any more convenient than just within shard_chooser.
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