On Optimistic Methods for Concurrency Control

H. T. Kung and John T. Robinson.
CMU

One-line Summary

Overview/Main Points

• Idea: all xacts have 3 phases

  read	validation	write
  all writes are private	check for conflicts	make write public

• Serial validation
  • record start_tn when xact starts
  • record read_set & write_set while running
  • obtain xact number (T_n) at the start of validation phase
  • do validation & write phase in critical section (one at time)

    Begin Critical Section
      finish_tn = current Tn
      valid = True
      for t from start_tn + 1 to finish_tn do
        if (write_set (t) ∧ read_set)
          valid = false;
    
      if valid then
        write phase
        current Tn += 1
        tn = current Tn
    End Critical Section
    

• Consider T_i & T_j. For T_i & T_j, if T_N(T_i) < T_N(T_j), then one of the followings must hold
  • T_i completes its write phase before T_j starts its read phase.
  • WS(T_i) ∧ RS(T_j) = Ø, and T_i completes its write phase before T_j starts its write phase.
  
• Goal: allow only serializable scheduler
• Techniques

Relevance

Flaws