4.2.03. A typical transaction pair

  • T1


  • read_lock(Y);
  • read_item(Y);
  • unlock(Y);


  • write_lock(X);
  • read_item(X);
  • X=X+Y;
  • write_item(X);
  • unlock(X);
  • T2


  • read_lock(X);
  • read_item(X);
  • unlock(X);


  • write_lock(Y);
  • read_item(Y);
  • Y=X+Y;
  • write_item(Y);
  • unlock(Y);
  • Violates rules of two phase locking
  • unlock occurs during locking/expanding phase