Regenerating code (RGC)[1] 

An \([n,k,d,\alpha,\beta,M]\) Regenerating Code \(\mathcal{C}\) is an erasure correcting code that encodes \(M\) symbols from \(GF(q)\) into an \(\alpha \times n\) matrix over \(GF(q)\), with each column of the matrix treated as a coordinate of a codeword.

The code has the following properties. Any \(k\) coordinates of the codeword can be used to recover the original \(M\) symbols. Any erased coordinate, i.e., the \(\alpha\) symbols of that coordinate, can be recovered by contacting any \(d\) other coordinates and collecting \(\beta\) symbols from each one of them, where \(k \le d \le n-1\) and \(\beta \le \alpha\).

The connection of such codes to distributed storage is as follows. A file of size \(M\) symbols is encoded using the code, and the \(\alpha\) symbols of each coordinate are stored in geographically separated storage nodes. Accessing any \(k\) such nodes gives back the original file. Node failures are considered erasures in the codeword, and any such failed node can be regenerated by contacting \(d\) surviving nodes and downloading \(\beta\) symbols from each of them.