Current File : //opt/alt/python37/lib64/python3.7/site-packages/sqlalchemy/ext/horizontal_shard.py
# ext/horizontal_shard.py
# Copyright (C) 2005-2019 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# 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 distribution.
"""
from .. import inspect
from .. import util
from ..orm.query import Query
from ..orm.session import Session
__all__ = ["ShardedSession", "ShardedQuery"]
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):
def iter_for_shard(shard_id):
context.attributes["shard_id"] = context.identity_token = shard_id
result = self._connection_from_session(
mapper=self._bind_mapper(), shard_id=shard_id
).execute(context.statement, self._params)
return self.instances(result, context)
if context.identity_token is not None:
return iter_for_shard(context.identity_token)
elif self._shard_id is not None:
return iter_for_shard(self._shard_id)
else:
partial = []
for shard_id in self.query_chooser(self):
partial.extend(iter_for_shard(shard_id))
# if some kind of in memory 'sorting'
# were done, this is where it would happen
return iter(partial)
def _execute_crud(self, stmt, mapper):
def exec_for_shard(shard_id):
conn = self._connection_from_session(
mapper=mapper,
shard_id=shard_id,
clause=stmt,
close_with_result=True,
)
result = conn.execute(stmt, self._params)
return result
if self._shard_id is not None:
return exec_for_shard(self._shard_id)
else:
rowcount = 0
results = []
for shard_id in self.query_chooser(self):
result = exec_for_shard(shard_id)
rowcount += result.rowcount
results.append(result)
return ShardedResult(results, rowcount)
def _identity_lookup(
self,
mapper,
primary_key_identity,
identity_token=None,
lazy_loaded_from=None,
**kw
):
"""override the default Query._identity_lookup method so that we
search for a given non-token primary key identity across all
possible identity tokens (e.g. shard ids).
"""
if identity_token is not None:
return super(ShardedQuery, self)._identity_lookup(
mapper,
primary_key_identity,
identity_token=identity_token,
**kw
)
else:
q = self.session.query(mapper)
if lazy_loaded_from:
q = q._set_lazyload_from(lazy_loaded_from)
for shard_id in self.id_chooser(q, primary_key_identity):
obj = super(ShardedQuery, self)._identity_lookup(
mapper, primary_key_identity, identity_token=shard_id, **kw
)
if obj is not None:
return obj
return None
def _get_impl(self, primary_key_identity, db_load_fn, identity_token=None):
"""Override the default Query._get_impl() method so that we emit
a query to the DB for each possible identity token, if we don't
have one already.
"""
def _db_load_fn(query, primary_key_identity):
# load from the database. The original db_load_fn will
# use the given Query object to load from the DB, so our
# shard_id is what will indicate the DB that we query from.
if self._shard_id is not None:
return db_load_fn(self, primary_key_identity)
else:
ident = util.to_list(primary_key_identity)
# build a ShardedQuery for each shard identifier and
# try to load from the DB
for shard_id in self.id_chooser(self, ident):
q = self.set_shard(shard_id)
o = db_load_fn(q, ident)
if o is not None:
return o
else:
return None
if identity_token is None and self._shard_id is not None:
identity_token = self._shard_id
return super(ShardedQuery, self)._get_impl(
primary_key_identity, _db_load_fn, identity_token=identity_token
)
class ShardedResult(object):
"""A value object that represents multiple :class:`.ResultProxy` objects.
This is used by the :meth:`.ShardedQuery._execute_crud` hook to return
an object that takes the place of the single :class:`.ResultProxy`.
Attribute include ``result_proxies``, which is a sequence of the
actual :class:`.ResultProxy` objects, as well as ``aggregate_rowcount``
or ``rowcount``, which is the sum of all the individual rowcount values.
.. versionadded:: 1.3
"""
__slots__ = ("result_proxies", "aggregate_rowcount")
def __init__(self, result_proxies, aggregate_rowcount):
self.result_proxies = result_proxies
self.aggregate_rowcount = aggregate_rowcount
@property
def rowcount(self):
return self.aggregate_rowcount
class ShardedSession(Session):
def __init__(
self,
shard_chooser,
id_chooser,
query_chooser,
shards=None,
query_cls=ShardedQuery,
**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.Engine` objects.
"""
super(ShardedSession, self).__init__(query_cls=query_cls, **kwargs)
self.shard_chooser = shard_chooser
self.id_chooser = id_chooser
self.query_chooser = query_chooser
self.__binds = {}
self.connection_callable = self.connection
if shards is not None:
for k in shards:
self.bind_shard(k, shards[k])
def _choose_shard_and_assign(self, mapper, instance, **kw):
if instance is not None:
state = inspect(instance)
if state.key:
token = state.key[2]
assert token is not None
return token
elif state.identity_token:
return state.identity_token
shard_id = self.shard_chooser(mapper, instance, **kw)
if instance is not None:
state.identity_token = shard_id
return shard_id
def connection(self, mapper=None, instance=None, shard_id=None, **kwargs):
if shard_id is None:
shard_id = self._choose_shard_and_assign(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._choose_shard_and_assign(
mapper, instance, clause=clause
)
return self.__binds[shard_id]
def bind_shard(self, shard_id, bind):
self.__binds[shard_id] = bind
Mini Shell