Parallel Snapshot Isolation


  • each data item has a preferred site
  • use per site timestamp <site, seqno>
    • vector timestamp <seqno1, seqno2, …>
    • each site maintains a vector timestamp indicating remote sites GotVTS
  • Tx protocol
    • get a startVTS, which has the highest seqno for each site
    • read local version that is “visible”.
      • data with version <site, seqno>: seqno < startVTS[site]
    • write buffered at local write-set
    • commit, use 2PC:
      • prepare phase, send to all preferred sites of objects in the write-set
        • if modified (has newer version), reply no
        • if being locked, reply no
        • otherwise, lock the data and reply yes
      • commit phase
        • assign a sequence number, apply changes locally
        • then propagate changes in the background
    • fast commit:
      • if all preferred sites are local, then no geo coordination needed
    • on propagation
      • the preferred site can release locks
      • wait until GotVTS >= startVTS, and GotVTS[site] = seq-1

Anomalies analysis

  • not possible for either SI or PSI
    • dirty read: non-committed value
    • non-repeatable read: read different values for the same object
    • lost updates: concurrent writes become “lost”
  • possible for both SI and PSI
    • short fork
      T1: Read(A)=0, Read(B)=0, Write(A)=1 
      T2: Read(A)=0, Read(B)=0, Write(B)=1 
      T3:                                  Read(A)=1, Read(B)=1
  • not possible for SI, but possible for PSI
    • long fork
      T1: R(A)=0, R(B)=0, W(A)=1 
      T2:                    R(A)=1, R(B)=0
      T3: R(A)=0, R(B)=0, W(B)=1 
      T4:                    R(A)=0, R(B)=1
      T5:                                  R(A)=1, R(B)=1