Distributed Commit in Asynchronous Systems

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1 Distributed Commit in Asynchronous Systems Minsoo Ryu Department of Computer Science and Engineering

2 2 Distributed Commit Problem - Either everybody commits a transaction, or nobody - This means consensus! 2

3 3 Example (Transaction) 3

4 4 Example: Straw Man Protocol 4

5 5 Problems with the Straw Man Protocol What could possibly go wrong? Not enough money in A s bank account? B s bank account no longer exists? A or B crashes before receiving message? The best-effort network to B fails? TC crashes after it sends debit to A but before sending to B? 5

6 Two-Phase Commit

7 7 Two-Phase Commit Developed for distributed databases (DDBMS) DDBMS is highly dependent on ability of all sites to be able to communicate reliably with one another Sites may fail Communication failures can result in network becoming split into two or more partitions Model Transaction coordinator (TC), aka coordinators, centrally coordinates operations that span multiple nodes Resource managers (RM), aka participants, manage individual resources on different nodes Operations (replicated or different) are sent to nodes in a transaction Atomicity: either everybody commits transaction, or nobody 7

8 Transaction Coordinator Resource Manager 8 Message Flow Prepare Yes or No Commit or Abort Done 8

9 9 2PC Algorithm (Phase 1) Prepare phase (voting phase) TC sends a query to commit message to all RMs and waits until it has received a reply from all RMs RMs check and execute the transaction up to the point where they will be asked to commit They each write an entry to their undo log and an entry to their redo log so that they will be able to abort or commit Each RM replies with an agreement message if the RM's actions succeeded, or an abort message if the RM experiences a failure that will make it impossible to commit 9

10 10 2PC Algorithm (Phase 2) Commit phase (completion or decision phase) Success: If the TC received an agreement message from all RMs The TC sends a commit message to all the cohorts Each RM completes the operation, and releases all the locks and resources held during the transaction Each RM sends an acknowledgment to the coordinator The TC completes the transaction when all acknowledgments have been received Failure: If the TC receives any abort message (or timeout expires) The TC sends a rollback message to all the RMs Each RM undoes the transaction using the undo log, and releases the resources and locks held during the transaction Each RM sends an acknowledgement to the coordinator The TC undoes the transaction when all acknowledgements have been received 10

11 11 State Diagrams Transaction Coordinator Resource Manager 11

12 12 Failure Recovery in 2PC Possible failures Nodes may crash (and possibly wake up later) Messages may be lost Timeouts Use timeouts to avoid process blocking when a process is waiting for a message from another process Write-ahead log Each node uses write-ahead log protocol, to achieve local fault recovery and rollback All nodes know their state before crash 12

13 13 RM Failures If a RM fails before voting, TC can timeout and decide abort If a RM fails after voting, if he voted commit, he must be prepared to commit the transaction when he recovers He must ask the coordinator or other participants what the decision was before doing so If he voted abort, he can abort the transaction when he recovers So BEFORE the participant votes, he must log on stable storage his position 13

14 14 CM Failures If CM fails before requesting votes, participants timeout and abort If CM fails after requesting votes (or after requesting some votes), RMs who did not get the vote request timeout, abort RMs who voted abort timeout and abort RMs who voted commit cannot unilaterally abort as all other RMs may have voted commit, and TC decided commit and some RM may have received the decision Must either wait until the TC recovers or contact ALL the other RMs The crash or unavailability of coordinator and one RM results in a BLOCK 14

15 Three-Phase Commit

16 16 Blocking Problem with 2PC If a RM have voted commit and is waiting for TC in READY state, it cannot abort unilaterally This is because all other RMs have voted commit, TC decided commit, and some RMs may have received the commit decision Must either wait until the TC recovers or contact ALL the other RMs The crash or unavailability of coordinator and one RM results in a BLOCK 2PC is safe, but not live due to the above blocking problem The blocking can be solved by the 3PC protocol 16

17 17 Example: Blocking with 2PC A/B/C/D voted for commit A received commit and fails B/C/D waits indefinitely for TC or A When TC wakes up, TC can send commit When A wakes up, B/C/D can ask A If TC is also a RM, as it typically is, then 2PC is vulnerable to a single-node failure 17

18 18 Three-Phase Commit (3PC) 3PC introduces an extra phase where RMs are told what the consensus is 3PC satisfies two conditions that are necessary and sufficient for a commit protocol to be nonblocking There is no single state from which it is possible to make a transition directly to either a COMMIT or an ABORT state There is no state in which it is not possible to make a final decision, and from which a transition to a COMMIT can be made 18

19 Transaction Coordinator Resource Manager 19 Message Flow 19

20 20 State Diagrams Transaction Coordinator Resource Manager 20

21 21 Dealing with Failure and Timeout Failure scenario A/B/C/D voted for commit A received precommit and fails TC fails and B/C/D are waiting Solution B/C/D timeout and abort When a new TC is elected, it will not send globalcommit until all RMs have acked that they are prepared to commit 21

22 22 Safety of 3PC Liveness (availability): Yep Doesn t block, it always makes progress by timing out Safety (correctness): Nope Can you think of scenarios in which original 3PC would result in inconsistent states between the replicas? Two examples of unsafety (network partitions) A hasn t crashed, it s just offline TC hasn t crashed, it s just offline 22

23 23 3PC with Network Partitions One example scenario: A receives precommit from TC Then, A gets partitioned from B/C/D and TC crashes None of B/C/D have received precommit, hence they all abort upon timeout A is prepared to commit, hence, according to protocol, after it times out, it unilaterally decides to commit Similar scenario with partitioned, not crashed, TC 23

24 24 Safety vs. Liveness So, 3PC is doomed for network partitions The way to think about it is that this protocol s design trades safety for liveness Remember that 2PC traded liveness for safety Can we design a protocol that s both safe and live? Well, it turns out that it s impossible in the most general case! 24

25 25 Fischer-Lynch-Paterson Impossibility Result It is impossible for a set of processors in an asynchronous system to agree on a binary value, even if only a single process is subject to an unannounced failure The core of the problem is asynchrony It makes it impossible to tell whether or not a machine has crashed (and therefore it will launch recovery and coordinate with you safely) or you just can t reach it now (and therefore it s running separately from you, potentially doing stuff in disagreement with you) 25

26 26 Paxos What FLP says: you can t guarantee both safety and progress when there is even a single fault at an inopportune moment What FLP doesn t say: in practice, how close can you get to the ideal (always safe and live)? Next: Paxos algorithm, which in practice gets close 26

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