- The sqlalchemy.orm.shard module now becomes an extension,

sqlalchemy.ext.horizontal_shard.   The old import
works with a deprecation warning.

1 files changed, 125 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/horizontal_shard.py b/lib/sqlalchemy/ext/horizontal_shard.py
new file mode 100644
index 000000000..78e3f5953
--- /dev/null
+++ b/lib/sqlalchemy/ext/horizontal_shard.py
@@ -0,0 +1,125 @@
+# 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
+