Update serializability in locking

Abstract

There are many concurrency control algorithms that use weak consistency of read-only transactions to enhance concurrency in database systems[3,8,18]. We identify these schedules (which become serializable when the weakly consistent read-only transactions are deleted from them) as update serializable schedules. In this paper, we study how update serializable schedules can be produced using locking and the advantages of allowing update serializable schedules in locking protocols. We show that if a locking protocol P which uses exclusive mode locks only is made heterogeneous, then the new locking protocol, called the heterogeneous locking protocol P, ensures update serializability. In addition, we also show that if the original locking protocol is deadlock-free, then a deadlock cycle in the heterogeneous locking protocol involves at least two read-only transactions. We also discuss the advantages that one gets when the guard locking protocol is made heterogeneous. The heterogeneous two phase locking protocol operates on general databases and ensures serializability. We present a simple non-two phase locking protocol on general databases that produces update serializable schedules which are not serializable. This protocol allows the read-only transactions to be non-two phase and restricts the update transactions to access exactly one data item.