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===========================
Set up database replication
===========================
You can create a replica of an existing database instance. When you make
subsequent changes to the original instance, the system automatically
applies those changes to the replica.
- Replicas are read-only.
- When you create a replica, do not specify the ``--users`` or
``--databases`` options.
- You can choose a smaller volume or flavor for a replica than for the
original, but the replica's volume must be big enough to hold the
data snapshot from the original.
This example shows you how to replicate a MySQL database instance.
Set up replication
~~~~~~~~~~~~~~~~~~
#. **Get the instance ID**
Get the ID of the original instance you want to replicate:
.. code-block:: console
$ openstack database instance list
+-----------+------------+-----------+-------------------+--------+-----------+------+
| id | name | datastore | datastore_version | status | flavor_id | size |
+-----------+------------+-----------+-------------------+--------+-----------+------+
| 97b...ae6 | base_1 | mysql | mysql-5.5 | ACTIVE | 10 | 2 |
+-----------+------------+-----------+-------------------+--------+-----------+------+
#. **Create the replica**
Create a new instance that will be a replica of the original
instance. You do this by passing in the ``--replica_of`` option with
the :command:`openstack database instance create` command. This example creates a replica
called ``replica_1``. ``replica_1`` is a replica of the original instance,
``base_1``:
.. code-block:: console
$ openstack database instance create replica_1 6 --size=5 --nic net-id=$netid \
--datastore_version mysql-5.5 \
--datastore mysql --replica_of ID_OF_ORIGINAL_INSTANCE
#. **Verify replication status**
Pass in ``replica_1``'s instance ID with the :command:`openstack database instance show` command
to verify that the newly created ``replica_1`` instance is a replica
of the original ``base_1``. Note that the ``replica_of`` property is
set to the ID of ``base_1``.
.. code-block:: console
$ openstack database instance show INSTANCE_ID_OF_REPLICA_1
+-------------------+--------------------------------------+
| Property | Value |
+-------------------+--------------------------------------+
| created | 2014-09-16T11:16:49 |
| datastore | mysql |
| datastore_version | mysql-5.5 |
| flavor | 6 |
| id | 49c6eff6-ef91-4eff-91c0-efbda7e83c38 |
| name | replica_1 |
| replica_of | 97b4b853-80f6-414f-ba6f-c6f455a79ae6 |
| status | BUILD |
| updated | 2014-09-16T11:16:49 |
| volume | 5 |
+-------------------+--------------------------------------+
Now pass in ``base_1``'s instance ID with the :command:`openstack database instance show` command
to list the replica(s) associated with the original instance. Note
that the ``replicas`` property is set to the ID of ``replica_1``. If
there are multiple replicas, they appear as a comma-separated list.
.. code-block:: console
$ openstack database instance show INSTANCE_ID_OF_BASE_1
+-------------------+--------------------------------------+
| Property | Value |
+-------------------+--------------------------------------+
| created | 2014-09-16T11:04:56 |
| datastore | mysql |
| datastore_version | mysql-5.5 |
| flavor | 6 |
| id | 97b4b853-80f6-414f-ba6f-c6f455a79ae6 |
| ip | 172.16.200.2 |
| name | base_1 |
| replicas | 49c6eff6-ef91-4eff-91c0-efbda7e83c38 |
| status | ACTIVE |
| updated | 2014-09-16T11:05:06 |
| volume | 5 |
| volume_used | 0.11 |
+-------------------+--------------------------------------+
#. **Detach the replica**
If the original instance goes down, you can detach the replica. The
replica becomes a standalone database instance. You can then take the
new standalone instance and create a new replica of that instance.
You detach a replica using the :command:`openstack database instance detach replica` command:
.. code-block:: console
$ openstack database instance detach replica INSTANCE_ID_OF_REPLICA
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