Distributed   Transactions Note for Distributed Systems Concepts and Design 5th Biaobiaoqi 2013-­‐‑08-­‐‑25 

Outline •  Flat and Nested Distributed Transactions •  Atomic Commit Protocols •  Concurrency Control in Distributed Transaction •  Deadlock Detection 

Distributed  Transactions •  Flat Transaction •  Nested Transaction •  Distributed Transaction: a flat or nested transaction that accesses objects managed by multiple servers. 

Example  for  Nested   Transaction 

Coordinator •  Roles o  Client, Coordinator, Participant •  Requests o  openTransaction, closeTransaction •  Join o  a new participant joins the transaction 

Atomic  Commit  Protocols •  Atomicity o  It requires that the servers participating in a distributed transaction either all commit it or all abort it. •  One-phase atomic commit protocol o  Coordinator to communicate the commit or abort request to all participants in the transaction and to keep on repeating the request until all have carried it out. o  It does not allow a server to make a decision to abort a transaction: •  Deadlock •  Optimistic concurrency control, validation phase failes •  Server crash 

2  Phase  Commit  Protocol •  Goal: o  Designed to allow any participant to abort its part of a transaction. •  1st phase(voting phase) o  Coordinator sends a canCommit? Request to all participants. o  Each participant votes Yes/No(committed/aborted) to coordinator.(back up in permanent storage) •  2nd phase(completion phase) o  Coordinator collects votes and make decision. o  Participants votes YES wait for doCommit or doAbbort request. If it’s commit case, participants make haveCommited call as confirmation. 

2PC  State  Transfer •  Coordinator: •  Participant: 

2PC  Failure  Model •  No Byzantine faults o  It’s assumed that an underlying protocol removes corrupt and duplicated messages •  Consensus cannot be reached in an asynchronous system if process sometimes fails. o  Process recovery from permanent storage and information held by other processes. 

Timeout  Action  in  2PC •  Uncertain state for participant o  After participant reply canCommit request, and waits for a long o  Causes •  Server crash or delayed messages. o  Resolution •  Participant sends getDecision request to coordinator, asking coordinator for decision on a transaction after some delay. •  Uncertain state for coordinator o  Coordinator waits for canCommit reply for a long time o  Causes •  Participants crash or delayed messages o  Resolution •  Coordinator sends doAbort request 

Performance  of  2PC •  Time costs o  If all goes well: 3N (haveCommited process are not counted) •  Blocking commit protocol o  à3-phase commit protocol •  Availability of single-node 

2PC  for  Nested   Transactions •  Relationship o  Parent transaction :coordinator o  Child sub-transaction :automatically joins parent •  Dependencies: o  Parent aborts à child will be forced abort o  Child aborts à parent can even commit(determined by business logic) •  Provisionally commit != prepared to commit 

Example 1 2 T 11 T 12 T 22 T 21 abort (at M) provisional commit (at N) provisional commit (at X) aborted (at Y) provisional commit (at N) provisional commit (at P) T T T 

Hierarchic  and  flat  2PC •  Hierarchic 2PC o  Multi-level nested protocol o  Parent sends canCommit to immediate children, and so on down the tree. o  Disadvantage •  Involves passing a series of messages down and up the tree •  Flat 2PC o  Coordinator of top-level transaction sends canCommit messages to all subtransacitons. o  Disadvantage •  Need to have the abort list in order to eliminate transactions whose parents have aborted. 

Concurrency  Control  in   Distributed  Transaction •  Concurrency Control o  Each sever applies local concurrency control; o  globally serialized is also needed. •  3 Kinds o  Locking o  Timestamp Ordering o  Optimistic Concurrency Control 

Locking •  Locks on an object are held locally. •  It cannot release any locks until it knows that the transaction has been committed or aborted at all the servers involved in the transaction. •  Distributed deadlock 

Timestamp  ordering •  Globally unique timestamps are needed. •  Same ordering of transactions can be achieved at all the servers. is used, in which server-id is less significant. •  Timestamps can be kept roughly synchronized through all coordinators by the use of synchronized local physical clocks 

Optimistic  Concurrency  Control •  Optimistic o  The likelihood of two clients’ transactions accessing the same object is low. •  3 Phase o  Working Phase •  Tentative version. o  Validation Phase •  Often executed in critical section •  After closeTransaction request is received. •  If validation fails, abort some transaction. o  Update Phase: •  Often executed in critical section •  Tentative versions are made permanent. •  Validation takes place during the first phase of the two-phase commit protocol. 

Deadlock •  PAID: Prevent, Avoid, Ignore, Detect •  Deadlock detection: find cycles in wait-for graph •  Simple solution: centralized deadlock detection. o  Poor availability, Lack of fault tolerance o  No ability to scale o  Transmission of local wait-for graphs is high •  Phantom deadlocks •  Edge chasing/path pushing o  Global wait-for graph is not constructed o  Forwarding messages called probes to find cycles 

Edge-­‐‑chasing  Algorithm •  1.Initiation o  When a server notes transaction T starts waiting for transaction U, where U is waiting to access an object at another server, it sending a probe containing the edge to the server of U. •  2.Detection o  Detection consists of receiving probes o  whether a deadlock has occurred o  whether to forward the probes. •  3.Resolution o  When a cycle is detected, a transaction in the cycle is aborted to break the deadlock. 

Edge-­‐‑chasing