We continue discussing the ZooKeeper. It is a co-ordination service with elements from group messaging, shared registers and distributed lock services. It provides a interface that guarantees wait-free property and FIFO execution of requests from each client.  Requests across all clients are also linearized.
Both the linearizable writes and the FIFO client order form the guarantees for ZooKeeper. These two guarantees also interact.  ZooKeeper handles both by designating a path as the "ready" ZNode. The other clients may make changes only when that ZNode exists.
ZNodes can be both regular and ephemeral. Regular nodes are the ones that clients can create and delete explicitly. Ephemeral nodes are the ones that the clients can create for the duration of the session. Sessions have an associated timeout. A session is initiated when the client connects and ends when the client closes or ZooKeeper deems faulty. Sessions come with associated state changes that inform the stage in which the execution of the operations are. Sessions also enable a client to move from one ZooKeeper server to another.
Perhaps the most important aspect of ZooKeeper is the interface it provides the clients. These include the following methods:
1) create (path, data, flags) : Used to create a ZNode with the name path and to store data array in it. It returns the name of the newly created ZNode.
2) delete(path, version) : delete the ZNode path if it is at the expected version
3) exists(data, path): Returns trues if ZNode with name path exists, and returns false otherwise
4) getData (path, watch) : returns the data and metadata for the node.
5) setData (path, data, version): writes the data object array to the node path if the version matches
6) getChildren(path, watch) - returns the set of names of the children of a node
7) sync(path) - Waits for all updates pending at the start of the operation to the concerned Zookeeper server.
#codingexercise
Find the position of the number closest to the midpoint of the range of numbers in a row-wise and column-wise sorted two dimensional array
Tuple<int, int> GetPosition(int [,] A, int r, int c)
{
int min = A[0,0];
int max = A[r-1, c-1];
int mid = (min+max)/2;
var t = new Tuple<int, int>();
int threshold = INT_MAX;
for (int i = 0; i < r-1; i++) // row wise
   for (int j = 0; j < c-1; j++) // column wise
    // we could optimize this to picking the element bottom-right, or bottom or right for the one closet to mid
   // here we simply traverse through the numbers
   { int distance = Math.Abs(A[i,j] - mid);
      if (distance <= threshold){
           threshold = distance;
           t.first = i;
           t.second = j;
   }
return t;
}