Content-addressable storage (CAS), also referred to as content-addressed storage or fixed-content storage, is a way to store information so it can be retrieved based on its content, not its name or location. It has been used for high-speed storage and retrieval of fixed content, such as documents stored for compliance with government regulations. Content-addressable storage is similar to content-addressable memory.
CAS systems work by passing the content of the file through a cryptographic hash function to generate a unique key, the "content address". The file system's directory stores these addresses and a pointer to the physical storage of the content. Because an attempt to store the same file will generate the same key, CAS systems ensure that the files within them are unique, and because changing the file will result in a new key, CAS systems provide assurance that the file is unchanged.
CAS became a significant market during the 2000s, especially after the introduction of the 2002 Sarbanes–Oxley Act in the United States which required the storage of enormous numbers of documents for long periods and retrieved only rarely. Ever-increasing performance of traditional file systems and new software systems have eroded the value of legacy CAS systems, which have become increasingly rare after roughly 2018. However, the principles of content addressability continue to be of great interest to computer scientists, and form the core of numerous emerging technologies, such as peer-to-peer file sharing, cryptocurrencies, and distributed computing.
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