Cells

Cells are disabled by default. All cell-related configuration options go under a [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 to nova.conf in the API cell:

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

[cells]
enable=True
name=api

Add the following 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 we have an API cell named api and a child cell named cell1. Within the api cell, we have the following RabbitMQ server info:

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

And in the child cell named cell1 we have the following RabbitMQ server info:

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

We would 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 above for all child cells.

In the child cell, we would 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 2.18, “Description of configuration options for cells”.

To determine the best cell for launching a new instance, Compute uses a set of filters and weights configured in /etc/nova/nova.conf. 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 currently keeps all inter-cell communication data, including user names and passwords, in the database. This is undesirable and unnecessary since cells data isn't updated very frequently. Instead, create a JSON file to input cells data specified via a [cells]cells_config option. When specified, the database is no longer consulted when reloading cells data. The file will need the columns present in the Cell model (excluding common database fields and the id column). The queue connection information must be specified 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
    }
}