CloudPatterns.org

Problem

Technical restrictions, licensing restrictions, or other reasons may prevent a cloud consumer from taking advantage of clustering and high availability technology and products. This can seriously jeopardize the availability and scalability of its cloud services and applications.

Solution

A system comprised of a set of mechanisms and relying on the use of heartbeat messages is established to emulate select features of clustering and high availability IT resources.

Figure 1 - Special heartbeat agents are employed to monitor heartbeat messages exchanged between the servers.

Application

Heartbeat messages are processed by a heartbeat monitor agents and are exchanged between:

• hypervisors
• each hypervisor and each virtual server
• each hypervisor and the central VIM

If an operating system is placed on a physical server, it needs to be converted into a virtual server prior to the issuance of heartbeat messages.

Figure 2 - The cloud architecture resulting from the application of this pattern.

1. A virtual server is created from the physical server.
2. The hypervisor proceeds to host the virtual server.
3. The primary virtual server is equipped with fault tolerance and maintains a synchronized state via the use of heartbeat messages.
4. The secondary server that shares the synchronized state is available in case the primary virtual server fails.

The application/service monitoring station monitors the servers and cloud services. In the event of failure, this station attempts recovery based on sequential pre-defined policies. If the primary server’s operating system fails, procedures are in place to avoid downtime.

Figure 3 - When the primary virtual server fails, along with its hosted cloud service, heartbeat messages are no longer transmitted. As a result, the hypervisor recognizes the failure and switches activity to the secondary virtual server that maintains the synchronized state. After the primary virtual server is back online, the hypervisor creates a new secondary for the new primary, and proceeds to save it as a synchronized non-active state.

NIST Reference Architecture Mapping

This pattern relates to the highlighted parts of the NIST reference architecture, as follows: