Abstract: High reactivity cannot be achieved without each object being replicated on every site, when assuming a nonnegligible network latency in a CSCW system. It is an austerity challenge for concurrency control, and can cause some new problems, such as intention violation. A new algorithm about concurrency control, based on document marking, uses state vector. It not only accords with the integrity consistency model, but also is easy to understand and more efficient.