Nowadays, there are many different high-availability solutions you can choose from. Some of them are implemented on completely different layers and offer different levels of high availability. Even though some of these solutions are fairly easy to implement and provide a significantly high level of availability, they cannot compete with complete high-availability solutions and fall short compared to them.
Virtualization is the first step to be mention. Virtualization has taken large steps forward in the last decade, and probably everyone is using it. It became really popular very quickly, mostly due to the fact that it significantly lowers the cost of server maintenance, boosts the server life cycle, and facilitates management of the servers. Almost every virtualization solution also provides an integrated high-availability option that allows virtualization hosts to be joined with virtualization clusters.
The solution instantly provides high availability of virtual machines running on top of it. Therefore, the virtualization high-availability solution is provided on the lower virtualization layer. However, this kind of high availability might still fall short in certain key points. It only manages to eliminate the hardware single points of failure. It does not implement high availability of the applications and services that the virtual machines are running.
Databases are also one of the components of IT infrastructure with built-in high-availability solutions. Database servers are known to provide mission-critical services, mostly due to the huge amount of important data they usually store. This is why common database server software products offer high-availability features bundled with the database server software. This means that it is possible to implement database high availability at the database application layer. Database server high availability can provide the top application layer with high availability, which does not cover all angles required for a complete high-availability solution.
A complete high-availability solution is considered to be a solution that is implemented at the operating system level and includes the application layer, of course backed up by hardware, power supply, and network connection redundancy. This kind of high-availability solution is resistant to hardware and application failures and allows you to achieve the highest possible availability of the service.