Single-Instance Storage (SIS) is a method of computer data organization by which a single file or asset can be accessed within a system by multiple users or "stores." Microsoft® owns a patent on one particular application of this technology, which is typically used in server systems such as the Microsoft® Exchange Server® email organization service. In a computer system, multiple machines and users may be connected to a larger server that includes a host system. Rather than having each computer contain the same files and documents, a single-instance storage system can be used to have these assets on the host system, or "SIS common store."
The basic purpose of single-instance storage is to reduce redundancy in a system by eliminating multiple copies of something whenever possible. One of the simplest ways in which this can be done is through an email server, such as the Microsoft® Exchange Server®. In this system, multiple users can all have accounts, called "stores," on the server through which to send, receive, and access email.
When someone in this server sends an email message to 100 other people on the same system, 100 copies are not actually generated. Through single-instance storage, each person is sent a "pointer" that directs them to a single copy of the message. None of the recipients is aware of this, however, as the pointer appears just like the actual email.
This reduces hosting sizes for documents, and speeds up delivery and processing of messages since less work is required by the system. The use of single-instance storage can extend to computer networks and servers in other environments. Each computer on a system requires certain files to function, such as those needed by the Operating System (OS). Rather than each computer having all of these files, however, a single copy can be kept in a common store and then the computers look to it when they are needed.
Single-instance storage is typically transparent and many computer users may not even realize they are accessing it. Users on a network that share data, for example, are likely to still open and view files that are hosted on a common system through the same methods as anyone else. When they open the file, however, they are actually activating a "pointer" that appears like a file to them. This redirects their system to the real file on the SIS common store. Single-instance storage can even be used on a smaller scale, allowing a system to better organize data for multiple users by eliminating copies of files.