2 - Phase Commit Protocol

Commit Protocol

In transaction processing, databases & comp. n/w, the 2 Phase commit is a type of atomic commitment protocol ( ACP ). It is distributed algo. that co-ordinates all the process that participate in distributed atomic transactions. ( commit/rollback)

This protocol achieves its goal even in many cases of temporary system failure ( involves process, n/w node, comm. etc) and thus widely utilized.

Recovery

To accommodate recovery from failure, the protocols participates use logging ( logs ) of the protocol state. Logs records which are typically slow to generate but survive failures are used by protocol for recovery failure.

2 Phases of Protocol

1. The commit-request phase : ( Voting Phase )

 In which a co-ordinator process attempts to prepare all the transaction’s participating processes to take the necessary steps for either committing or aborting the transaction & to vote either

                               YES ==> Commit                                 NO ==> Abort

YES  :   if the transaction participates local portion execution has ended properly

NO   :   if the problem has been detected with the local portion

2. Commit Phase

Based on voting; the co-ordinator decides whether to commit or rollback & notifies the result to all cohorts. The cohorts then fallow with the needed actions ( commit/abort ) with their local transaction resources & their respective portion in transaction.

Basic Algorithm

Assumptions:

                               Master site  :  One node is designated as co-ordinator

                               Cohorts        :   Rest of the nodes

Protocol assumes that there is a stable-storage at each node with write-ahead log. ( No data loss/no crash in log of each node & two can comm. with each other )

1. Commit Request Phase ( Voting Phase )

The co-ordinator sends a query to commit message to all cohorts & waits until it receive a reply from all cohorts.

The cohorts executes the transaction upto the point, where they will be asked to commit ( Entry written to both undo & redo log buffer )

Each cohort replies with an agreement message ( Yes/No)

                               Yes ==> if the cohorts action succeeded

                               No  ==> if cohorts actions fails ( impossible to commit )

2.Commit Phase ( Completion Phase )

 2.1)  Success

•    The co-ordinator receives “ YES “ message from all cohorts

•    Co-ordinator     =============>    Cohorts (all)

                                          Commit msg.

•     Each cohorts complete the operation & releases all the locks & resources held during transaction.

•     Each Cohorts    =================>  Co-ordinator

                                          Send Acknowledgment

•     Co-ordinator completes the transaction when all acknowledgment have been received.

 

2.2)  Failure ( “NO” )

•     If cohorts votes “NO” during the commit-request phase ( or the co-ordinator timeout expires )

•     Co-ordinator     =============>    Cohorts (all)

                                             Rollback msg.

•     Each cohorts undo the transaction using undo log buffer & releases all the locks & resources held    during transaction.

      

•    Each Cohorts    =================>  Co-ordinator

                                        Send Acknowledgment

•     Co-ordinator undoes the transaction when all acknowledgment have been received.

Disadvantage of 2PC

•  It is a blocking protocol.

• If Co-ordinator fails permanently, some cohorts never resolve their transactions.

          Eg.

        After a cohort has sent an agreement message to co-ordinator cohort is blocked until is commit/rollback msg. is received back by co-ordinator.