At one time, applications were constructed as a monolithic mass of code that implemented all of the functions required to implement a complete solution including user interface (if it faced users), application processing, data management and storage management. Some of the more sophisticated applications also included tools for application management and security. Offering higher levels of availability or the ability for an application to fail over to another machine in the network usually required the assistance of another piece of software, a clustering monitor.VMScluster still is one of the best examples of SSI clustering.
A very few suppliers took this concept to another level and created an multi-system environment that acted, for all practical uses, as a single computer. This was called a single system image (SSI) cluster. An individual could log into a machine and not know which of the machines in the cluster he/she was accessing. When that individual started an application, it wasn't clear where it was running, what machine was managing storage or, if connections were required outside of the datacenter, which machine was actually connected to the outside network. Early versions of this technology could only link together 16 machines. It was later possible to link together literally thousands of machines ranging from little desktop system, to desk side systems, to work group systems, to very large systems in the datacenter.
Digital Equipment Corporation's VAXcluster (now called a VMScluster), offered in the early 1980s, was one of the best examples of SSI clustering software. Although the underlying hardware and operating environment software have evolved a great deal from VAX-11/780s and VAX/VMS 3.0, HP is still making this tool available today. Click here to go back to the future.
After close to 30 years of evolution, this software still is one of the best examples of SSI clustering. During the last moments of independent life as a corporation, Digital offered the code that made VMSclusters work to Microsoft for inclusion in its Wolfpack project. Rather than embark on that rather difficult journey, Microsoft chose to implement a high availability/failover system rather than a true single system image cluster.
At one point, HP promised to provide SSI clustering on its HP/UX systems. Later on when the enormity of that task became clear, the company backed away from that promise and offered an improved version of ServiceGuard that, even today, is only a high availability/failover environment.
In the late 1990s, Sun's Solaris Cluster moved from being merely a high availability/failover environment to a full SSI implementation.
In the early 2000s, several companies, including Cassatt, Qlusters, and a few others, implemented similar concepts using industry standard systems, commercial off the shelf hardware and the Linux operating environment. These products faithfully created a SSI cluster supported by many individual blades or complete systems. They've also added capabilities to include virtual systems running in one of any number of virtual machine environments.
Although these companies are still offering the capabilities of a full SSI cluster, they've all changed their messages and marketing tactics to focus on system management. It's hard to find a mention of SSI on their Web sites or in their marketing literature.
Why don't we hear more about the SSI concept today? The reason is pretty clear. Architects of today's IT solutions seldom build a monolithic block of code for a computing solution. So, deploying an environment designed to virtualize the functions of these applications and to implement high availability and reliability is no longer in favor.
It is far more likely that each of the functions of an application, user interface, application processing, data management and storage management will be implemented as network services and these services may be hosted on several machines in the organization's network. This service oriented architecture (SOA) concept has come to replace single system image as the approach to major application design.
Does your organization still use single system image clusters? What functions do these installations support for your organization? Is your organization planning to eventually replace its SSI installations?