# Connections

Connections aim to ease the use of multiple sessions with cqlengine. Connections can be set on a model class, per query or using a context manager.

## Register a new connection

To use cqlengine, you need at least a default connection. If you initialize cqlengine’s connections with with `connection.setup`, a connection will be created automatically. If you want to use another cluster/session, you need to register a new cqlengine connection. You register a connection with `register_connection()`:

    from cassandra.cqlengine import connection

    connection.setup(['127.0.0.1')
    connection.register_connection('cluster2', ['127.0.0.2'])

`register_connection()` can take a list of hosts, as shown above, in which case it will create a connection with a new session. It can also take a session argument if you’ve already created a session:

    from cassandra.cqlengine import connection
    from cassandra.cluster import Cluster

    session = Cluster(['127.0.0.1']).connect()
    connection.register_connection('cluster3', session=session)

## Change the default connection

You can change the default cqlengine connection on registration:

    from cassandra.cqlengine import connection

    connection.register_connection('cluster2', ['127.0.0.2'] default=True)

or on the fly using `set_default_connection()`

    connection.set_default_connection('cluster2')

## Unregister a connection

You can unregister a connection using `unregister_connection()`:

    connection.unregister_connection('cluster2')

## Management

When using multiples connections, you also need to sync your models on all connections (and keyspaces) that you need operate on. Management commands have been improved to ease this part. Here is an example:

    from cassandra.cqlengine import management

    keyspaces = ['ks1', 'ks2']
    conns = ['cluster1', 'cluster2']

    # registers your connections
    # ...

    # create all keyspaces on all connections
    for ks in keyspaces:
        management.create_simple_keyspace(ks, connections=conns)

    # define your Automobile model
    # ...

    # sync your models
    management.sync_table(Automobile, keyspaces=keyspaces, connections=conns)

## Connection Selection

cqlengine will select the default connection, unless your specify a connection using one of the following methods.

### Default Model Connection

You can specify a default connection per model:

    class Automobile(Model):
        __keyspace__ = 'test'
        __connection__ = 'cluster2'
        manufacturer = columns.Text(primary_key=True)
        year = columns.Integer(primary_key=True)
        model = columns.Text(primary_key=True)

    print len(Automobile.objects.all())  # executed on the connection 'cluster2'

### QuerySet and model instance

You can use the `using()` method to select a connection (or keyspace):

    Automobile.objects.using(connection='cluster1').create(manufacturer='honda', year=2010, model='civic')
    q = Automobile.objects.filter(manufacturer='Tesla')
    autos = q.using(keyspace='ks2', connection='cluster2').all()

    for auto in autos:
        auto.using(connection='cluster1').save()

### Context Manager

You can use the ContextQuery as well to select a connection:

    with ContextQuery(Automobile, connection='cluster1') as A:
        A.objects.filter(manufacturer='honda').all()  # executed on 'cluster1'

### BatchQuery

With a BatchQuery, you can select the connection with the context manager. Note that all operations in the batch need to use the same connection.

    with BatchQuery(connection='cluster1') as b:
        Automobile.objects.batch(b).create(manufacturer='honda', year=2010, model='civic')