RTO/RPO requirements
Applications with high-availability requirements usually have very short RPO/RTO requirements. For those environments, some form of automated application recovery is often deployed. Two distinct architectures have been available in this space: fault-tolerant computing (FTC) and clustered computing for availability (CCA). FTC uses fully redundant servers with specialized operating systems that run a mirror copy of the application across both "halves" of the server, providing instant and fully transparent recovery in the event of a hardware failure. CCA uses separate server "nodes" connected across a network, as well as fault-management software that monitors for failures and can switch an application workload (including an application, its clients and data) to another node in the cluster. Both approaches do a good job of addressing planned and unplanned downtime, allowing companies to manage application availability to very high levels. FTC platforms usually provide higher overall application availability than CCA architectures.

CCA comes in two flavors: shared disk and shared nothing. In the shared-disk model, two or more nodes share a set of physical disks, limiting these to local configurations. In the shared-nothing model, some form of storage-centric replication is used to keep two physically separate data stores (the source and the target) in sync across a network.

While FTC and CCA solutions do a good job of managing high-availability requirements, there were several issues with these approaches. Early FTC designs from companies like Tandem Computers, Sequoia Systems and Stratus Technologies were proprietary and didn't support mainstream apps. More recently, FTC companies like Hewlett-Packard (HP) Co. and Stratus built systems using commodity hardware and software, but with a proprietary software layer to connect the mainstream operating system to the redundant hardware architecture. This results in higher costs and lengthier development cycles for new releases. CCA is generally more complex than FTC because it requires custom scripting, strict change-control requirements and sophisticated administrators; however, it uses off-the-shelf hardware and software, making it generally more applicable to mainstream applications. But the complexity of clusters makes them less attractive, especially to smaller shops.

				Defining ACC solutions
				Several distinct capabilities set application continuity computing (ACC) products apart from existing fault-tolerant computing (FTC) and clustered computing for availability (CCA) alternatives. Simplicity of deployment. The ACC model is built around pre-packaged, application-specific hardware/software solutions targeted for easy deployment. Transparent recovery. Because ACC products maintain a hot standby on a shadow server, recovery occurs within seconds for local configurations, and the same application capabilities are supported in both pre- and post-failover modes. Based on industry-standard hardware/software. ACC products use commodity hardware/software components and leverage the native data validation capabilities of the application.