= oVirt Engine API Python SDK
== Introduction
This project contains the Python SDK for the oVirt Engine API.
== Important
Note that most of the code of this SDK is automatically generated. If
you just installed the package then you will have everything already,
but if you downloaded the source then you will need to generate it,
follow the instructions in the README.adoc file of the parent
directory.
== Installation
The SDK can be installed in CentOS Linux 8 and CentOS Stream using the RPM packages provided by the oVirt project. To do so install the oVirt release package:
Then install the SDK packages. For Python 3:
For other operating systems (and also for CentOS) you can
install the SDK using the pip command, which will download the source
from https://pypi.python.org/pypi[PyPI], build and install it.
The SDK uses http://www.xmlsoft.org[libxml2] for parsing and rendering XML. The part of the SDK that interacts with that library is written in C. This means that before building you must make sure you have the C compiler and the required header and libraries files installed in your system. For example, if you are using distributions like Fedora, or CentOS:
gcc \ libxml2-devel \ python3-devel
If you are using distributions like Debian, or Ubuntu:
gcc \ libxml2-dev \ python3-dev
== Usage
=== Packages
The following are the Python modules that are most frequently needed in order to use the SDK:
ovirtsdk4::
This is the top level module. It most important element is the
Connection class, as is the mechanism to connect to the server and to
get the reference to the root of the services tree.
+
The Error class, is the base exception class that the
SDK will raise when it needs to report any error.
For certain kinds of errors there are specific error classes, extending the base error class:
AuthError - Raised when authentication or authorization fail.
ConnectionError - Raised when the name of the server can't be resolved,
and when the server is down or unreachable.
NotFoundError - Raised when the requested object doesn't exist.
TimeoutError - Raised when an operation times out.
ovirtsdk4.types::
This module contains the classes that implement the types used in the
API. For example, the ovirtsdk4.types.Vm Python class is the
implementation of the virtual machine type. These classes are just
containers of data, they don't contain any logic.
+
Instances of these classes are used as parameters and return values of
service methods. The conversion to/from the underlying representation
is handled transparently by the SDK.
ovirtsdk4.services::
ovirtsdk4.services.VmsService Python
class is the implementation of the service that manages the collection
of virtual machines of the system.
+
Instances of these classes are automatically created by the SDK when a
service is located. For example, a new instance of the VmsService
class will be automatically created by the SDK when doing the following:
+
[source,python]+ Avoid creating instances of these classes manually, as the parameters of the constructors, and in general all the methods except the service locators and service methods (described later) may change in the future.
There are other modules, like ovirtsdk4.http, ovirtsdk4.readers and
ovirtsdk4.writers. These are used to implement the HTTP communication,
and to for XML parsing and rendering. Refrain from using them, as they
are internal implementation details that may change in the future:
backwards compatibility isn't guaranteed.
=== Connecting to the server
To connect to the server import the ovirtsdk4 module. That will give
to the Connection class. This is the entry point of the SDK, and gives
you access to the root of the tree of services of the API:
import ovirtsdk4 as sdk
The connection holds expensive resources, including a pool of HTTP connections to the server and an authentication token. It is very important to free these resources when they are no longer in use:
Once a connection is closed it can't be reused.
The ca.pem file is required when connecting to a server protected
with TLS. In an usual oVirt installation it will be in
/etc/pki/ovirt-engine/ca.pem. If you don't specify ca_file, then
system wide CA certificate store will be used.
If something fails when trying to create the connection (authentication
failure, communication failure, etc) the SDK will raise a
ovirtsdk4.Error exception containing the details.
=== Using types
The classes in the ovirtsdk4.types module are pure data containers,
they don't have any logic or operations. Instances can be created and
modified at will.
Creating or modifying one of this instances does not have any effect in the server side, unless one they are explicitly passed to a call to one of the service methods described below. Changes in the server side are not automatically reflected in the instances that already exist in memory.
The constructors of these classes have multiple optional arguments, one for each attribute of the type. This is intended to simplify creation of objects using nested calls to multiple constructors. For example, to create an instance of a virtual machine, with an specification of the cluster and template that it should use, and the memory in bytes it should have:
from ovirtsdk4 import types
Using the constructors in this way is recommended, but not mandatory. You can also create the instance with no arguments in the call to the constructor, and then populate the object step by step, using the setters, or using a mix of both approaches:
Attributes that are defined as lists of objects in the specification of
the API are implemented as Python lists. For example, the
custom_properties attributes of the
http://ovirt.github.io/ovirt-engine-api-model/master/#types/vm[Vm]
type is defined as a list of objects of type CustomProperty, so when
using it in the SDK it will be a Python list:
Attributes that are defined as enumerated values in the specification of
the API are implemented as enum in Python, using the native support
for enums in Python 3, and using the
https://pypi.python.org/pypi/enum34[enum34] package in Python 2.7. For
example, the status attribute of the Vm type is defined using the
http://ovirt.github.io/ovirt-engine-api-model/master/#types/vm_status[VmStatus]
enum:
NOTE: In the specification of the API the values of enum types appear in lower case, because that is what is used in the XML or JSON documents. But in Python it is common practice to use upper case for this kind of constants, so that is how they are defined in the Python SDK: all upper case.
Reading the attributes of instances of types is done using the corresponding properties:
=== Using links
Some of the attributes of types are defined as links in the specification of the API. This is done to indicate that their value won't usually be populated when retrieving the representation of that object, only a link will be returned instead. For example, when retrieving a virtual machine, the XML returned by the server will look like this:
Check the http://ovirt.github.io/ovirt-engine-sdk/master[reference] documentation of the SDK to find out the details.
If the object can't be retrieved, for whatever the reason, the SDK will
raise a ovirtsdk4.Error exception, containing the details of the
failure. This includes the situation when the object doesn't actually
exist. Note that the exception will be raised when calling the get
service method, the call to the service locator method never fails, even
if the object doesn't exist, because it doesn't send any request to the
server. For example:
vm_service = vms_service.vm_service('junk')
==== Using the list methods
These service methods are used to retrieve the representations of the objects of the collection. For example, to retrieve the complete collection of virtual machines of the system:
The result will be a Python list containing the instances of
corresponding types. For example, in this case, the result will be a
list of instances of the Python class ovirtsdk4.types.Vm.
The list methods of some services support additional parameters. For
example, almost all the top level collections support a search
parameter that can be used ask the server to filter the results, and a
max parameter that can be used to limit the number of results returned
by the server. For example, to get the list of virtual machines whose
name starts with my, and to get at most 10 results:
NOTE: Not all the list methods support these parameters, and some
list methods may support other additional parameters. Check the
http://ovirt.github.io/ovirt-engine-sdk/master[reference] documentation
of the SDK to find out the details.
If list of results is empty, for whatever the reason, the returned value
will be an empty Python list, it will never be None.
If there is an error while trying to retrieve the result, then the SDK
will raise an ovirtsdk4.Error exception containing the details of the
failure.
==== Using the add methods
These service methods add new elements to the collection. 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.
For example, to add a new virtual machine named myvm:
from ovirtsdk4 import types
If the object can't be created, for whatever the reason, the SDK will
raise an ovirtsdk4.Error exception containing the details of the
failure. It will never return None.
It is very important to understand that the Python object returned by
this add method is an instance of the relevant type, it isn't a
service, just a container of data. In this particular example the
returned object will be an instance of the ovirtsdk4.types.Vm class.
If once the virtual machine is created you need to perform some
operation on it, like retrieving it again, or starting it, you will
first need to find the service that manages it, calling the
corresponding service locator:
vm = vms_service.add( ... )
vm_service = vms_service.vm_service(vm.id)
Note that the creation of most objects is an asynchronous task. That
means, for example, that when creating a new virtual machine the add
method will return before the virtual machine is completely created
and ready to be used. It is good practice to poll the status of the
object till it is completely created. For a virtual machine that means
checking till the status is down. So the recommended approach to create
a virtual machine is the following:
vm = vms_service.add( ... )
vm_service = vms_service.vm_service(vm.id)
In the above loop it is very important to retrieve the object each time,
using the get method, otherwise the status attribute won't be
updated.
==== Using the update methods
These service 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.
For example, to update the name of a virtual machine from myvm to
newvm:
from ovirtsdk4 import types
vm = vms_service.list(search='name=myvm')[0] vm_service = vms_service.vm_service(vm.id)
When performing updates, try to avoid sending the complete representation of the object, send only the attributes that you want to update. For example, try to avoid this:
vm = vm_service.get()
vm.name = 'newvm'
The problem with that is double. First you are sending much more information than what the server needs, thus wasting resources. Second, and more important, the server will try to update all the attributes of the object, even those that you didn't need to change. Usually that isn't a problem, but has caused many unexpected bugs in the server side in the past.
The update methods of some services support additional parameters that
control how or what to update. For example, for virtual machines you may
want to update its current state, or the state that will be used the
next time it is started. To do so the update method of the service
that manages a virtual machine supports a
http://ovirt.github.io/ovirt-engine-api-model/master/#services/vm/methods/update/parameters/next_run[next_run]
boolean parameter:
If the update can't be performed, for whatever the reason, the SDK will
raise an ovirtsdk4.Error exception containing the details of the
failure. It will never return None.
The Python object returned by this update method is an instance of the
relevant type, it isn't a service, just a container of data. In this
particular example the returned object will be an instance of the
ovirtsdk4.types.Vm class.
==== Using the remove methods
These service methods remove existing objects. They usually don't receive any parameters, as they are methods of the services that manage single objects, therefore the service already knows what object to remove.
For example, to remove the virtual machine with identifier 123:
vm_service = vms_service.vm_service('123')
The remove methods of some services support additional parameters that
control how or what to remove. For example, for virtual machines it is
possible to remove the virtual machine while preserving the disks.
To do so the remove method of the service that manages a virtual machine supports a
http://ovirt.github.io/ovirt-engine-api-model/master/#services/vm/methods/remove[detach_only]
boolean parameter:
The remove methods return None if the object is removed
successfully. It does not return the removed object. If the object
can't removed, for whatever the reason, the SDK will raise an
ovirtsdk4.Error exception containing the details of the failure.
==== Using action methods
These service methods perform miscellaneous operations. For example, the service that manages a virtual machine has methods to start and stop it:
Many of these methods include parameters that modify the operation. For example, the method that starts a virtual machine supports a http://ovirt.github.io/ovirt-engine-api-model/master/#services/vm/methods/start/parameters/use_cloud_init[use_cloud_init] parameter that indicates if you want to start it using https://cloudinit.readthedocs.io/cloud-init[cloud-init]:
Most action methods return None when they succeed, and raise a
ovirtsdk4.Error when they fail. But a few action methods return
values. For example, the service that manages a storage domains has an
http://ovirt.github.io/ovirt-engine-api-model/master/#services/storage_domain/methods/is_attachedd[is_attached]
action method that checks if the storage domain is already attached to a
data center. That method returns a boolean:
Check the http://ovirt.github.io/ovirt-engine-sdk[reference] documentation of the SDK to see the action methods supported by each service, the parameters that they support, and the values that they return.
== More information
The reference documentation of the API is available http://ovirt.github.io/ovirt-engine-api-model[here].
The reference documentation of the SDK is available http://ovirt.github.io/ovirt-engine-sdk[here].
There is a collection of examples that show how to use the SDK https://github.com/oVirt/ovirt-engine-sdk/tree/master/sdk/examples[here].