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Time and Global State (Berkeley Algorithm (Internal synchronization when…

- - - - A cut C is consistent if, for each event it contains, it also contains all the events that happened-before that event.
      - • A consistent global state is one that corresponds to a consistent cut.
        • A run is a total ordering of all the events in a global history that is consistent with each local history’s ordering.
        • A linearization or consistent run is an ordering of the events in a global history that is consistent with this happened-before relation. ,
- - - - Lamport’s clock: L(e)<L(e’) we cannot conclude that e->e’.
      - Vector clock to overcome the above problem.
      - N processes is an array of N integers. Each process keeps its own vector clock Vi, which it uses to timestamp local events.
      - VC1: initially, Vi[j] = 0, for i,j = 1,2…N
        VC2: just before pi timestamps an event, it sets Vi[i] = vi[i]+1 VC3: pi includes the value t= Vi in every message it sends
        VC4: when pi receives a timestamp t in a message, it sets Vi[j]=max(Vi[j], t[j])for j =1,2…,N. Merge operation.
- - - - • Primary server: connected directly to a time source.
        •Secondary servers are synchronized with primary server.
        • Third servers are synchronized with secondary servers. Such subnet can reconfigure as servers become unreachable or failures occur.
    - - NTP servers synchronize in one of three modes:
        
        Multicast mode: for high-speed LAN. One or more servers periodically multicasts the time to servers connected by LAN, which set their times assuming small delay. Achieve low accuracy.
        
        Procedure call: similar to Cristian’s algorithm. One server receives request, replying with its timestamp. Higher accuracy than multicast or multicast is not supported.
        
        Symmetric mode: used by servers that supply time in LAN and by higher level of synchronization subnet. Highest accuracy. A pair of servers operating in symmetric mode exchange messages bearing timing information.