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.
