openstack-manuals/doc/config-ref-rst/source/compute/cells.rst

=====
Cells
=====

``Cells`` functionality enables you to scale an OpenStack Compute
cloud in a more distributed fashion without having to use complicated
technologies like database and message queue clustering.
It supports very large deployments.

When this functionality is enabled, the hosts in an OpenStack Compute
cloud are partitioned into groups called cells.
Cells are configured as a tree. The top-level cell should have a host
that runs a ``nova-api`` service, but no ``nova-compute`` services.
Each child cell should run all of the typical ``nova-*`` services in
a regular Compute cloud except for ``nova-api``. You can think of
cells as a normal Compute deployment in that each cell has its own
database server and message queue broker.

The ``nova-cells`` service handles communication between cells and
selects cells for new instances. This service is required for every
cell. Communication between cells is pluggable, and currently the
only option is communication through RPC.

Cells scheduling is separate from host scheduling.
``nova-cells`` first picks a cell. Once a cell is selected and the
new build request reaches its ``nova-cells`` service, it is sent
over to the host scheduler in that cell and the build proceeds as
it would have without cells.

.. warning::

   Cell functionality is currently considered experimental.

Cell configuration options
~~~~~~~~~~~~~~~~~~~~~~~~~~

Cells are disabled by default. All cell-related configuration
options appear in the ``[cells]`` section in ``nova.conf``.
The following cell-related options are currently supported:

enable
  Set to ``True`` to turn on cell functionality. Default is ``false``.

name
  Name of the current cell. Must be unique for each cell.

capabilities
  List of arbitrary ``key=value`` pairs defining capabilities of the current
  cell. Values include ``hypervisor=xenserver;kvm,os=linux;windows``.

call_timeout
  How long in seconds to wait for replies from calls between cells.

scheduler_filter_classes
  Filter classes that the cells scheduler should use.
  By default, uses ``nova.cells.filters.all_filters`` to map to
  all cells filters included with Compute.

scheduler_weight_classes
  Weight classes that the scheduler for cells uses. By default,
  uses ``nova.cells.weights.all_weighers`` to map to all cells
  weight algorithms included with Compute.

ram_weight_multiplier
  Multiplier used to weight RAM. Negative numbers indicate that
  Compute should stack VMs on one host instead of spreading out
  new VMs to more hosts in the cell. The default value is 10.0.

Configure the API (top-level) cell
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The cell type must be changed in the API cell so that requests can
be proxied through nova-cells down to the correct cell properly.
Edit the ``nova.conf`` file in the API cell, and specify ``api``
in the ``cell_type`` key:

.. code-block:: ini

   [DEFAULT]
   compute_api_class=nova.compute.cells_api.ComputeCellsAPI
   ...

   [cells]
   cell_type= api

Configure the child cells
~~~~~~~~~~~~~~~~~~~~~~~~~

Edit the ``nova.conf`` file in the child cells, and specify
``compute`` in the ``cell_type`` key:

.. code-block:: ini

   [DEFAULT]
   # Disable quota checking in child cells. Let API cell do it exclusively.
   quota_driver=nova.quota.NoopQuotaDriver

   [cells]
   cell_type = compute

Configure the database in each cell
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before bringing the services online, the database in each cell
needs to be configured with information about related cells.
In particular, the API cell needs to know about its immediate
children, and the child cells must know about their immediate agents.
The information needed is the ``RabbitMQ`` server credentials
for the particular cell.

Use the :command:`nova-manage cell create` command to add this
information to the database in each cell:

.. code-block:: console

   # nova-manage cell create -h
   usage: nova-manage cell create [-h] [--name <name>]
                                  [--cell_type <parent|api|child|compute>]
                                  [--username <username>] [--password <password>]
                                  [--broker_hosts <broker_hosts>]
                                  [--hostname <hostname>] [--port <number>]
                                  [--virtual_host <virtual_host>]
                                  [--woffset <float>] [--wscale <float>]

   optional arguments:
     -h, --help            show this help message and exit
     --name <name>         Name for the new cell
     --cell_type <parent|api|child|compute>
                           Whether the cell is parent/api or child/compute
     --username <username>
                           Username for the message broker in this cell
     --password <password>
                           Password for the message broker in this cell
     --broker_hosts <broker_hosts>
                           Comma separated list of message brokers in this cell.
                           Each Broker is specified as hostname:port with both
                           mandatory. This option overrides the --hostname and
                           --port options (if provided).
     --hostname <hostname>
                           Address of the message broker in this cell
     --port <number>       Port number of the message broker in this cell
     --virtual_host <virtual_host>
                           The virtual host of the message broker in this cell
     --woffset <float>
     --wscale <float>

As an example, assume an API cell named ``api`` and a child
cell named ``cell1``.

Within the ``api`` cell, specify the following ``RabbitMQ``
server information:

.. code-block:: ini

   rabbit_host=10.0.0.10
   rabbit_port=5672
   rabbit_username=api_user
   rabbit_password=api_passwd
   rabbit_virtual_host=api_vhost

Within the ``cell1`` child cell, specify the following
``RabbitMQ`` server information:

.. code-block:: ini

   rabbit_host=10.0.1.10
   rabbit_port=5673
   rabbit_username=cell1_user
   rabbit_password=cell1_passwd
   rabbit_virtual_host=cell1_vhost

You can run this in the API cell as root:

.. code-block:: console

   # nova-manage cell create --name cell1 --cell_type child \
     --username cell1_user --password cell1_passwd --hostname 10.0.1.10 \
     --port 5673 --virtual_host cell1_vhost --woffset 1.0 --wscale 1.0

Repeat the previous steps for all child cells.

In the child cell, run the following, as root:

.. code-block:: console

   # nova-manage cell create --name api --cell_type parent \
     --username api_user --password api_passwd --hostname 10.0.0.10 \
     --port 5672 --virtual_host api_vhost --woffset 1.0 --wscale 1.0

To customize the Compute cells, use the configuration
option settings documented in the table :ref:`nova-cells`.

Cell scheduling configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To determine the best cell to use to launch a new instance,
Compute uses a set of filters and weights defined in the
``/etc/nova/nova.conf`` file. The following options are
available to prioritize cells for scheduling:

scheduler_filter_classes
  List of filter classes. By default ``nova.cells.filters.all_filters``
  is specified, which maps to all cells filters included with Compute
  (see the section called :ref:`Filters <compute-scheduler-filters>`).

scheduler_weight_classes
  List of weight classes.
  By default ``nova.cells.weights.all_weighers`` is specified,
  which maps to all cell weight algorithms included with Compute.
  The following modules are available:

  * ``mute_child``. Downgrades the likelihood of child cells being chosen
    for scheduling requests, which haven't sent capacity or capability
    updates in a while. Options include ``mute_weight_multiplier``
    (multiplier for mute children; value should be negative).
  * ``ram_by_instance_type``. Select cells with the most RAM capacity
    for the instance type being requested. Because higher weights win,
    Compute returns the number of available units for the instance type
    requested. The ``ram_weight_multiplier`` option defaults to 10.0
    that adds to the weight by a factor of 10.
    Use a negative number to stack VMs on one host instead of spreading
    out new VMs to more hosts in the cell.
  * ``weight_offset``. Allows modifying the database to weight a
    particular cell. You can use this when you want to disable a
    cell (for example, '0'), or to set a default cell by making its
    ``weight_offset`` very high (for example, '999999999999999').
    The highest weight will be the first cell to be scheduled for
    launching an instance.

Additionally, the following options are available for the cell scheduler:

scheduler_retries
  Specifies how many times the scheduler tries to launch a new
  instance when no cells are available (default=10).

scheduler_retry_delay
  Specifies the delay (in seconds) between retries (default=2).

As an admin user, you can also add a filter that directs builds to
a particular cell. The ``policy.json`` file must have a line with
``"cells_scheduler_filter:TargetCellFilter" : "is_admin:True"``
to let an admin user specify a scheduler hint to direct a build to
a particular cell.

Optional cell configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cells store all inter-cell communication data, including user names
and passwords, in the database. Because the cells data is not updated
very frequently, use the ``[cells]cells_config`` option to specify
a JSON file to store cells data. With this configuration,
the database is no longer consulted when reloading the cells data.
The file must have columns present in the Cell model (excluding
common database fields and the ``id`` column). You must specify the
queue connection information through a ``transport_url`` field,
instead of ``username``, ``password``, and so on.
The ``transport_url`` has the following form:

.. code-block:: console

   rabbit://USERNAME:PASSWORD@HOSTNAME:PORT/VIRTUAL_HOST

The scheme can be either ``qpid`` or ``rabbit``, as shown previously.
The following sample shows this optional configuration:

.. code-block:: json

   {
       "parent": {
           "name": "parent",
           "api_url": "http://api.example.com:8774",
           "transport_url": "rabbit://rabbit.example.com",
           "weight_offset": 0.0,
           "weight_scale": 1.0,
           "is_parent": true
       },
       "cell1": {
           "name": "cell1",
           "api_url": "http://api.example.com:8774",
           "transport_url": "rabbit://rabbit1.example.com",
           "weight_offset": 0.0,
           "weight_scale": 1.0,
           "is_parent": false
       },
       "cell2": {
           "name": "cell2",
           "api_url": "http://api.example.com:8774",
           "transport_url": "rabbit://rabbit2.example.com",
           "weight_offset": 0.0,
           "weight_scale": 1.0,
           "is_parent": false
       }
   }