Chapter 3. Common concepts

The API uses the type concept to describe the different kinds of objects accepted and returned.

There are three relevant kinds of types:

Many of the types used by the API represent identified objects, objects that have an unique identifier and exist independently of other objects. The types used to describe those objects extend the Identified type, which contains the following set of common attributes:

Objects are the individual instances of the types supported by the API. For example, the virtual machine with identifier 123 is an object of the Vm type.

A collection is a set of objects of the same type.

The state of objects needs to be represented when it is transferred beetween the client and the server. The API supports XML and JSON as the representation of the state of objects, both for input and output.

<vm id="123" href="/ovirt-engine/api/vms/123">
  <name>myvm</name>
  <description>My VM</description>
  <memory>1073741824</memory>
  ...
</vm>

<vms>
  <vm id="123" href="/ovirt-engine/api/vms/123">
    <name>yourvm</name>
    <description>Your VM</description>
    <memory>1073741824</memory>
    ...
  </vm>
  <vm id="456" href="/ovirt-engine/api/vms/456">
    <name>myname</name>
    <description>My description</description>
    <memory>2147483648</memory>
    ...
  </vm>
  ...
</vms>

{
  "id": "123",
  "href": "/ovirt-engine/api/vms/123",
  "name": "myvm",
  "description": "My VM",
  "memory": 1073741824,
  ...
}

{
  "vm": [
    {
      "id": "123",
      "href": "/ovirt-engine/api/vms/123",
      "name": "myvm",
      "description": "My VM",
      "memory": 1073741824,
      ...
    },
    {
      "id": "456",
      "href": "/ovirt-engine/api/vms/456",
      "name": "yourvm",
      "description": "Your VM",
      "memory": 2147483648,
      ...
    },
  ]
}

Services are the parts of the server responsible for retrieving, adding updating, removing and executing actions on the objects supported by the API.

There are two relevant kinds of services:

Each service is accessible via a particular path within the server. For example, the service that manages the collection of virtual machines available in the system is available in the via the path /vms, and the service that manages the virtual machine 123 is available via the path /vms/123.

All kinds of services have a set of methods that represent the operations that they can perform. The services that manage collections of objects usually have the list and add methods. The services that manage specific objects usually have the get, update and remove methods. In addition, services may also have action methods, that represent less common operations. For example, the Vm service has a start method that is used to start a virtual machine.

For the more usual methods there is a direct mapping between the name of the method and the name of the HTTP method:

The path used in the HTTP request is the path of the service, with the /ovirt-engine/api prefix.

For example, the request to list the virtual machines should be like this, using the HTTP GET method and the path /vms:

GET /ovirt-engine/api/vms

For action methods the HTTP method is always POST, and the name of the method is added as a suffix to the path. For example, the request to start virtual machine 123 should look like this, using the HTTP POST method and the path /vms/123/start:

POST /ovirt-engine/api/vms/123/start

Each method has a set of parameters.

Parameters are classified into two categories:

For example, the operation that adds a virtual machine (see here) has three parameters: vm, clone and clone_permissions. The main parameter is vm, as it describes the object that is added. The clone and clone_permissions parameters are secondary parameters.

The main parameter, when used for input, must be included in the body of the HTTP request. For example, when adding a virtual machine, the vm parameter, of type Vm must be included in the request body. So the complete request to add a virtual machine, including all the HTTP details, must look like this:

POST /ovirt-engine/api/vms HTTP/1.1
Host: myengine.example.com
Authorization: Bearer fqbR1ftzh8wBCviLxJcYuV5oSDI=
Content-Type: application/xml
Accept: application/xml

<vm>
  <name>myvm</name>
  <description>My VM</description>
  <cluster>
    <name>Default</name>
  </cluster>
  <template>
    <name>Blank</name>
  </template>
</vm>

When used for output, the main parameters are included in the response body. For example, when adding a virtual machine, the vm parameter will be included in the response body. So the complete response body will look like this:

HTTP/1.1 201 Created
Content-Type: application/xml

<vm href="/ovirt-engine/api/vms/123" id="123">
  <name>myvm</name>
  <description>My VM</description>
  ...
</vm>

Secondary parameters are only allowed for input (except for action methods, which are described later), and they must be included as query parameters. For example, when adding a virtual machine with the clone parameter set to true, the complete request must look like this:

POST /ovirt-engine/api/vms?clone=true HTTP/1.1
Host: myengine.example.com
Authorization: Bearer fqbR1ftzh8wBCviLxJcYuV5oSDI=
Content-Type: application/xml
Accept: application/xml

<vm>
  <name>myvm</name>
  <description>My VM</description>
  <cluster>
    <name>Default</name>
  </cluster>
  <template>
    <name>Blank</name>
  </template>
</vm>

Action methods only have secondary parameters. They can be used for input and output, and they should be included in the request body, wrapped with an action element. For example, the action method used to start a virtual machine (see here) has a vm parameter to describe how the virtual machine should be started, and a use_cloud_init parameter to specify if cloud-init should be used to configure the guest operating system. So the complete request to start virtual machine 123 using cloud-init will look like this when using XML:

POST /ovirt-engine/api/vms/123/start HTTP/1.1
Host: myengine.example.com
Authorization: Bearer fqbR1ftzh8wBCviLxJcYuV5oSDI=
Content-Type: application/xml
Accept: application/xml

<action>
  <use_cloud_init>true</use_cloud_init>
  <vm>
    <initialization>
      <nic_configurations>
        <nic_configuration>
          <name>eth0</name>
          <on_boot>true</on_boot>
          <boot_protocol>static</boot_protocol>
          <ip>
            <address>192.168.0.100</address>
            <netmask>255.255.255.0</netmask>
            <gateway>192.168.0.1</netmask>
          </ip>
        </nic_configuration>
      </nic_configurations>
      <dns_servers>192.168.0.1</dns_servers>
    </initialization>
  </vm>
</action>

The list method of some services has a search parameter that can be used to specify search criteria. When used, the server will only return objects within the collection that satisfy those criteria. For example, the following request will return only the virtual machine named myvm:

GET /ovirt-engine/api/vms?search=name%3Dmyvm

GET /ovirt-engine/api/vms?max=1

GET /ovirt-engine/api/vms?search=name%3Dmyvm

GET /ovirt-engine/api/vms?search=name%3D=myvm&case_sensitive=true

(criteria) [sortby (element) asc|desc]

GET /ovirt-engine/api/vms?search=name%3Dmyvm

GET /ovirt-engine/api/vms?search=name%3Dmyvm*

GET /ovirt-engine/api/vms?search=page%201&max=10

GET /ovirt-engine/api/vms?search=page%202&max=10

GET /ovirt-engine/api/vms?search=sortby%20name%20page%202&max=10

The API returns references to related objects as links. For example, when a virtual machine is retrieved it contains links to its disk attachments and network interface cards:

<vm id="123" href="/ovirt-engine/api/vms/123">
  ...
  <link rel="diskattachments" href="/ovirt-engine/api/vms/123/diskattachments"/>
  <link rel="nics" href="/ovirt-engine/api/vms/123/nics"/>
  ...
</vm>

The complete description of those linked objects can be retrieved by sending separate requests:

GET /ovirt-engine/api/vms/123/diskattachments
GET /ovirt-engine/api/vms/123/nics

However, in some situations it is more convenient for the application using the API to retrieve the linked information in the same request. This is useful, for example, when the additional network round trips introduce an unacceptable overhead, or when the multiple requests complicate the code of the application in an unacceptable way. For those use cases the API provides a follow parameter that allows the application to retrieve the linked information using only one request.

The value of the follow parameter is a list of strings, separated by commas. Each of those strings is the path of the linked object. For example, to retrieve the disk attachments and the NICs in the example above the request should be like this:

GET /ovirt-engine/api/vms/123?follow=disk_attachments,nics

That will return an response like this:

<vm id="123" href="/ovirt-engine/api/vms/123">
  ...
  <disk_attachments>
    <disk_attachment id="456" href="/ovirt-engine/api/vms/123/diskattachments/456">
      <active>true</active>
      <bootable>true</bootable>
      <interface>virtio_scsi</interface>
      <pass_discard>false</pass_discard>
      <read_only>false</read_only>
      <uses_scsi_reservation>false</uses_scsi_reservation>
      <disk id="789" href="/ovirt-engine/api/disks/789"/>
    </disk_attachment>
    ...
  </disk_attacments>
  <nics>
    <nic id="234" href="/ovirt-engine/api/vms/123/nics/234">
      <name>eth0</name>
      <interface>virtio</interface>
      <linked>true</linked>
      <mac>
        <address>00:1a:4a:16:01:00</address>
      </mac>
      <plugged>true</plugged>
    </nic>
    ...
  </nics>
  ...
</vm>

The path to the linked object can be a single word, as in the previous example, or it can be a sequence of words, separated by dots, to request nested data. For example, the previous example used disk_attachments in order to retrieve the complete description of the disk attachments, but each disk attachment contains a link to the disk, which wasn’t followed. In order to also follow the links to the disks, the following request can be used:

GET /ovirt-engine/api/vms/123?follow=disk_attachments.disk

That will result in the following response:

<vm id="123" href="/ovirt-engine/api/vms/123">
  <disk_attachments>
    <disk_attachment id="456" href="/ovirt-engine/api/vms/123/diskattachments/456">
      <active>true</active>
      <bootable>true</bootable>
      <interface>virtio_scsi</interface>
      <pass_discard>false</pass_discard>
      <read_only>false</read_only>
      <uses_scsi_reservation>false</uses_scsi_reservation>
      <disk id="789" href="/ovirt-engine/api/disks/789">
        <name>mydisk</name>
        <description>My disk</description>
        <actual_size>0</actual_size>
        <format>raw</format>
        <sparse>true</sparse>
        <status>ok</status>
        <storage_type>image</storage_type>
        <total_size>0</total_size>
        ...
      </disk>
    </disk_attachment>
    ...
  </disk_attachments>
  ...
</vm>

The path can be made as deep as needed. For example, to also get the statistics of the disks:

GET /ovirt-engine/api/vms/123?follow=disk_attachments.disk.statistics

Multiple path elements and multiple paths can be combined. For example, to get the disk attachments and the network interface cards, both with their statistics:

GET /ovirt-engine/api/vms/123?follow=disk_attachments.disk.statistics,nics.statistics

Many of the services that manage a single object provide a reference to a permissions service that manages the permissions assigned to that object. Each permission contains links to the user or group, the role and the object. For example, the permissions assigned to a specific virtual machine can be retrieved sending a request like this:

GET /ovirt-engine/api/vms/123/permissions

The response body will look like this:

<permissions>
  <permission id="456" href="/ovirt-engien/api/vms/123/permissions/456">
    <user id="789" href="/ovirt-engine/api/users/789"/>
    <role id="abc" href="/ovirt-engine/api/roles/abc"/>
    <vm id="123" href="/ovirt-engine/api/vms/123"/>
  </permission>
  ...
</permissions>

A permission is added to an object sending a POST request with a permission representation to this service. Each new permission requires a role and a user.

Some errors require further explanation beyond a standard HTTP status code. For example, the API reports an unsuccessful object state update or action with a fault in the response body. The fault contains the reason and detail attributes. For example, when the server receives a request to create a virtual machine without the mandatory name attribute it will respond with the following HTTP response line:

HTTP/1.1 400 Bad Request

And the following response body:

<fault>
  <reason>Incomplete parameters</reason>
  <detail>Vm [name] required for add</detail>
</fault>