Release: 1.4.39 | Release Date: June 24, 2022

# SQLAlchemy 1.4 Documentation

# Horizontal Sharding

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 distribution.

## API Documentation

Object Name | Description
--- | ---
ShardedQuery |
ShardedSession |

class sqlalchemy.ext.horizontal_shard.ShardedSession(shard_chooser, id_chooser, execute_chooser=None, shards=None, query_cls=<class 'sqlalchemy.ext.horizontal_shard.ShardedQuery'>, **kwargs)

  Class signature

  class `sqlalchemy.ext.horizontal_shard.ShardedSession` (`sqlalchemy.orm.session.Session`)

  method `sqlalchemy.ext.horizontal_shard.ShardedSession.`__init__(shard_chooser, id_chooser, execute_chooser=None, shards=None, query_cls=<class 'sqlalchemy.ext.horizontal_shard.ShardedQuery'>, **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.

      - execute_chooser –

        For a given `ORMExecuteState`, 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.

        Changed in version 1.4: The `execute_chooser` parameter supersedes the `query_chooser` parameter.

      - shards – A dictionary of string shard names to `Engine` objects.

  method `sqlalchemy.ext.horizontal_shard.ShardedSession.`connection_callable(mapper=None, instance=None, shard_id=None, **kwargs)

    Provide a `Connection` to use in the unit of work flush process.

  method `sqlalchemy.ext.horizontal_shard.ShardedSession.`get_bind(mapper=None, shard_id=None, instance=None, clause=None, **kw)

    Return a “bind” to which this `Session` is bound.

    The “bind” is usually an instance of `Engine`, except in the case where the `Session` has been explicitly bound directly to a `Connection`.

    For a multiply-bound or unbound `Session`, the `mapper` or `clause` arguments are used to determine the appropriate bind to return.

    Note that the “mapper” argument is usually present when `Session.get_bind()` is called via an ORM operation such as a `Session.query()`, each individual INSERT/UPDATE/DELETE operation within a `Session.flush()`, call, etc.

    The order of resolution is:

    1. if mapper given and `Session.binds` is present, locate a bind based first on the mapper in use, then on the mapped class in use, then on any base classes that are present in the `__mro__` of the mapped class, from more specific superclasses to more general.

    2. if clause given and `Session.binds` is present, locate a bind based on `Table` objects found in the given clause present in `Session.binds`.

    3. if `Session.binds` is present, return that.

    4. if clause given, attempt to return a bind linked to the `MetaData` ultimately associated with the clause.

    5. if mapper given, attempt to return a bind linked to the `MetaData` ultimately associated with the `Table` or other selectable to which the mapper is mapped.

    6. No bind can be found, `UnboundExecutionError` is raised.

    Note that the `Session.get_bind()` method can be overridden on a user-defined subclass of `Session` to provide any kind of bind resolution scheme. See the example at Custom Vertical Partitioning.

    Parameters

      - mapper – Optional `mapper()` mapped class or instance of `Mapper`. The bind can be derived from a `Mapper` first by consulting the “binds” map associated with this `Session`, and secondly by consulting the `MetaData` associated with the `Table` to which the `Mapper` is mapped for a bind.

      - clause – A `ClauseElement` (i.e. `select()`, `text()`, etc.). If the `mapper` argument is not present or could not produce a bind, the given expression construct will be searched for a bound element, typically a `Table` associated with bound `MetaData`.

    See also

    Partitioning Strategies (e.g. multiple database backends per Session)

    `Session.binds`

    `Session.bind_mapper()`

    `Session.bind_table()`

class sqlalchemy.ext.horizontal_shard.ShardedQuery(*args, **kwargs)

  Class signature

  class `sqlalchemy.ext.horizontal_shard.ShardedQuery` (`sqlalchemy.orm.Query`)

  method `sqlalchemy.ext.horizontal_shard.ShardedQuery.`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.

    The shard_id can be passed for a 2.0 style execution to the bind_arguments dictionary of `Session.execute()`:

        results = session.execute(
            stmt,
            bind_arguments={"shard_id": "my_shard"}
        )