Degrees of Isolation
Serializability = Degree 3 Isolation
Degree 2 Isolation (a.k.a. cursor stability)
- Data manager holds read-lock(x) only while reading x, but holds write locks till commit (as in 2PL)
- E.g. when scanning records in a file, each get-next-record releases lock on current record and gets lock on next one
- read(x) is not “repeatable” within a transaction, e.g.,rl1[x] r1[x] ru1[x] wl2[x] w2[x] wu2[x] rl1[x] r1[x] ru1[x]
- Degree 2 is commonly used by ISAM file systems
- Degree 2 is often a DB system’s default behavior!And customers seem to accept it!!!