197 lines
7.2 KiB
Python
197 lines
7.2 KiB
Python
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# ext/horizontal_shard.py
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# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
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# <see AUTHORS file>
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#
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# This module is part of SQLAlchemy and is released under
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# the MIT License: http://www.opensource.org/licenses/mit-license.php
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"""Horizontal sharding support.
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Defines a rudimental 'horizontal sharding' system which allows a Session to
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distribute queries and persistence operations across multiple databases.
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For a usage example, see the :ref:`examples_sharding` example included in
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the source distribution.
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"""
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from .. import inspect
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from .. import util
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from ..orm.session import Session
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from ..orm.query import Query
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__all__ = ['ShardedSession', 'ShardedQuery']
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class ShardedQuery(Query):
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def __init__(self, *args, **kwargs):
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super(ShardedQuery, self).__init__(*args, **kwargs)
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self.id_chooser = self.session.id_chooser
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self.query_chooser = self.session.query_chooser
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self._shard_id = None
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def set_shard(self, shard_id):
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"""return a new query, limited to a single shard ID.
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all subsequent operations with the returned query will
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be against the single shard regardless of other state.
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"""
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q = self._clone()
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q._shard_id = shard_id
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return q
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def _execute_and_instances(self, context):
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def iter_for_shard(shard_id):
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context.attributes['shard_id'] = context.identity_token = shard_id
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result = self._connection_from_session(
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mapper=self._mapper_zero(),
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shard_id=shard_id).execute(
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context.statement,
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self._params)
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return self.instances(result, context)
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if self._shard_id is not None:
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return iter_for_shard(self._shard_id)
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else:
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partial = []
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for shard_id in self.query_chooser(self):
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partial.extend(iter_for_shard(shard_id))
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# if some kind of in memory 'sorting'
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# were done, this is where it would happen
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return iter(partial)
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@classmethod
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def _identity_lookup(
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cls, session, mapper, primary_key_identity, identity_token=None,
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**kw):
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"""override the default Query._identity_lookup method so that we
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search for a given non-token primary key identity across all
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possible identity tokens (e.g. shard ids).
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"""
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if identity_token is not None:
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return super(ShardedQuery, cls)._identity_lookup(
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session, mapper, primary_key_identity,
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identity_token=identity_token,
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**kw
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)
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else:
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q = cls([mapper], session)
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for shard_id in q.id_chooser(q, primary_key_identity):
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obj = super(ShardedQuery, cls)._identity_lookup(
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session, mapper, primary_key_identity,
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identity_token=shard_id,
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**kw
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)
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if obj is not None:
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return obj
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return None
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def _get_impl(
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self, primary_key_identity, db_load_fn, identity_token=None):
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"""Override the default Query._get_impl() method so that we emit
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a query to the DB for each possible identity token, if we don't
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have one already.
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"""
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def _db_load_fn(query, primary_key_identity):
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# load from the database. The original db_load_fn will
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# use the given Query object to load from the DB, so our
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# shard_id is what will indicate the DB that we query from.
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if self._shard_id is not None:
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return db_load_fn(self, primary_key_identity)
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else:
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ident = util.to_list(primary_key_identity)
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# build a ShardedQuery for each shard identifier and
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# try to load from the DB
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for shard_id in self.id_chooser(self, ident):
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q = self.set_shard(shard_id)
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o = db_load_fn(q, ident)
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if o is not None:
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return o
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else:
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return None
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if identity_token is None and self._shard_id is not None:
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identity_token = self._shard_id
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return super(ShardedQuery, self)._get_impl(
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primary_key_identity, _db_load_fn, identity_token=identity_token)
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class ShardedSession(Session):
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def __init__(self, shard_chooser, id_chooser, query_chooser, shards=None,
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query_cls=ShardedQuery, **kwargs):
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"""Construct a ShardedSession.
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:param shard_chooser: A callable which, passed a Mapper, a mapped
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instance, and possibly a SQL clause, returns a shard ID. This id
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may be based off of the attributes present within the object, or on
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some round-robin scheme. If the scheme is based on a selection, it
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should set whatever state on the instance to mark it in the future as
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participating in that shard.
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:param id_chooser: A callable, passed a query and a tuple of identity
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values, which should return a list of shard ids where the ID might
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reside. The databases will be queried in the order of this listing.
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:param query_chooser: For a given Query, returns the list of shard_ids
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where the query should be issued. Results from all shards returned
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will be combined together into a single listing.
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:param shards: A dictionary of string shard names
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to :class:`~sqlalchemy.engine.Engine` objects.
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"""
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super(ShardedSession, self).__init__(query_cls=query_cls, **kwargs)
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self.shard_chooser = shard_chooser
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self.id_chooser = id_chooser
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self.query_chooser = query_chooser
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self.__binds = {}
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self.connection_callable = self.connection
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if shards is not None:
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for k in shards:
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self.bind_shard(k, shards[k])
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def _choose_shard_and_assign(self, mapper, instance, **kw):
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if instance is not None:
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state = inspect(instance)
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if state.key:
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token = state.key[2]
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assert token is not None
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return token
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elif state.identity_token:
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return state.identity_token
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shard_id = self.shard_chooser(mapper, instance, **kw)
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if instance is not None:
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state.identity_token = shard_id
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return shard_id
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def connection(self, mapper=None, instance=None, shard_id=None, **kwargs):
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if shard_id is None:
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shard_id = self._choose_shard_and_assign(mapper, instance)
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if self.transaction is not None:
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return self.transaction.connection(mapper, shard_id=shard_id)
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else:
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return self.get_bind(
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mapper,
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shard_id=shard_id,
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instance=instance
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).contextual_connect(**kwargs)
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def get_bind(self, mapper, shard_id=None,
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instance=None, clause=None, **kw):
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if shard_id is None:
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shard_id = self._choose_shard_and_assign(
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mapper, instance, clause=clause)
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return self.__binds[shard_id]
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def bind_shard(self, shard_id, bind):
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self.__binds[shard_id] = bind
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