Eucalyptus includes a high availability (HA) computing capability. High availability was overwhelmingly the most frequently requested improvement over Eucalyptus 2 by the community; but it's one that causes the most confusion.
Eucalyptus consists of a number of "always-on" web services that interact with each other. In a high availability computing configuration, each service can be configured with a running-but-inactive spare. If and when an active component fails or becomes disconnected from the other Eucalyptus services, the system will enable the spare on the fly and keep running. In addition, when the failed or disconnected component is restored to full functionality, Eucalyptus will automatically re-incorporate it into the running system and re-establish the automatic failover capability for that component.
There are six Eucalyptus-internal service components for high availability computing.
The Cloud Controller (CLC)
The CLC handles credentials, all request management except for requests to Walrus, and VM state management.
Walrus implements Put/Get object storage with append-only semantics and eventual consistency (similar to AWS S3).
The Cluster Controller (CC)
The CC manages IP addresses, network provisioning (Layer 2 and Layer 3) and VM inventory and scheduling for a single availability zone.
The Storage Controller (SC)
The SC implements Eucalyptus Block Storage (EBS) — dynamic storage volume management — for a single availability zone.
The VMWare Broker (VMWb)
The VMWb acts as a proxy for a VMWare installation (ESX-based or vSphere) so that Eucalyptus commands can be actuated in a VMWare virtualization environment.
The Node Controller (NC)
The NC actualizes VM and volume actions on a specific hypervisor (other than the VMWare hypervisors).
High availability Eucalyptus implements active-spare pairings for all components except NCs . Therfore the platform itself is resilient to the unavailability or failure of the internal services that implement it.
Several reference architectures exist that document a scalable web services use case with high availability: Scalable Web Services Small with HA and Scalable Web Services Large with HA. Learn more about Eucalyptus high availability in this recorded webinar.