Cells

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:

The compute API class must be changed in the API cell so that requests can be proxied through nova-cells down to the correct cell properly. Add the following line to nova.conf in the API cell:

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

[cells]
enable=True
name=api

Add the following lines to nova.conf in the child cells, replacing cell1 with the name of each cell:

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

[cells]
enable=True
name=cell1

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

# nova-manage cell create -h
Options:
  -h, --help            show this help message and exit
  --name=<name>         Name for the new cell
  --cell_type=<parent|child>
                        Whether the cell is a parent or child
  --username=<username>
                        Username for the message broker in this cell
  --password=<password>
                        Password for the message broker in this cell
  --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>
                        (weight offset) It might be used by some cell scheduling code in the future
  --wscale=<float>
                        (weight scale) It might be used by some cell scheduling code in the future

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:

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:

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:

# 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:

# 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 Table 2.19, “Description of cell configuration options”.

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:

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

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.

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:

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:

{
    "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
    }
}