Content-addressable storage

Content-addressable storage, also referred to as associative storage or abbreviated CAS, is a mechanism for storing information that can be retrieved based on its content, not its storage location. It is typically used for high-speed storage and retrieval of fixed content, such as documents stored for compliance with government regulations. Roughly speaking, content-addressable storage is the permanent-storage analogue to content-addressable memory.

Content Addressable Storage (CAS) and Fixed Content Storage (FCS) are different acronyms for the same type of technology. The CAS / FCS technology is intended to store data that does not change (fixed) in time. The difference is that typically CAS exposes a digest generated by a cryptographic hash function (such as SHA-1 or MD5) from the document it refers to. If the hash function is weak, this method could be subject to collisions in an adversarial environment (different documents returning the same hash). The main advantages of CAS / FCS technology is that the location of the actual data and the number of copies is unknown to the user.

When being contrasted with content-addressed storage, a typical local or networked storage device is referred to as location-addressed. In a location-addressed storage device, each element of data is stored onto the physical medium, and its location recorded for later use. The storage device often keeps a list, or directory, of these locations. When a future request is made for a particular item, the request includes only the location (for example, path and file names) of the data. The storage device can then use this information to locate the data on the physical medium, and retrieve it. When new information is written into a location-addressed device, it is simply stored in some available free space, without regard to its content. The information at a given location can usually be altered or completely overwritten without any special action on the part of the storage device.

Within the scope of this discussion, a good way to think of the above is as container-addressed storage.

The Content Addressable File Store (CAFS) was a hardware device developed and sold by International Computers Limited (ICL) in the 1970s and 1980s that provided location-addressed disk storage with built-in search capability. The search logic was incorporated into the disk controller. A query expressed in a high-level query language could be compiled into a search specification that was then sent to the disk controller for execution. Files could also be accessed via the conventional location-addressing mechanism, permitting CAFS to support an IDMS CODASYL database and also support content addressing of the same records.

...
Wikipedia