# SQLAlchemy 0.8 Documentation

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Release: 0.8.1 | Release Date: April 27, 2013

# 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.