Serializable transactions

Transactions groups a set of read/write operations together

  • e.g. 
    begin_tx
v <-- read(checking)
v' <-- read(saving)
write(checking, v-10)
write(saving,v'+10)
commit_tx
  • Transactions Properties: ACID
    • Atomicity, Consistency, Isolation, Durability
  • Two goals:
      1. handle failure (A, D). A machine crash and later re-starts.
        • can be achieved by write-ahead-logging
      1. handle concurrency (I): serializability

Concurrency control

T1:
transfer $10 from checking to saving

T2:
withdaw $20 from checking

T3:
read the sum of checking, saving

Bad interleaving
T1: Read(C)=100,             Read(S)              Write(C, 90), Write(S, 110)
T2:              Read(C)=100          Write(C, 80)
  • What’s the expected outcome?
    • Isolation: T1 happens either before or after T2 (sometimes called before-after atomicity)
  • Ideal isolation semantics: serializability
  • Definition: execution of a set of transactions is equivalent to some serial order
    • Two executions are equivalent if they have the same effect on database and produce the same output
    • typically, people mean strict serializability, i.e. the equivalent serial order cannot re-order commit-to-begin_tx ordering.

(Conflict) serializability

  • An execution schedule is an ordering of read/write/commit/abort operations (executed by the local system)
  • e.g. 
    R_1(C), R_2(C), R_1(S), W_2(C), Commit2, W_1(C), W_1(S), Commit1
  • Two schedules are equivalent if they:
    • contain the same operations
    • order conflicting operations the same way:
  • A pair of operations conflict if they access the same data item and one is a write.
  • Examples: 
    Serializable? R_1(C), R_1(S), R_2(C), W_2(C), Commit2, W_1(C), W_1(S), Commit1
No
Serialiazable? R_1(C), R_1(S), R_3(C), R_3(S), Commit3, W_1(C), W_1(S), Commit1
equivalent to R_3(C), R_3(S), Commit3, R_1(C), R_1(S), W_1(C), W_1(S), Commit1
Serialiazable? R_1(C), R_1(S), W_1(C), R_3(C), R_3(S), Commit3, W_1(S), Commit1
no.
  • How to check if a schedule is serializable?
    • Check if Serialization graph is cycle-free