2009-04-15 20:42:50 +00:00
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<h1>The libvirt API concepts</h1>
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<p> This page describes the main principles and architecture choices
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behind the definition of the libvirt API:
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<ul>
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<li>
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<a href="#Objects">Objects exposed</a>
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</li>
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<li>
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<a href="#Functions">Functions and naming conventions</a>
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</li>
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<li>
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<a href="#Driver">The drivers</a>
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</li>
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<li>
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<a href="#Remote">Daemon and remote access</a>
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</li>
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</ul>
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<h2><a name="Objects" id="Objects">Objects exposed</a></h2>
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<p> As defined in the <a href="goals.html">goals section</a>, libvirt
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API need to expose all the resources needed to manage the virtualization
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support of recent operating systems. The first object manipulated though
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the API is <code>virConnectPtr</code> which represent a connection to
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an hypervisor. Any application using libvirt is likely to start using the
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API by calling one of <a href="html/libvirt-libvirt.html#virConnectOpen"
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>the virConnectOpen functions</a>. You will note that those functions take
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a name argument which is actually an URI to select the right hypervisor to
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open, this is needed to allow remote connections and also select between
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different possible hypervisors (for example on a Linux system it may be
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possible to use both KVM and LinuxContainers on the same node). A NULL
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name will default to a preselected hypervisor but it's probably not a
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wise thing to do in most cases. See the <a href="uri.html">connection
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URI</a> page for a full descriptions of the values allowed.<p>
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<p> Once the application obtained a <code class='docref'>virConnectPtr</code>
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connection to the
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hypervisor it can then use it to manage domains and related resources
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available for virtualization like storage and networking. All those are
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exposed as first class objects, and connected to the hypervisor connection
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(and the node or cluster where it is available).</p>
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<p class="image">
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<img alt="first class objects exposed by the API"
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src="libvirt-object-model.png"/>
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</p>
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<p> The figure above shows the five main objects exported by the API:</p>
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<ul>
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<li>virConnectPtr: represent a connection to an hypervisor.</li>
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<li>virDomainPtr: represent one domain either active or defined (i.e.
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existing as permanent config file and storage but not currently running
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on that node). The function <code class='docref'>virConnectListDomains</code>
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allows to list all the IDs for the domains active on this hypervisor.</li>
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<li>virNetworkPtr: represent one network either active or defined (i.e.
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existing as permanent config file and storage but not currently activated.
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The function <code class='docref'>virConnectListNetworks</code>
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allows to list all the virtualization networks actived on this node.</li>
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<li>virStorageVolPtr: represent one storage volume, usually this is used
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as a block device available to one of the domains. The function
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<code class="docref">virStorageVolLookupByPath</code> allows to find
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the object based on its path on the node.</li>
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<li>virStoragePoolPtr: represent a storage pool, i.e. a logical area
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which can be used to allocate and store storage volumes. The function
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<code class="docref">virStoragePoolLookupByVolume</code> allows to find
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the storage pool containing a given storage volume.</li>
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</ul>
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<p> Most object manipulated by the library can also be represented using
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XML descriptions. This is used primarily to create those object, but is
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also helpful to modify or save their description back.</p>
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<p> Domains, network and storage pools can be either <code>active</code>
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i.e. either running or available for immediate use, or
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<code>defined</code> in which case they are inactive but there is
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a permanent definition available in the system for them. Based on this
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thay can be activated dynamically in order to be used.</p>
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<p> Most kind of object can also be named in various ways:<p>
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<ul>
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<li>by their <code>name</code>, an user friendly identifier but
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whose unicity cannot be garanteed between two nodes.</li>
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<li>by their <code>ID</code>, which is a runtime unique identifier
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provided by the hypervisor for one given activation of the object,
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but it becomes invalid once the resource is deactivated.</li >
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<li>by their <code>UUID</code>, a 16 bytes unique identifier
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as defined in <a href="http://www.ietf.org/rfc/rfc4122.txt">RFC 4122</a>,
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which is garanteed to be unique for long term usage and across a
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set of nodes.</li>
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</ul>
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2009-11-06 15:04:19 +00:00
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2009-04-15 20:42:50 +00:00
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<h2><a name="Functions" id="Functions">Functions and naming
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conventions</a></h2>
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<p> The naming of the functions present in the library is usually
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made of a prefix describing the object associated to the function
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and a verb describing the action on that object.</p>
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<p> For each first class object you will find apis
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for the following actions:</p>
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<ul>
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2009-11-06 15:04:19 +00:00
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<li><b>Lookup</b>:...LookupByName,
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2009-04-15 20:42:50 +00:00
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<li><b>Enumeration</b>:virConnectList... and virConnectNumOf...:
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those are used to enumerate a set of object available to an given
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hypervisor connection like:
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<code class='docref'>virConnectListDomains</code>,
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<code class='docref'>virConnectNumOfDomains</code>,
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<code class='docref'>virConnectListNetworks</code>,
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<code class='docref'>virConnectListStoragePools</code>, etc.</li>
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<li><b>Description</b>: ...GetInfo: those are generic accessor providing
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a set of informations about an object, they are
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<code class='docref'>virNodeGetInfo</code>,
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<code class='docref'>virDomainGetInfo</code>,
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<code class='docref'>virStoragePoolGetInfo</code>,
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<code class='docref'>virStorageVolGetInfo</code>.</li>
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<li><b>Accessors</b>: ...Get... and ...Set...: those are more specific
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accessors to query or modify the given object, like
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<code class='docref'>virConnectGetType</code>,
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<code class='docref'>virDomainGetMaxMemory</code>,
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<code class='docref'>virDomainSetMemory</code>,
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<code class='docref'>virDomainGetVcpus</code>,
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<code class='docref'>virStoragePoolSetAutostart</code>,
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<code class='docref'>virNetworkGetBridgeName</code>, etc.</li>
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<li><b>Creation</b>: </li>
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<li><b>Destruction</b>: ... </li>
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</ul>
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<p> For more in-depth details of the storage related APIs see
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2009-11-06 15:04:19 +00:00
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<a href="storage.html">the storage management page</a>,
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2009-04-15 20:42:50 +00:00
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<h2><a name="Driver" id="Driver">The libvirt drivers</a></h2>
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<p></p>
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<p class="image">
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<img alt="The libvirt driver architecture"
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src="libvirt-driver-arch.png"/>
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</p>
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<h2><a name="Remote" id="Remote">Daemon and remote access</a></h2>
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<p></p>
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<p class="image">
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<img alt="The libvirt daemon and remote architecture"
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src="libvirt-daemon-arch.png"/>
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</p>
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</body>
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</html>
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