SQLAlchemy 0.7 Documentation

Release: 0.7.10 | Release Date: February 7, 2013 | Download PDF

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Horizontal Sharding

Horizontal sharding support.

Defines a rudimental ‘horizontal sharding’ system which allows a Session to distribute queries and persistence operations across multiple databases.

For a usage example, see the Horizontal Sharding example included in the source distribution.

API Documentation

class sqlalchemy.ext.horizontal_shard.ShardedSession(shard_chooser, id_chooser, query_chooser, shards=None, query_cls=<class 'sqlalchemy.ext.horizontal_shard.ShardedQuery'>, **kwargs)
__init__(shard_chooser, id_chooser, query_chooser, shards=None, query_cls=<class 'sqlalchemy.ext.horizontal_shard.ShardedQuery'>, **kwargs)

Construct a ShardedSession.

Parameters:
  • shard_chooser – A callable which, passed a Mapper, a mapped instance, and possibly a SQL clause, returns a shard ID. This id may be based off of the attributes present within the object, or on some round-robin scheme. If the scheme is based on a selection, it should set whatever state on the instance to mark it in the future as participating in that shard.
  • id_chooser – A callable, passed a query and a tuple of identity values, which should return a list of shard ids where the ID might reside. The databases will be queried in the order of this listing.
  • query_chooser – For a given Query, returns the list of shard_ids where the query should be issued. Results from all shards returned will be combined together into a single listing.
  • shards – A dictionary of string shard names to Engine objects.
class sqlalchemy.ext.horizontal_shard.ShardedQuery(*args, **kwargs)
set_shard(shard_id)

return a new query, limited to a single shard ID.

all subsequent operations with the returned query will be against the single shard regardless of other state.