We continue discussing the ZooKeeper. It incorporates elements from group messaging, shared registers and distributed lock services in a replicated, centralized service. It provides a interface that guarantees wait-free property and FIFO execution of requests from each client.  Requests across all clients are also linearized.
Caching is also used to improve performance. The id of the leader is useful to cache because it is frequently required and avoids asking other servers.  ZooKeeper uses a watch mechanism to enable clients to cache data without managing the client cache directly.  With this mechanism, a client can watch for an update to a given data object, and receive notification about update.
ZooKeeper organizes data objects around in a hierarchical namespace
Chubby service provides strong locking and synchronization guarantees.  Locks come in useful to implement leader election and group membership.  ZooKeeper does not implement primitives, rather it implements a co-ordination service with an interface. Such a choice led to the implementation of a co-ordination kernel that enables new primitives without changing the service core. Exposing primitives also had an inherent issue. The processing of requests depended on responses and failure detection of other clients. On the other hand, ZooKeeper uses a wait free data objects organized hierarchically as in filesystems.
#codingexercise
Print all possible decodings of integer sequence
private int GetCount(string A, int n)
{
int count = 0;
if ( n ==0 || n == 1) return 1;
if (A[n-1] > '0')
   count = GetCount(A, n-1);
if(A[n-1] == '1' || A[n-2] == '2' && A[n-1] <= '7')
    count += GetCount(A, n -2);
return count;
}