# horizontal_shard.py
# Copyright (C) the SQLAlchemy authors and contributors
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""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 :ref:`examples_sharding` example included in
the source distrbution.
"""
import sqlalchemy.exceptions as sa_exc
from sqlalchemy import util
from sqlalchemy.orm.session import Session
from sqlalchemy.orm.query import Query
__all__ = ['ShardedSession', 'ShardedQuery']
class ShardedSession(Session):
def __init__(self, shard_chooser, id_chooser, query_chooser, shards=None, **kwargs):
"""Construct a ShardedSession.
:param 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.
:param 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.
:param 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.
:param shards: A dictionary of string shard names to :class:`~sqlalchemy.engine.base.Engine`
objects.
"""
super(ShardedSession, self).__init__(**kwargs)
self.shard_chooser = shard_chooser
self.id_chooser = id_chooser
self.query_chooser = query_chooser
self.__binds = {}
self._mapper_flush_opts = {'connection_callable':self.connection}
self._query_cls = ShardedQuery
if shards is not None:
for k in shards:
self.bind_shard(k, shards[k])
def connection(self, mapper=None, instance=None, shard_id=None, **kwargs):
if shard_id is None:
shard_id = self.shard_chooser(mapper, instance)
if self.transaction is not None:
return self.transaction.connection(mapper, shard_id=shard_id)
else:
return self.get_bind(mapper,
shard_id=shard_id,
instance=instance).contextual_connect(**kwargs)
def get_bind(self, mapper, shard_id=None, instance=None, clause=None, **kw):
if shard_id is None:
shard_id = self.shard_chooser(mapper, instance, clause=clause)
return self.__binds[shard_id]
def bind_shard(self, shard_id, bind):
self.__binds[shard_id] = bind
class ShardedQuery(Query):
def __init__(self, *args, **kwargs):
super(ShardedQuery, self).__init__(*args, **kwargs)
self.id_chooser = self.session.id_chooser
self.query_chooser = self.session.query_chooser
self._shard_id = None
def set_shard(self, 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.
"""
q = self._clone()
q._shard_id = shard_id
return q
def _execute_and_instances(self, context):
if self._shard_id is not None:
result = self.session.connection(
mapper=self._mapper_zero(),
shard_id=self._shard_id).execute(context.statement, self._params)
return self.instances(result, context)
else:
partial = []
for shard_id in self.query_chooser(self):
result = self.session.connection(
mapper=self._mapper_zero(),
shard_id=shard_id).execute(context.statement, self._params)
partial = partial + list(self.instances(result, context))
# if some kind of in memory 'sorting'
# were done, this is where it would happen
return iter(partial)
def get(self, ident, **kwargs):
if self._shard_id is not None:
return super(ShardedQuery, self).get(ident)
else:
ident = util.to_list(ident)
for shard_id in self.id_chooser(self, ident):
o = self.set_shard(shard_id).get(ident, **kwargs)
if o is not None:
return o
else:
return None
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