• Safety (ME1) and Liveness (ME2) are guaranteed
• Ordering (ME3) is not guaranteed
  • Network delays may reorder requests
• Performance and availability of server are the main drawbacks

Ring-Based Algorithm

• Pass a token (in one direction) around a ring

• Ring is logical, not necessarily physical link

  • Every process $p_i$ has connection to process $p_{(i+1)\ mod\ N}$

• Token arrival

  • Only process in possession of token may access critical region
  • Pass token on to neighbour if: no request upon arrival of token, or when exiting critical region

• Algorithm satisfies ME1, ME2, but not ME3

  

• Performance

  • Bandwidth
    • Enter: 1 to N msg
    • Exit: 1 msg
  • Client delay
    • 0 to N msg transmissions delays
  • Synchronization delay
    • 1 to N-1 msg transmission delays