1. RELIABILITY

2. Reading
Textbook: Ch. 12.4
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CSCE Spring 2011

3. Design Issues Placing of data and programs (DBMS and application)
Network issues
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CSCE Spring 2011

4. Data Distribution Fully redundant Partially redundant
Partitioned: each data item is stored at exactly one site
How can we guaranty atomicity of transactions?
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CSCE Spring 2011

5. Distributed Processing
Site A
Site B T2
Site C T3 T1
Distributed DBMS
Site D T4
T: must be atomic
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CSCE Spring 2011

6. Distributed Reliability Protocols
Ensure that distributed transactions are atomic
Protocol evaluation:
Max. number of messages
Max. number of log forces
Checkpoint: for recovery, indicates the latest consistent point (undo has to go back to this point, recovery can start from this point)
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CSCE Spring 2011

7. Reliability Protocols
2-Phase Commit
Presumed Commit
Early-Prepare Commit
Presumed Abort
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CSCE Spring 2011

8. Two-Phase Commit 1. Coordinator Participant
User initiates transaction T at the site that acts as a coordinator C. C sends the appropriate work request to the participants. (N number of messages)
Executes the work request and sends reply to C. (N number of messages)
User issues a commit command. C sends a prepare-to-commit message to each participant. (N number of messages)
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CSCE Spring 2011

9. Two-Phase Commit 2. Coordinator Participant Window of
Uncertainty (If the participant voted yes, it cannot decode whether to commit or abort until hears from C)
If the participant is able to commit, it forces a prepare log record and sends a yes vote to C. (N forces, N messages)
If the participant cannot commit, it forces an abort log record and sends a no vote to C. (N forces, N messages)
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CSCE Spring 2011
CSCE Spring 2011 9

10. Two-Phase Commit 3. Coordinator Participant
If all votes were yes, C forces a commit log record and sends a commit message to all participants (1 force, N messages)
At this point the user can be told that the transaction has been committed.
If even one node voted no, C forces an abort log record that contains transaction membership and sends an abort message to each participant that voted yes or didn’t send a vote. At this point, the user can be told that the transaction has been aborted. W Of U R T Y 10
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CSCE Spring 2011

11. Two-Phase Commit 4. Coordinator Participant
When the participant receives a commit message from C, it forces a commit log record, sends an ACK to C, and forgets about the transaction. (N forces, N messages)
If it receives an abort message, the participant forces an abort log record, sends an ACK to C, and forgets about the transaction.
After receiving ACKs from all participants, C writes an end-of-transaction record to the log and forgets about the transaction.
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CSCE Spring 2011
CSCE Spring 2011 11

12. Efficiency of 2-Phase Commit
Total number of messages: 6N
Log forces; 2N +1
Can be combined with 2-Phase locking
Farkas
CSCE Spring 2011

13. Next Lectures
Replica Update Protocols
Farkas
CSCE Spring 2011