2.3. Services

The API provides a set of services, each associated with a server path. For example, the service that manages the collection of virtual machines in the system is located in /vms, and the service that manages the virtual machine with identifier 123 is located in /vms/123.

In the Ruby software development kit, the root of that tree of services is implemented by the system service. It is retrieved by calling the system_service method of the connection:

system_service = connection.system_service

Once you have the reference to the system service, you can use it to retrieve references to other services, using the *_service methods (called service locators).

For example, to retrieve a reference to the service that manages the collection of virtual machines in the system, you can use the vms_service service locator:

vms_service = system_service.vms_service

To retrieve a reference to the service that manages the virtual machine with identifier 123, use the service locator of the vm_service service. The service locator uses the virtual machine identifier as a parameter:

vm_service = vms_service.vms_service('123')

After you have located the service you want, you can call its service methods. These methods send requests to the server and do the real work.

Services that manage collections of objects usually have the list and add methods.

Services that manage a single object usually have the get, update, and remove methods.

Services may have additional action methods, which perform actions other than retrieving, creating, updating, or removing. These methods are most commonly found in services that manage a single object.

The get method retrieves the representation of a single object.

The following example locates and retrieves the representation of the virtual machine with identifier 123:

# Find the service that manages the virtual machine:
vms_service = system_service.vms_service
vm_service = vms_service.vm_service('123')

# Retrieve the representation of the virtual machine:
vm = vm_service.get

The result will be an instance of the corresponding type. In this case, the result is an instance of the Ruby class Vm.

The get method of some services supports additional parameters that control how to retrieve the representation of the object, or which representation to retrieve, if there is more than one.

For example, you may want to retrieve a virtual machine’s future state, after boot-up. The get method of the service that manages a virtual machine supports a next_run Boolean parameter:

# Retrieve the representation of the virtual machine; not the
# current one, but the one that will be used after the next
# boot:
vm = vm_service.get(next_run: true)

See the reference documentation of the software development kit for details.

If the object cannot be retrieved, the software development kit will raise an Error exception, containing details of the failure. This will occur if you try to retrieve a non-existent object.

In the following examples, the service locator method will succeed, while the get method will raise an exception:

# Find the service that manages a virtual machine that does
# not exist. This will succeed.
vm_service = vms_service.vm_service('non_existent_VM')

# Retrieve the virtual machine. This will raise an exception.
vm = vm_service.get

The list method retrieves the representations of multiple objects in a collection.

# Find the service that manages the collection of virtual
# machines:
vms_service = system_service.vms_service
vms = vms_service.list

The result is a Ruby array containing the instances of the corresponding types. In the above example, the response is a list of instances of the Ruby class Vm.

The list method of some services supports additional parameters.

For example, almost all of the top-level collections support a search parameter to filter the results, and a max parameter to limit the number of results returned by the server.

vms = vms_service.list(search: 'name=my*', max: 10)

If the list of results is empty, the returned value will be an empty Ruby array. It will never be nil.

If the list of results cannot be retrieved, the SDK will raise an Error exception containing the details of the failure.

Add methods add new elements to collections. They receive an instance of the relevant type describing the object to add, send the request to add it, and return an instance of the type describing the added object.

# Add the virtual machine:
vm = vms_service.add(
  OvirtSDK4::Vm.new(
    name: 'myvm',
    cluster: {
      name: 'mycluster'
    },
    template: {
      name: 'mytemplate'
    }
  )
)

If you need to perform an action on the virtual machine you just added, you must locate the service that manages it and call the service locator:

# Add the virtual machine:
vm = vms_service.add(
  ...
)

# Find the service that manages the virtual machine:
vm_service = vms_service.vm_service(vm.id)

# Start the virtual machine:
vm_service.start

The creation of most objects is an asynchronous task. For example, if you create a new virtual machine, the add method will return the virtual machine before the virtual machine is completely created and ready to be used. You should poll the status of the object until it is completely created. For a virtual machine that means checking until the status is DOWN.

The recommended approach is to create a virtual machine, locate the service that manages the new virtual machine, and retrieve the status repeatedly until the virtual machine status is DOWN, indicating that all the disks have been created.

# Add the virtual machine:
vm = vms_service.add(
  ...
)

# Find the service that manages the virtual machine:
vm_service = vms_service.vm_service(vm.id)

# Wait until the virtual machine is DOWN, indicating that all the
# disks have been created:
loop do
  sleep(5)
  vm = vm_service.get
  break if vm.status == OvirtSDK4::VmStatus::DOWN
end

If the object cannot be created, the SDK will raise an Error exception containing the details of the failure. It will never return nil.

Update methods update existing objects. They receive an instance of the relevant type describing the update to perform, send the request to update it, and return an instance of the type describing the updated object.

In the following example, the service locator method locates the service managing the virtual machine and the update method updates its name:

# Find the virtual machine and the service that
# manages it:
vm = vms_service.list(search: 'name=myvm').first
vm_service = vms_service.vm_service(vm.id)

# Update the name:
updated_vm = vms_service.update(
  OvirtSDK4::Vm.new(
    name: 'newvm'
  )
)

When you update an object, update only the attributes you want to update:

vm = vm_service.get
vm.name = 'newvm'

Do not update the entire object:

# Retrieve the current representation:
vms_service.update(vm)

Updating all attributes of the virtual machine is a waste of resources and can introduce unexpected bugs on the server side.

Update methods of some services support additional parameters that can be used to control how or what to update. For example, you may want to update the memory of a virtual machine, not in its current state, but the next time it is started. The update method of the service that manages a virtual machine supports a next_run Boolean parameter:

vm = vm_service.update(
  OvirtSDK4::Vm.new(
    memory: 1073741824
  ),
  next_run: true
)

If the update cannot be performed, the SDK will raise an Error exception containing the details of the failure. It will never return nil.

Remove methods remove existing objects. They normally do not support parameters because they are methods of services that manage single objects, and the service already knows what object to remove.

vm_service = vms_service.vm_service('123')
vms_service.remove

Some remove methods support parameters that control how or what to remove. For example, it is possible to remove a virtual machine while preserving its disks, using the detach_only Boolean parameter:

vm_service.remove(detach_only: true)

The remove method returns nil if the object is removed successfully. It does not return the removed object.

If the object cannot be removed, the SDK will raise an Error exception containing the details of the failure.

There are additional action methods, apart from the methods described above. The service that manages a virtual machine has methods to start and stop it.

vm_service.start

Some action methods include parameters that modify the operation. For example, the start method supports a use_cloud_init parameter.

vm_service.start(use_cloud_init: true)

Most action methods return nil when they succeed, and raise an Error when they fail. Some action methods, however, return values. For example, the service that manages storage domains has an is_attached action method that checks whether the storage domain is already attached to a data center. The is_attached action method returns a Boolean value:

sds_service = system_service.storage_domains_service
sd_service = sds_service.storage_domain_service('123')
if sd_service.is_attached
  ...
end

See the reference documentation of the software development kit to see the action methods supported by each service, their parameters, and return values.