Sebastian Obermeier, University of Paderborn Germany. mbusiness Deadlines pre-acp Blocking Bi-State-Termination

Transcription

1 Sebastian Obermeier, University of Paderborn Germany

2 I sell my BMW for o.k. it is reserved for you o.k. I want to buy it Which problems does network partioning cause for this business? next slides 2/29

3 I sell my BMW for o.k. it is reserved for you o.k. I want to buy it? I am bound, I sold my BMW. Why does he not react? Has he already sold it to someone else? Do I have to look for another car? 3/29

4 I sell my BMW for o.k. it is reserved for you? I am bound, I sold my BMW. o.k. I want to buy it Nice that this is now my BMW Why does he not pay? 4/29

5 I sell my BMW for o.k. it is reserved for you I am bound, I sold my BMW.? o.k. I want to buy it? did he sell it to someone else? Shall I buy another car? The problem: Without network connection partners can not decide whether or not they agreed on the business 5/29

6 in mobile systems: network split occurs more often blocking possible (without network connection partners can not agree on commit)?? in fixed-wired networks: network split is rare often a different channel is available, e.g. phone, no blocking 6/29

7 instead of being blocked without knowing the Commit decision (alternative "solutions") move all data about Commit decision to fixed-wired network each connected user has access to Commit decision every user takes only his own risk of being not connected business contract is valid, only if commited by fixed wired network one-side binding (of the weaker business partner): customer is bound (blocking possible) until confirmed, e.g. booking a flight via internet seller is bound until confirmed by customer e.g. written offers 7/29

8 All messages until deadline may get lost ask for confirmation?? I sell my BMW for , if you want it, you should let me know by 4 pm. I want to buy it, if I can have it let me know by 4 pm Question: Is it possible to set up deadlines (e.g. 4 p.m.) for Commit decisions when network partitioning may occur? 8/29

9 A B A: General Alexander, B: General Bonaparte A -> B: Hey Bona, attack at break of dawn B -> A: Good Idea, Al, see you tomorrow A receives message, but suddenly realizes: Bona does not know whether I have received his message. Maybe he does not dare to attack Therefore: Message of A stating the reception of the Acknowledgement. B receives the message and thinks: Al does not know whether I have received his message. Maybe he does not dare to attack Therefore: Message of B, Even if not message loss occurs: The problem is not solvable 9/29

10 Assume, there is an ACP with deadlines. Case 1: ACP contains at least one message. there must be a "last message" M to a receiver R, i.e. R does not send a message back to S after receiving M. S's commit decision can not depend on whether M reaches R. As M can get lost, i.e. M may not reach R before the deadline, and as R's and S's commit decisions are identical, R's decision (like S's decision) can not depend on whether M reaches R M can be deleted without changing R's and S's commit decision The same holds for all other messages The decision must be the same as if no message was exchanged Case 2: ACP contains no message. a safe decision for commit is not possible. 10/29

11 Resource Blocking even before DB votes for commit:... DB1 DBn Coordinator readytovotecommit? ready ready VoteCommit cancommit? Commit! 11/29

12 sub-transaction on DB is ready to vote commit: DB enters non-blocking suspend state DB still controls resources DB still can abort sub-transaction e.g. after time-out DB1... DBn readytovotecommit? Coordinator Coordinator coordinator asks for confirmation when all votes have arrived DB is only blocked for confirmation not blocked in suspend state + reduces blocking time ready VoteCommit ready cancommit? Commit! + reduces risk to be blocked [ EDBT-PIM 2006 ] 12/29

13 Additional state, if T in suspend and J executed, test for conflict: WS(T) (RS(J) U WS(J)) = Ø, or RS(T) WS(J) = Ø In suspend, RM is not bound to its vote RM must vote again 2 Votes: pre-vote and vote RM aborts or repeats transaction in case of conflicting transactions no blocking of conflicting transactions wait for global commit (blocking) commit commit invokesubtransaction coordinator: suspend! coordinator: Vote! abort successful, send pre-vote read-phase suspend (non-blocking) abort repeat abort send unsuccessful 13/29

14 DB has sent vote uncertain about transaction execution new state old state If concurrent transactions request conflicting locks Blocking 14/29

15 old state after transaction abort conflicting transaction 25 remaining seats new state after transaction commit 25 remaining seats 23 remaining seats reserve 3 seats 25 remaining seats 22 remaining seats 23 remaining seats commit 2nd transaction 20 remaining seats 15/29

16 Terminate a pending transaction T, which is uncertain about its result, into two states T committed T aborted Following transactions operate on both: before-image and after-image of the Bi-State terminated transaction [IDEAS 2007] 16/29

17 Lock Point Initiator received results ACP starts Transaction sequence fix 17/29

18 Locked by Tx Phase 1 Phase 2 Acquire locks for Tx Vote Request Conflicting Ty must wait Continue Ty VoteCommit DoCommit, release locks [IDEAS 2007] 18/29

19 Locked by Tx Phase 1 Phase 2 Execute Ty Commit Ty Acquire locks for Tx Vote Request Bi-State Terminate Tx release locks of Tx Acquire Locks for Tx Release Locks for Tx VoteCommit DoCommit (Tx) [IDEAS 2007] 19/29

20 ID Name Location After Before 5 Miller Paderborn 7 Smith Paderborn After: IDs of Transactions that inserted the tuple Before: IDs of Transactions that deleted the tuple 20/29

21 T1 : INSERT INTO Persons VALUES(5, Miller, Paderborn ) ID Name Location After Before 5 Miller Paderborn 1 21/29

22 T4 : DELETE * FROM Persons WHERE ID = 7 ID Name Location After Before 5 Miller Paderborn 1 7 Smith Paderborn 4 22/29

23 T5 : UPDATE Persons SET Location = PB WHERE Location = Paderborn ID Name Location After Before 5 Miller Paderborn 1 7 Smith Paderborn 4 23/29

24 T5 : UPDATE Persons SET Location = PB WHERE Location = Paderborn ID Name Location After Before 5 Miller Paderborn 1 7 Smith Paderborn 4 5 Miller Paderborn Smith Paderborn Copy affected tuples and mark as deleted 24/29

25 T5 : UPDATE Persons SET Location = PB WHERE Location = Paderborn ID Name Location After Before 5 Miller PB Smith PB Miller Paderborn Smith Paderborn Copy affected tuples and mark as deleted 2. Execute update on after-image 25/29

26 Transaction T commits Delete tuples with T appearing in column Before Delete all strings T in column After Transaction T aborts Delete tuples with T appearing in column After Delete all strings T in column Before 26/29

27 ID Name Location After Before 5 Miller PB Smith PB Miller Paderborn Smith Paderborn 4 5 T5 Commits ID Name Location After Before 5 Miller PB Smith PB Miller Paderborn Smith Paderborn /29

28 T6 : SELECT * FROM Persons WHERE Location = PB ID Name Location After Before 5 Miller PB 1 7 Smith PB 4 ID Name Location ID Name Location 5 Miller PB 5 Miller PB T1 Commit, T4 Commit 7 Smith PB T1 Commit, T4 Abort ID Name Location 7 Smith PB ID Name Ort T1 Abort, T4 Abort T1 Abort, T4 Commit 28/29

29 Transactions Atomic Commit Protocols 2PC 3PC CLCP mbusiness business with mobile partners Deadlines Blocking before Atomic Commit Protocol start Bi-State-Termination 29/29