This document discusses two-phase commit protocols for distributed transactions. It defines key terms like transaction manager and cohorts. It explains the two phases - the commit request phase where the coordinator sends commit messages and waits for agreements, and the commit phase where it sends a commit or abort message based on agreements. It provides an example transaction and flow chart. The basic two-phase commit algorithm is described where the coordinator coordinates committing or aborting based on responses in each phase.

1. Nadar Saraswathi college of Arts and Science
Distributed of operating system
two phase & non blocking
commit protocols
presented by
K. Diana Devi
M.sc(cs)

2. Contents
 Definition
 Terms Used
 Transaction Manager
 Terms Used
 Example
 Transaction Chart
 Types of Phase
 Basic Algorithm
 Conclusion of 2 phase commit

3. Definition
The protocols results in either all nodes co
Transaction or aborting, even in the case of site
Terms Used
One node is designated the coordinator(or
manager , which is the master site and the rest
In the network are called cohorts.

4. Transaction Manager
1. Transaction manager responsible for
maintaining a log for
recovery purposes.
2. Starting the execution of transactions that
originate at the site
3. Coordinating the termination of each
transaction at the site.
Terms Used
log -> It is a file listing changes to the
database, stored in a

5. Example
client
Transfer $1000 from
A:$3000 to B:$2000
Transaction Coordinator
BANK A
BANK B

6. Transaction chart
Clie
nt Transaction coordinator
Bank A
Bank B
start
Result
prepare
rA
rB
outcome
prepare
If rA==yes && rB==yes
Outcome=“commit”
Else
Outcome=“abort”
B commits upon
receiving”commit”

7. Types of phases
1. Commit Request Phase
2. Commit phase
Basic Algorithm
During phase1, initially the coordinator
sends a query to commit messages to all
cohorts. Then it waits for all report back with
the agreement message. The cohorts, if the
transaction was successful, write an entry to
the log.

8. Basic Algorithm
During phase2, if the coordinator
receives an agree message from all
cohorts, then it writes a commit record into
its log and sends a commit message to all
the cohorts. If all agreement messages do
not come back the coordinator sends an
abort message.
Next the coordinator waits for the
acknowledgement from the
cohorts. When acks are received from all
cohorts the coordinator writes a complete

9. If the cohorts receives a commit message, it
releases all the locks
and resources held during the transaction and
sends an
acknowledgement to the coordinator.
If the messages is abort, then the cohort undoes
the transaction
With the undo log and releases the resources
and locks held during
The transaction. Then it sends an
acknowledgement.

10. Conclusion of 2 phase commit
Other processor completing for resources
locks held by the blocked processor will
have to wait for the locks to be released.
If the coordinator fails permanently it will go
back to Rollback state.
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