Writing Cloud Driver Modules

Salt Cloud runs on a module system similar to the main Salt project. The modules inside saltcloud exist in the salt/cloud/clouds directory of the salt source.

There are two basic types of cloud modules. If a cloud host is supported by libcloud, then using it is the fastest route to getting a module written. The Apache Libcloud project is located at:

http://libcloud.apache.org/

Not every cloud host is supported by libcloud. Additionally, not every feature in a supported cloud host is necessarily supported by libcloud. In either of these cases, a module can be created which does not rely on libcloud.

All Driver Modules

The following functions are required by all driver modules, whether or not they are based on libcloud.

The __virtual__() Function

This function determines whether or not to make this cloud module available upon execution. Most often, it uses get_configured_provider() to determine if the necessary configuration has been set up. It may also check for necessary imports, to decide whether to load the module. In most cases, it will return a True or False value. If the name of the driver used does not match the filename, then that name should be returned instead of True. An example of this may be seen in the Azure module:

https://github.com/saltstack/salt/tree/develop/salt/cloud/clouds/msazure.py

The get_configured_provider() Function

This function uses config.is_provider_configured() to determine wither all required information for this driver has been configured. The last value in the list of required settings should be followed by a comma.

Libcloud Based Modules

Writing a cloud module based on libcloud has two major advantages. First of all, much of the work has already been done by the libcloud project. Second, most of the functions necessary to Salt have already been added to the Salt Cloud project.

The create() Function

The most important function that does need to be manually written is the create() function. This is what is used to request a virtual machine to be created by the cloud host, wait for it to become available, and then (optionally) log in and install Salt on it.

A good example to follow for writing a cloud driver module based on libcloud is the module provided for Linode:

https://github.com/saltstack/salt/tree/develop/salt/cloud/clouds/linode.py

The basic flow of a create() function is as follows:

  • Send a request to the cloud host to create a virtual machine.
  • Wait for the virtual machine to become available.
  • Generate kwargs to be used to deploy Salt.
  • Log into the virtual machine and deploy Salt.
  • Return a data structure that describes the newly-created virtual machine.

At various points throughout this function, events may be fired on the Salt event bus. Four of these events, which are described below, are required. Other events may be added by the user, where appropriate.

When the create() function is called, it is passed a data structure called vm_. This dict contains a composite of information describing the virtual machine to be created. A dict called __opts__ is also provided by Salt, which contains the options used to run Salt Cloud, as well as a set of configuration and environment variables.

The first thing the create() function must do is fire an event stating that it has started the create process. This event is tagged salt/cloud/<vm name>/creating. The payload contains the names of the VM, profile, and provider.

A set of kwargs is then usually created, to describe the parameters required by the cloud host to request the virtual machine.

An event is then fired to state that a virtual machine is about to be requested. It is tagged as salt/cloud/<vm name>/requesting. The payload contains most or all of the parameters that will be sent to the cloud host. Any private information (such as passwords) should not be sent in the event.

After a request is made, a set of deploy kwargs will be generated. These will be used to install Salt on the target machine. Windows options are supported at this point, and should be generated, even if the cloud host does not currently support Windows. This will save time in the future if the host does eventually decide to support Windows.

An event is then fired to state that the deploy process is about to begin. This event is tagged salt/cloud/<vm name>/deploying. The payload for the event will contain a set of deploy kwargs, useful for debugging purposed. Any private data, including passwords and keys (including public keys) should be stripped from the deploy kwargs before the event is fired.

If any Windows options have been passed in, the salt.utils.cloud.deploy_windows() function will be called. Otherwise, it will be assumed that the target is a Linux or Unix machine, and the salt.utils.cloud.deploy_script() will be called.

Both of these functions will wait for the target machine to become available, then the necessary port to log in, then a successful login that can be used to install Salt. Minion configuration and keys will then be uploaded to a temporary directory on the target by the appropriate function. On a Windows target, the Windows Minion Installer will be run in silent mode. On a Linux/Unix target, a deploy script (bootstrap-salt.sh, by default) will be run, which will auto-detect the operating system, and install Salt using its native package manager. These do not need to be handled by the developer in the cloud module.

The salt.utils.cloud.validate_windows_cred() function has been extended to take the number of retries and retry_delay parameters in case a specific cloud host has a delay between providing the Windows credentials and the credentials being available for use. In their create() function, or as a sub-function called during the creation process, developers should use the win_deploy_auth_retries and win_deploy_auth_retry_delay parameters from the provider configuration to allow the end-user the ability to customize the number of tries and delay between tries for their particular host.

After the appropriate deploy function completes, a final event is fired which describes the virtual machine that has just been created. This event is tagged salt/cloud/<vm name>/created. The payload contains the names of the VM, profile, and provider.

Finally, a dict (queried from the provider) which describes the new virtual machine is returned to the user. Because this data is not fired on the event bus it can, and should, return any passwords that were returned by the cloud host. In some cases (for example, Rackspace), this is the only time that the password can be queried by the user; post-creation queries may not contain password information (depending upon the host).

The libcloudfuncs Functions

A number of other functions are required for all cloud hosts. However, with libcloud-based modules, these are all provided for free by the libcloudfuncs library. The following two lines set up the imports:

from salt.cloud.libcloudfuncs import *   # pylint: disable=W0614,W0401
import salt.utils.functools

And then a series of declarations will make the necessary functions available within the cloud module.

get_size = salt.utils.functools.namespaced_function(get_size, globals())
get_image = salt.utils.functools.namespaced_function(get_image, globals())
avail_locations = salt.utils.functools.namespaced_function(avail_locations, globals())
avail_images = salt.utils.functools.namespaced_function(avail_images, globals())
avail_sizes = salt.utils.functools.namespaced_function(avail_sizes, globals())
script = salt.utils.functools.namespaced_function(script, globals())
destroy = salt.utils.functools.namespaced_function(destroy, globals())
list_nodes = salt.utils.functools.namespaced_function(list_nodes, globals())
list_nodes_full = salt.utils.functools.namespaced_function(list_nodes_full, globals())
list_nodes_select = salt.utils.functools.namespaced_function(list_nodes_select, globals())
show_instance = salt.utils.functools.namespaced_function(show_instance, globals())

If necessary, these functions may be replaced by removing the appropriate declaration line, and then adding the function as normal.

These functions are required for all cloud modules, and are described in detail in the next section.

Non-Libcloud Based Modules

In some cases, using libcloud is not an option. This may be because libcloud has not yet included the necessary driver itself, or it may be that the driver that is included with libcloud does not contain all of the necessary features required by the developer. When this is the case, some or all of the functions in libcloudfuncs may be replaced. If they are all replaced, the libcloud imports should be absent from the Salt Cloud module.

A good example of a non-libcloud driver is the DigitalOcean driver:

https://github.com/saltstack/salt/tree/develop/salt/cloud/clouds/digitalocean.py

The create() Function

The create() function must be created as described in the libcloud-based module documentation.

The get_size() Function

This function is only necessary for libcloud-based modules, and does not need to exist otherwise.

The get_image() Function

This function is only necessary for libcloud-based modules, and does not need to exist otherwise.

The avail_locations() Function

This function returns a list of locations available, if the cloud host uses multiple data centers. It is not necessary if the cloud host uses only one data center. It is normally called using the --list-locations option.

salt-cloud --list-locations my-cloud-provider

The avail_images() Function

This function returns a list of images available for this cloud provider. There are not currently any known cloud providers that do not provide this functionality, though they may refer to images by a different name (for example, "templates"). It is normally called using the --list-images option.

salt-cloud --list-images my-cloud-provider

The avail_sizes() Function

This function returns a list of sizes available for this cloud provider. Generally, this refers to a combination of RAM, CPU, and/or disk space. This functionality may not be present on some cloud providers. For example, the Parallels module breaks down RAM, CPU, and disk space into separate options, whereas in other providers, these options are baked into the image. It is normally called using the --list-sizes option.

salt-cloud --list-sizes my-cloud-provider

The script() Function

This function builds the deploy script to be used on the remote machine. It is likely to be moved into the salt.utils.cloud library in the near future, as it is very generic and can usually be copied wholesale from another module. An excellent example is in the Azure driver.

The destroy() Function

This function irreversibly destroys a virtual machine on the cloud provider. Before doing so, it should fire an event on the Salt event bus. The tag for this event is salt/cloud/<vm name>/destroying. Once the virtual machine has been destroyed, another event is fired. The tag for that event is salt/cloud/<vm name>/destroyed.

This function is normally called with the -d options:

salt-cloud -d myinstance

The list_nodes() Function

This function returns a list of nodes available on this cloud provider, using the following fields:

  • id (str)
  • image (str)
  • size (str)
  • state (str)
  • private_ips (list)
  • public_ips (list)

No other fields should be returned in this function, and all of these fields should be returned, even if empty. The private_ips and public_ips fields should always be of a list type, even if empty, and the other fields should always be of a str type. This function is normally called with the -Q option:

salt-cloud -Q

The list_nodes_full() Function

All information available about all nodes should be returned in this function. The fields in the list_nodes() function should also be returned, even if they would not normally be provided by the cloud provider. This is because some functions both within Salt and 3rd party will break if an expected field is not present. This function is normally called with the -F option:

salt-cloud -F

The list_nodes_select() Function

This function returns only the fields specified in the query.selection option in /etc/salt/cloud. Because this function is so generic, all of the heavy lifting has been moved into the salt.utils.cloud library.

A function to call list_nodes_select() still needs to be present. In general, the following code can be used as-is:

def list_nodes_select(call=None):
    '''
    Return a list of the VMs that are on the provider, with select fields
    '''
    return salt.utils.cloud.list_nodes_select(
        list_nodes_full('function'), __opts__['query.selection'], call,
    )

However, depending on the cloud provider, additional variables may be required. For instance, some modules use a conn object, or may need to pass other options into list_nodes_full(). In this case, be sure to update the function appropriately:

def list_nodes_select(conn=None, call=None):
    '''
    Return a list of the VMs that are on the provider, with select fields
    '''
    if not conn:
        conn = get_conn()   # pylint: disable=E0602

    return salt.utils.cloud.list_nodes_select(
        list_nodes_full(conn, 'function'),
        __opts__['query.selection'],
        call,
    )

This function is normally called with the -S option:

salt-cloud -S

The show_instance() Function

This function is used to display all of the information about a single node that is available from the cloud provider. The simplest way to provide this is usually to call list_nodes_full(), and return just the data for the requested node. It is normally called as an action:

salt-cloud -a show_instance myinstance

Actions and Functions

Extra functionality may be added to a cloud provider in the form of an --action or a --function. Actions are performed against a cloud instance/virtual machine, and functions are performed against a cloud provider.

Actions

Actions are calls that are performed against a specific instance or virtual machine. The show_instance action should be available in all cloud modules. Actions are normally called with the -a option:

salt-cloud -a show_instance myinstance

Actions must accept a name as a first argument, may optionally support any number of kwargs as appropriate, and must accept an argument of call, with a default of None.

Before performing any other work, an action should normally verify that it has been called correctly. It may then perform the desired feature, and return useful information to the user. A basic action looks like:

def show_instance(name, call=None):
'''
Show the details from EC2 concerning an AMI
'''
if call != 'action':
    raise SaltCloudSystemExit(
        'The show_instance action must be called with -a or --action.'
    )

return _get_node(name)

Please note that generic kwargs, if used, are passed through to actions as kwargs and not **kwargs. An example of this is seen in the Functions section.

Functions

Functions are called that are performed against a specific cloud provider. An optional function that is often useful is show_image, which describes an image in detail. Functions are normally called with the -f option:

salt-cloud -f show_image my-cloud-provider image='Ubuntu 13.10 64-bit'

A function may accept any number of kwargs as appropriate, and must accept an argument of call with a default of None.

Before performing any other work, a function should normally verify that it has been called correctly. It may then perform the desired feature, and return useful information to the user. A basic function looks like:

def show_image(kwargs, call=None):
    '''
    Show the details from EC2 concerning an AMI
    '''
    if call != 'function':
        raise SaltCloudSystemExit(
            'The show_image action must be called with -f or --function.'
        )

    params = {'ImageId.1': kwargs['image'],
              'Action': 'DescribeImages'}
    result = query(params)
    log.info(result)

    return result

Take note that generic kwargs are passed through to functions as kwargs and not **kwargs.