The following image depicts the logical relationship between
Eucalyptus components in a generalized deployment.
The cloud components, Cloud Controller (CLC) and Walrus, communicate
with cluster components, the Cluster Controllers (CCs) and Storage
Controllers (SCs). The CCs and SCs, in turn, communicate with the
Node Controllers (NCs). The networks between machines hosting these
components must be able to allow TCP connections between them.
However, if the CCs are on separate network interfaces (one for the
network on which the cloud components are hosted and another for the
network that NCs use) the CCs will act as software routers between
these networks in some networking configurations. So each cluster
can use an internal private network for its NCs and the CCs will
route traffic from that network to a network shared by the cloud
Virtual machines (VMs) run on the machines that host NCs. You can use
the CCs as software routers for traffic between clients outside
Eucalyptus and VMs. Or the VMs can use the routing framework already
in place without CC software routers. However, depending on the
layer-2 isolation characteristics of your existing network, you
might not be able to implement all of the security features
supported by Eucalyptus.
If you configure Eucalyptus for high availability (HA), you must
have primary and secondary cloud and cluster components. In the
event of a failure, the secondary component becomes the primary
Eucalyptus HA uses a service called an Arbitrator that monitors
connectivity between a user and a user-facing component (CLC,
Walrus, and CC). An Arbitrator approximates reachability to a
user. Each Arbitrator uses ICMP messages to periodically test
reachability to an external entity (for example, a network
gateway or border router) or to an external site (for example,
An Arbitrator is not required in HA. However, it is nice to have
in order to test connectivity with a user.
If all Arbitrators fail to reach a monitored entity, Eucalyptus
assumes there is a loss of connectivity between a user and the
component. At that point a failover occurs. To allow for normal
outages and maintenance, we recommend that you register more
than one Arbitrator for each user-facing component.