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Python SDK for oVirt Engine API

Project Description
= 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 Fedora 24 and CentOS 7 using the RPM packages
provided by the oVirt project. To do so install the oVirt release package:

# dnf install http://resources.ovirt.org/pub/yum-repo/ovirt-release41.rpm

Then install the SDK packages. For Python 2:

# dnf install python-ovirt-engine-sdk4

For Python 3:

# dnf install python3-ovirt-engine-sdk4

For other operating systems (and also for Fedora and 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:

# dnf -y install \
gcc \
libxml2-devel \
python-devel

For Python 3:

# dnf -y install \
gcc \
libxml2-devel \
python3-devel

If you are using distributions like Debian, or Ubuntu:

# apt-get --assume-yes install \
gcc \
libxml2-dev \
python-dev

For Python 3:

# apt-get --assume-yes install \
gcc \
libxml2-dev \
python3-dev

NOTE: The examples above use the `dnf` command, which is the default in
Fedora 24. In CentOS 7 you may need to use the `yum` command, as `dnf`
is optional.

== 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.
+
It also contains the `Error` class, which is the exception that the SDK
will raise when it needs to report any error.

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::

This module contains the classes that implement the _services_ supported
by the API. For example, the `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]
----
vms_service = connection.system_service().vms_service()
----
+
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:

[source,python]
----
import ovirtsdk4 as sdk

# Create a connection to the server:
connection = sdk.Connection(
url='https://engine.example.com/ovirt-engine/api',
username='admin@internal',
password='...',
ca_file='ca.pem',
)
----

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:

[source,python]
----
# Close the connection to the server:
connection.close()
----

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 it should have:

[source,python]
----
from ovirtsdk4 import types

vm = types.Vm(
name='myvm',
cluster=types.Cluster(
name='mycluster'
),
template=types.Template(
name='mytemplate'
),
memory=1073741824
)
----

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:

[source,python]
----
vm = types.Vm()
vm.name = 'myvm'
vm.cluster = types.Cluster(name='mycluster')
vm.template = types.Template(name='mytemplate')
vm.memory=1073741824
----

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:

[source,python]
----
vm = types.Vm(
name='myvm',
custom_properties=[
types.CustomProperty(...),
types.CustomProperty(...),
...
]
)
----

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:

[source,python]
----
if vm.status == types.VmStatus.DOWN:
...
elif vm.status == types.VmStatus.IMAGE_LOCKED:
....
----

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:

[source,python]
----
print("vm.name: %s" % vm.name)
print("vm.memory: %s" % vm.memory)
for custom_property in vm.custom_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:

[source,python]
----
<vm id="123" href="/ovirt-engine/api/vms/123">
<name>myvm</name>
<link rel="diskattachments" href="/ovirt-engine/api/vms/123/diskattachments/>
...
</vm>
----

That link is available as `vm.diskattachments`, but it doesn't contain
the actual disk attachments. To get the actual data the `Connection`
class provides a `follow_link` method that uses the value of the `href`
XML attribute to retrieve the actual data. For example, to retrieve the
details of the disks of the virtual machine, you can first follow the
link to the disk attachments, and then follow the link to each of the
disks:

[source,python]
----
# Retrieve the virtual machine:
vm = vm_service.get()

# Follow the link to the disk attachments, and then to the disks:
attachments = connection.follow_link(vm.disk_attachments)
for attachment in attachments:
disk = connection.follow_link(attachment.disk)
print("disk.alias: " % disk.alias)
----

=== Locating services

The API provides a set of _services_, each associated to a particular
path within the URL space of the server. For example, the service that
manages the collection of virtual machines of the system lives in
`/vms`, and the service that manages the virtual machine with identifier
`123` lives in `/vms/123`.

In the SDK the root of that tree of services is implemented by the
_system service_. It is obtained calling the `system_service` method
of the connection:

[source,python]
----
system_service = connection.system_service()
----

Once you have the reference to this system service you can use it to get
references to other services, calling the `+*_service+` methods (called
_service locators_) of the previous service. For example, to get a
reference to the service that manages the collection of virtual machines
of the system use the `vms_service` service locator:

[source,python]
----
vms_service = system_service.vms_service()
----

To get a reference to the service that manages the virtual machine with
identifier `123`, use the `vm_service` service locator of the service
that manages the collection of virtual machines. It receives as a
parameter the identifier of the virtual machine:

[source,python]
----
vm_service = vms_service.vms_service('123')
----

IMPORTANT: Calling the service locators doesn't send any request to the
server. The Python objects that they return are pure services, they
don't contain any data. For example, the `vm_service` Python object
obtained in the previous example is *not* the representation of a
virtual machine. It is the service that can be used to retrieve, update,
delete, start and stop that virtual machine.

=== Using services

Once you have located the service you are interested on, you can start
calling its _service methods_, the methods that send requests to the
server and do the real work.

The services that manage collections of object usually have the
following `list` and `add` methods.

The services that manage a single object usually have the `get`,
`update` and `remove` methods.

Both kind of services can also have additional _action methods_, which
perform actions other than retrieving, creating, updating or removing.
Most frequently they available in services that manage a single object.

==== Using the _get_ methods

These service methods are used to retrieve the representation of a
single object. For example, to retrieve the representation of the
virtual machine with identifier `123`:

[source,python]
----
# 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. For example,
in this case, the result will be an instance of the Python class
`ovirtsdk4.types.Vm`.

The `get` methods of some services support additional parameters that
control how to retrieve the representation of the object, or what
representation to retrieve in case there are multiple representations.
For example, for virtual machines you may want to retrieve its current
state, or the state that will be used the next time it is started, as
they may be different. To do so the `get` method of the service that
manages a virtual machine supports a
http://ovirt.github.io/ovirt-engine-api-model/master/#services/vm/methods/get/parameters/next_run[next_run]
boolean parameter:

[source,python]
----
# 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)
----

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:

[source,python]
----
# Find the service that manages a virtual machine that does
# not exist. This will succeed.
vm_service = vms_service.vm_service('junk')

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

==== 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:

[source,python]
----
# Find the service that manages the collection of virtual
# machines:
vms_service = system_service.vms_service()
vms = vms_service.list()
----

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:

[source,python]
----
vms = vms_service.list(search='name=my*', max=10)
----

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`:

[source,python]
----
from ovirtsdk4 import types

# Add the virtual machine:
vm = vms_service.add(
vm=types.Vm(
name='myvm',
cluster=types.Cluster(
name='mycluster'
),
template=types.Template(
name='mytemplate'
)
)
)
----

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:

[source,python]
----
# Add the virtual machine:
vm = vms_service.add(
...
)

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

# Perform some other operation on the virtual machine, like
# starting it:
vm_service.start()
----

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:

[source,python]
----
# 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 till the virtual machine is down, which means that it is
# completely created:
while True:
time.sleep(5)
vm = vm_service.get()
if vm.status == types.VmStatus.DOWN:
break
----

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`:

[source,python]
----
from ovirtsdk4 import types

# Find the virtual machine, and then the service that
# manages it:
vm = vms_service.list(search='name=myvm')[0]
vm_service = vms_service.vm_service(vm.id)

# Update the name:
updated_vm = vms_service.update(
vm=types.Vm(
name='newvm'
)
)
----

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:

[source,python]
----
# Retrieve the current representation:
vm = vm_service.get()

# Update the representation, in memory, no request sent
# to the server:
vm.name = 'newvm'

# Send the update. Do *not* do this.
vms_service.update(vm)
----

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:

[source,python]
----
# Update the memory of the virtual machine 1 GiB, but not the current
# one, the one it will have after the next boot:
vm = vm_service.update(
vm=types.Vm(
memory=1073741824
),
next_run=True
)
----

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`:

[source,python]
----
# Find the service that manages the virtual machine:
vm_service = vms_service.vm_service('123')

# Remove the virtual machine:
vms_service.remove()
----

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:

[source,python]
----
# Remove the virtual machine, but preserve the disks:
vm_service.remove(detach_only=True)
----

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:

[source,python]
----
# Start the virtual machine:
vm_service.start()
----

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]:

[source,python]
----
# Start the virtual machine:
vm_service.start(cloud_init=True)
----

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:

[source,python]
----
# Check if the storage domain is attached to a data center:
sds_service = system_service.storage_domains_service()
sd_service = sds_service.storage_domain_service('123')
if sd_service.is_attached():
...
----

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].
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