Robustness against Parallel Snapshot Isolation Giovanni Bernardi - PowerPoint PPT Presentation

Robustness against Parallel Snapshot Isolation Giovanni Bernardi jointly with Andrea Cerone Alexey Gotsman 1 / 22

strong consistency weak consistency Eventual Consistency, Sequential DB Causal Consistency, Parallel Snapshot Isolation High Latency Low Latency 2 / 22

P = wr ( y , 1) txn { rd ( x ); rd ( y ) } wr ( x , 1) �≈ Sequential DB 3 / 22

P = wr ( y , 1) txn { rd ( x ); rd ( y ) } wr ( x , 1) wr x 1 wr y 1 �≈ Sequential DB 4 / 22

P = wr ( y , 1) txn { rd ( x ); rd ( y ) } wr ( x , 1) wr x 1 wr y 1 �≈ Sequential DB 5 / 22

P = wr ( y , 1) txn { rd ( x ); rd ( y ) } wr ( x , 1) wr x 1 rd x 0; rd y 1 rd x 1; rd y 0 wr y 1 �≈ Sequential DB 6 / 22

Robustness against psi behaviours of P behaviours of P ? = psi sc 7 / 22

Definition psi : TransHB + Ext + NoConflict T wr ( m , z ) T rd ( n , y ) Transactions: R , S , T , . . . H = { T 1 , T 2 , T 3 , T 4 } , finite Histories: H , H ′ , . . . ( H , hb) 8 / 22

Definition psi : TransHB + Ext + NoConflict T wr ( m , z ) T rd ( n , y ) Transactions: R , S , T , . . . H = { T 1 , T 2 , T 3 , T 4 } , finite Histories: H , H ′ , . . . ( H , hb) ∀ R , S , T ∈ H . R hb S and S hb T ⇒ R hb T hb rd y 1; wr x 1 hb wr y 1 rd x 1; rd y 1 hb causal message delivery 9 / 22

Definition psi : TransHB + Ext + NoConflict T wr ( m , z ) T rd ( n , y ) Transactions: R , S , T , . . . H = { T 1 , T 2 , T 3 , T 4 } , finite Histories: H , H ′ , . . . ( H , hb) ∀ T ∈ H . T rd ( x , n ) ⇒ lastwrite ( x , T , hb) wr ( x , n ) . . . hb rd y 1 ; wr x 1 hb wr y 1 rd x 1 ; rd y 1 hb reads return value written last according to hb or default initial 10 / 22

Definition psi : TransHB + Ext + NoConflict T wr ( m , z ) T rd ( n , y ) Transactions: R , S , T , . . . H = { T 1 , T 2 , T 3 , T 4 } , finite Histories: H , H ′ , . . . ( H , hb) ∀ S , T ∈ H . T , S wr ( x , ) = ⇒ S = T or S hb T or T hb S wr x 3 hb wr x 5 write-write conflict detection 11 / 22

Definition sc : TransHB + Ext + TotalHB T wr ( m , z ) T rd ( n , y ) Transactions: R , S , T , . . . H = { T 1 , T 2 , T 3 , T 4 } , finite Histories: H , H ′ , . . . ( H , hb) S = T or S hb T or T hb S ∀ S , T ∈ H . hb rd z 0 hb wr y 5 hb rd y 5; wr z 7 hb hb hb wr x 3 serial execution 12 / 22

Robustness against psi behaviours of P behaviours of P ? = psi sc Idea: define φ s.t. ∀ ( H , hb) ∈ psi . φ ( H , hb) implies ∃ hb ′ . ( H , hb ′ ) ∈ sc show a total hb ′ over H s.t. easy reads return values written last according to hb ′ hard 13 / 22

Dynamic dependency graph function λ DDG ( H , hb) = ( H , − → ) ◮ T overwrites value written by S on x S hb T ww , x S T ◮ T reads the value written by S on x S hb T wr , x S T ◮ S overwrites a value read by T ww , x wr x , 1 S rd x , 0 R T rw , x rw , x wr , x 14 / 22

Dynamic dependency graph function λ DDG ( H , hb) = ( H , − → ) wr x 1 ◮ T overwrites value written by S on x hb rd x 0; rd y 1 ww , x rd x 1; rd y 0 S T hb ◮ T reads the value written by S on x wr y 1 wr , x S T rw edges ⇒ DDG ( H , hb) may contain cycles ◮ S overwrites a value read by T ww , x wr x , 1 S rd x , 0 R T rw , x rw , x wr , x 15 / 22

Dynamic dependency graph function λ DDG ( H , hb) = ( H , − → ) wr x 1 ◮ T overwrites value written by S on x wr , x rw , y rd x 0; rd y 1 ww , x rd x 1; rd y 0 S T rw , x wr , y ◮ T reads the value written by S on x wr y 1 wr , x S T rw edges ⇒ DDG ( H , hb) may contain cycles ◮ S overwrites a value read by T ww , x wr x , 1 S rd x , 0 R T rw , x rw , x wr , x 16 / 22

Robustness against psi behaviours of P behaviours of P ? = psi sc ∀ ( H , hb) ∈ psi . 1. if acyclic ( DDG ( H , hb)) then ∃ hb ′ . ( H , hb ′ ) ∈ sc 2. if ∃ total po hb ′ . hb ′ contains edges DDG ( H , hb) then reads return values written last according to hb ′ or default initial 17 / 22

Robustness against psi behaviours of P behaviours of P ? = psi sc ∀ ( H , hb) ∈ psi . 1. if acyclic ( DDG ( H , hb)) then ∃ hb ′ . ( H , hb ′ ) ∈ sc 2. if ∃ total po hb ′ . hb ′ contains edges DDG ( H , hb) then reads return values written last according to hb ′ or default initial As it is, useless for static analysis! 18 / 22

Robustness criterion Cycle π in DDG ( H , hb) PSI-critical if rw , x rw , y S ′ → T ′ ◮ π contains S − − − → T , − − − ◮ x � = y for all ( H , hb) ∈ psi no critical cycles in DDG ( H , hb) implies ∃ hb ′ . ( H , hb ′ ) ∈ sc 19 / 22

wr x 1 wr , x rw , y rd x 0; rd y 1 rd x 1; rd y 0 rw , x wr , y wr y 1 20 / 22

in this talk First robustness criterion for psi on going work Reasoning techniques for geo-replicated DBs ◮ Uniform axiomatisation weak consistency levels Paper to appear at CONCUR’15 ◮ Systematic investigation robustness/chopping First chopping criterion for si ask Andrea ◮ Checking robustness of applications against psi TPC-C, RUBiS, . . . 21 / 22

That’s the story. Thank you :-) Questions? 22 / 22