We continue reading the paper "Erasure coding in Windows Azure storage"  by Cheng Huang, Huseyin Simitci, Yikang Xu, Aaron Ogus, Brad Calder, Parikshit Gopalan, Jin Li, and Sergey Yekhanin.
Erasure coding is a data protection method where data is broken into fragments, expanded and encoded with redundant data pieces and stored across a different set of locations or media. This paper talks about Erasure coding in windows Azure storage WAS. It introduces new codes called the Local Reconstruction Codes.A (k,l,r) LRC divides k data fragments into l local groups. It encodes l local parities, one for each local group and r global parities.  We were discussing Local Reconstruction codes in windows azure storage.  The placement of the fragments was based on two factors: load which favors the less occupied extent nodes and reliability which avoids placing fragments into the same correlated domain.  Correlated domains can be those from a fault such as on a rack that brings down a few nodes together or they can be on an upgrade domain which categorizes a group of nodes that are taken offline. Let us now see how to place a fragment in WAS.  A WAS stamp consists of twenty racks There are sixteen fragments possible for an extent and each is placed on a different rack for maximum reliability. If each fragment were similarly placed on a different upgrade domain, there would be one too many and resulting in slow down.  There are up to 10 upgrade domains used. In order to place the fragments across upgrade domains, the local group property of LRC is exploited and the group fragments belonging to the different local groups are placed into the same upgrade domains. The local data fragments are placed in the same upgrade domain. Similarly the local data parities are placed in the same upgrade domain.  The global parities are placed in separate upgrade domains.
#codingexercise
After detecting the overlaps from two strings, find if a new given string is a concatenation of two or more overlaps

Void Combine (List<List<char>> overlaps, List<List<char>> b, List<char> target,  int start, int level)

{

For (int I = start ; I < overlaps.Length; i++)

{

b[level] = overlap[i];
if (b.toString() == target.toString()) Console.WriteLine('True');

If (I < overlaps.Length)

Combine(overlaps,b, target, start+1,level+1)

B[level] = NULL;

}