High-level architecture

The XAPI Toolstack manages a cluster of hosts, network switches and storage on behalf of clients such as XenCenter and Xen Orchestra.

The most fundamental concept is of a Resource pool: the whole cluster managed as a single entity. The following diagram shows a cluster of hosts running xapi, all sharing some storage:

A Resource Pool A Resource Pool

At any time, at most one host is known as the pool coordinator (formerly known as “master”) and is responsible for coordination and locking resources within the pool. When a pool is first created a coordinator host is chosen. The coordinator role can be transferred

  • on user request in an orderly fashion (xe pool-designate-new-master)
  • on user request in an emergency (xe pool-emergency-transition-to-master)
  • automatically if HA is enabled on the cluster.

All hosts expose an HTTP, XML-RPC and JSON-RPC interface running on port 80 and with TLS on port 443, but control operations will only be processed on the coordinator host. Attempts to send a control operation to another host will result in a XenAPI redirect error message. For efficiency the following operations are permitted on non-coordinator hosts:

  • querying performance counters (and their history)
  • connecting to VNC consoles
  • import/export (particularly when disks are on local storage)

Since the coordinator host acts as coordinator and lock manager, the other hosts will often talk to the coordinator. Non-coordinator hosts will also talk to each other (over the same HTTP and RPC channels) to

  • transfer VM memory images (VM migration)
  • mirror disks (storage migration)

Note that some types of shared storage (in particular all those using vhd) require coordination for disk GC and coalesce. This coordination is currently done by xapi and hence it is not possible to share this kind of storage between resource pools.

The following diagram shows the software running on a single host. Note that all hosts run the same software (although not necessarily the same version, if we are in the middle of a rolling update).

A Host A Host

The XAPI Toolstack expects the host to be running Xen on x86. The Xen hypervisor partitions the host into Domains, some of which can have privileged hardware access, and the rest are unprivileged guests. The XAPI Toolstack normally runs all of its components in the privileged initial domain, Domain 0, also known as “the control domain”. However there is experimental code which supports “driver domains” allowing storage and networking drivers to be isolated in their own domains.