= oVirt Engine API Ruby SDK :reference: http://www.rubydoc.info/gems/ovirt-engine-sdk/OvirtSDK4
== Introduction
This project contains the Ruby SDK for the oVirt Engine API.
== Important
Note that most of the code of this SDK is automatically generated. If you
just installed the gem 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:
Then install the SDK package:
For other operating systems (and also for Fedora and CentOS) you can
install the SDK using the gem command, which will download the source
from https://rubygems.org[RubyGems], build and install it:
$ gem install ovirt-engine-sdk
The SDK uses http://www.xmlsoft.org[libxml2] for parsing and rendering XML and https://curl.haxx.se/libcurl[libcurl] for HTTP transfers. The parts of the SDK that interact with those libraries are 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
libcurl-devel
libxml2-devel
redhat-rpm-config
ruby
ruby-devel
rubygems
rubygems-devel
NOTE: The installation of the ruby, ruby-devel, rubygems and
rubygems-devel packages is necessary if you are going to use the Ruby
version included in your distribution. If you are using tools like
rbenv or rvm then this is not necessary.
If you are using distributions like Debian, or Ubuntu:
gcc
libcurl4-openssl-dev
libxml2-dev
ruby
ruby-dev
NOTE: The installation of the ruby and ruby-dev packages is
necessary if you are going to use the Ruby version included in your
distribution. If you are using tools like rbenv or rvm then this is
not necessary.
Some Linux distributions, like Debian and Ubuntu, provide multiple
versions of libcurl, compiled with support for different TLS
libraries: OpenSSL, NSS and GnuTLS. Currently the SDK only supports
OpenSSL, so make sure that you have that version of libcurl installed.
For example, in Ubuntu 16.04 if you have the NSS version installed, you
will have to remove it and then install the OpenSSL version:
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
=== Modules and classes
All the classes of the SDK are inside the {reference}[OvirtSDK4] module. Within the module there are several kinds of classes:
Connection::
The {reference}/Connection[Connection] class, as is the mechanism to connect to the server and to get the reference to the root of the services tree.
Errors::
The {reference}/Error[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: +
Types::
These classes implement the types used by the API. For example, the {reference}/Vm[Vm] Ruby 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.
Services::
VmsService
class will be automatically created by the SDK when doing the following:
+
[source,ruby]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 classes, like HTTP client classes, readers and writers. These are used to implement the HTTP communication, and 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 require the ovirtsdk4 file. That will give to
the {reference}/Connection[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:
connection = OvirtSDK4::Connection.new( 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:
The connection and all the services that have been obtained from it can't be used after the connection has been closed.
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 {reference}/Error[Error] exception containing the details.
=== Using types
The type classes 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 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:
The hashes passed to these constructors are processed recursively. For
example, in the above code instead of explicitly calling the constructor
for the Cluster and Template classes it is also possible to use
plain hashes:
The SDK will internally convert those hashes to the required classes, so the result will be exactly the same.
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 Ruby arrays. 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 Ruby array:
Attributes that are defined as enumerated values in the specification of
the API are implemented as constatns within a module that has the same
name than the enumerated type. 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 Ruby it is common practice to use upper case for this kind of constants, so that is how they are defined in the Ruby SDK: all upper case.
Reading the attributes of instances of types is done using the corresponding attribute readers:
=== 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:
That link is available as vm.disk_attachments, but it doesn't contain
the actual disk attachments. To get the actual data the
{reference}/Connection[Connection] class provides a
{reference}/Connection#follow_link-instance_method[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:
vm = vm_service.get
=== 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 {reference}/Connection#system_service-instance_method[system_service] method of the connection:
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
{reference}/SystemService#vms_service-instance_method[vms_service]
service locator:
To get a reference to the service that manages the virtual machine with
identifier 123, use the
{reference}/VmsService#vm_service-instance_method[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:
IMPORTANT: Calling the service locators doesn't send any request to the
server. The Ruby objects that they return are pure services, they
don't contain any data. For example, the vm_service Ruby 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 list
and add methods.
The services that manage a single object usually have the get,
update and remove methods.
Both kinds of services can also have additional action methods, which perform actions other than retrieving, creating, updating or removing. Most frequently they are 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:
vms_service = system_service.vms_service vm_service = vms_service.vm_service('123')
The result will be an instance of the corresponding type. For example, in this case, the result will be an instance of the Ruby class {reference}/Vm[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:
Check the {reference}[reference] documentation of the SDK to find out the details.
If the object can't be retrieved, for whatever reason, the SDK will
raise an {reference}/Error[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 Ruby array containing the instances of the corresponding types. For example, in this case, the result will be a list of instances of the Ruby class {reference}/Vm[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
{reference}[reference] documentation of the SDK to find out the details.
If list of results is empty, for whatever reason, the returned value
will be an empty Ruby array, it will never be nil.
If there is an error while trying to retrieve the result, then the SDK will raise an {reference}/Error[Error] exception containing the details of the failure.
==== Using the add methods
These service 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.
For example, to add a new virtual machine named myvm:
If the object can't be created, for whatever reason, the SDK will
raise an {reference}/Error[Error] exception containing the details of
the failure. It will never return nil.
It is very important to understand that the Ruby 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 {reference}/Vm[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:
vm = vms_service.list(search: 'name=myvm').first 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 reason, the SDK will
raise an {reference}/Error[Error] exception containing the details of
the failure. It will never return nil.
The Ruby 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
{reference}/Vm[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 nil if the object is removed successfully.
It does not return the removed object. If the object can't be removed,
for whatever reason, the SDK will raise an {reference}/Vm[Vm]
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 nil when they succeed, and raise a
{reference}/Error[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 {reference}[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 {reference}[here].
There is a collection of examples that show how to use the SDK https://github.com/oVirt/ovirt-engine-sdk-ruby/tree/master/sdk/examples[here].
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