Parallel Snapshot Isolation

Design

• 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