Quorum

Quorum :
    * Understanding importance of quorum
    * Perform Quorum calculation.
    * Configure cluster votes.
    
    -> what is Quorum?
        In order for a cluster function the nodes must be in agreement on certain facts, like:
        *which machines are currently cluster member.
        *where services are running.
        *which machine are using which resources. etc.
    for that all importance of voting comes.
    every node has a vote, and only if more than half of all possible votes are successfully cast "wikk the cluster will be operational"
    Quorum : the minimum number of votes needed to achieve more than half of the votes.
    Quorate "if number of votes is achieved the cluster"
    ---------------
    split brain:
    ---
    caused if service dead on one node but still FS mounted and the other went up and mounted the same FS.
    
    why do we need Quorum ?
    ---
    if we have 5 nodes and service went down on 3 of 5 so the cluster will be down as it is not quorate so we need quorum to reach the votes to the half number of the running nodes.
    
    ----
    -> Calculation Quorum.
    first :
    Expected votes:
        number of votes expected if all cluster nodes are fully operational and communicating with each other.
    Total votes:
        number of votes currently present. this can be lower than the number of expected votes(if some nodes are down or not communicating)
        
    
    Quorum = floor(expected votes / 2 + 1)
    floor() mean : always round down.
    
    Example:
        Quorum = floor(3/2 +1)
        Quorum = floor(1.5 + 1)
        Quorum = floor(2.5)
        Quorum = 2
        
        
    -> Displaying Quorum.
    
    in /etc/cluster/cluster.conf
        under clusternode > votes
        
    ccs_tool lsnode
    cman_tool status
    clustat
    clustat -Q && echo Quorate || echo Not Quotrate
    "clustat -Q " will exit with 0 if quorate and 1 for not quorate
====
Note
====
    If you use SNMP monitoring in your infrastructure, you can use this special cluster plugin that Redhat High Availability ship (net-snmp)
    "/usr/share/doc/cluster-snmp-*/README.snmpd"
    
    
    
    -> Configure Quorum votes.
        Edit Quorum votes with luci.
        Edit votes manually:
            from /etc/cluster/cluster.conf
            <clusternode name="node1" nodeid="1" votes="2"/>
            "DONT FORGET TO CHANGE VERSION NUMBER"
            cman_tool version -r
            cman_tool status
            

Practise:
=========
* Displaying Quorum and votes.
* Adding more votes to a node.