Horizontal Shard

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 distrbution.

API Documentation

class sqlalchemy.ext.horizontal_shard.ShardedSession(shard_chooser, id_chooser, query_chooser, shards=None, **kwargs)

__init__(shard_chooser, id_chooser, query_chooser, shards=None, **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)

__init__(*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.