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. For example, when many clients elect a leader, the leader may take some time to complete its work before it can notify others. During this time, the other clients may not make any changes. And if the leader dies, the partial changes may not be used. Chubby service does not handle the latter requirement. ZooKeeper handles both by designating a path as the "ready" ZNode. The other clients may make changes only when that ZNode exists. Therefore the leader is only responsible for deleting ready, making changes and creating ready. If the leader dies, the ready is not available, so the other clients won't use the changes. If there are a large number of clients, then the latency per client ZNode change will aggregate but because the requests are asynchronous and the changes are pipelined. it does not take that much time.
There are two types of nodes a client can create
clients can create and delete the nodes explicitly and these are called regular ZNodes
clients can create the nodes for the duration of a session and these are called ephemeral ZNodes
#codingexercise
Find the majority element in an array.
using Moore's voting algorithm:
int GetMajorityCandidate(List<int> A)
{
int mindex = 0;
int count = 1;
for (int i = 1; i < A.Count; i++)
{
if (A[mindex] == A[i])
     count++;
else
     count--;
if count == 0;
{
   mindex = i;
   count = 1;
}
}
return A[mindex];
}
bool IsMajorityCandidate(List<int>A,  int candidate)
{
int i, count = 0;
for (int i = 0; i < A.Count; i++)
     if (A[i] == candidate)
         count ++;
if (count > n/2)
   return true;
return false;
}