# The libvirt API concepts

This page describes the main principles and architecture choices behind the definition of the libvirt API:

- Objects exposed
- Functions and naming conventions
- The libvirt drivers
- Daemon and remote access

## Objects exposed

As defined in the goals section, libvirt API need to expose all the resources needed to manage the virtualization support of recent operating systems. The first object manipulated though the API is `virConnectPtr` which represent a connection to an hypervisor. Any application using libvirt is likely to start using the API by calling one of the virConnectOpen functions. You will note that those functions take a name argument which is actually an URI to select the right hypervisor to open, this is needed to allow remote connections and also select between different possible hypervisors (for example on a Linux system it may be possible to use both KVM and LinuxContainers on the same node). A NULL name will default to a preselected hypervisor but it's probably not a wise thing to do in most cases. See the connection URI page for a full descriptions of the values allowed.

Once the application obtained a `virConnectPtr` connection to the hypervisor it can then use it to manage domains and related resources available for virtualization like storage and networking. All those are exposed as first class objects, and connected to the hypervisor connection (and the node or cluster where it is available).

The figure above shows the five main objects exported by the API:

- virConnectPtr: represent a connection to an hypervisor.
- virDomainPtr: represent one domain either active or defined (i.e. existing as permanent config file and storage but not currently running on that node). The function `virConnectListDomains` allows to list all the IDs for the domains active on this hypervisor.
- virNetworkPtr: represent one network either active or defined (i.e. existing as permanent config file and storage but not currently activated. The function `virConnectListNetworks` allows to list all the virtualization networks actived on this node.
- virStorageVolPtr: represent one storage volume, usually this is used as a block device available to one of the domains. The function `virStorageVolLookupByPath` allows to find the object based on its path on the node.
- virStoragePoolPtr: represent a storage pool, i.e. a logical area which can be used to allocate and store storage volumes. The function `virStoragePoolLookupByVolume` allows to find the storage pool containing a given storage volume.

Most object manipulated by the library can also be represented using XML descriptions. This is used primarily to create those object, but is also helpful to modify or save their description back.

Domains, network and storage pools can be either `active` i.e. either running or available for immediate use, or `defined` in which case they are inactive but there is a permanent definition available in the system for them. Based on this thay can be activated dynamically in order to be used.

Most kind of object can also be named in various ways:

- by their `name`, an user friendly identifier but whose unicity cannot be garanteed between two nodes.
- by their `ID`, which is a runtime unique identifier provided by the hypervisor for one given activation of the object, but it becomes invalid once the resource is deactivated.
- by their `UUID`, a 16 bytes unique identifier as defined in RFC 4122, which is garanteed to be unique for long term usage and across a set of nodes.

## Functions and naming conventions

The naming of the functions present in the library is usually made of a prefix describing the object associated to the function and a verb describing the action on that object.

For each first class object you will find apis for the following actions:

- Lookup:...LookupByName,
- Enumeration:virConnectList... and virConnectNumOf...: those are used to enumerate a set of object available to an given hypervisor connection like: `virConnectListDomains`, `virConnectNumOfDomains`, `virConnectListNetworks`, `virConnectListStoragePools`, etc.
- Description: ...GetInfo: those are generic accessor providing a set of informations about an object, they are `virNodeGetInfo`, `virDomainGetInfo`, `virStoragePoolGetInfo`, `virStorageVolGetInfo`.
- Accessors: ...Get... and ...Set...: those are more specific accessors to query or modify the given object, like `virConnectGetType`, `virDomainGetMaxMemory`, `virDomainSetMemory`, `virDomainGetVcpus`, `virStoragePoolSetAutostart`, `virNetworkGetBridgeName`, etc.
- Creation:
- Destruction: ...

For more in-depth details of the storage related APIs see the storage management page.

## The libvirt drivers

## Daemon and remote access