1. Transaction Communication
Ela Sharda

2. Outline Transaction Concurrency Transparency ACID properties
Distributed Transactions
Actors
Activity Log
Shadow Paging
The Two Phase Commit Protocol
References

3. Transaction
Fundamental unit of interconnection b/w client and server processes in database system.
Database Transaction : Sequence of synchronous request/reply operations that satisfy ACID properties.
Communication Transaction: Set of asynchronous request/reply communications
- have ACID properties.
- without sequential constraint of operation.

4. Concurrency Transparency
Enables several processes to operate concurrently using shared resources without interference between them.
Execution of a transaction appears to take place in a critical section.
But the operations from different transactions are interleaved to gain more concurrency.

5. ACID Properties[1]
Transactions are communications with ACID property, ACID mainly concerned with concurrency transparency of distributed system.
- Atomicity
- Consistency
- Isolation
- Durability

6. Atomicity A transaction is atomic unit of processing.
Either all the operations in a transaction are performed in its entirety or not performed at all.
It is the responsibility of transaction recovery subsystem to ensure atomicity.

7. Consistency
Complete execution of transaction results from one consistent state to another.
Execution of interleaved transaction is equal to serial execution of transactions.
It is also referred as serializability.

8. Isolation
A transaction should appear as it is being executed in isolation form other transactions.
Execution of the transaction should not be interfered with any other transactions executing concurrently.
- Partial results of an incomplete transaction are not visible to others before the transaction is successfully committed.

9. Durability
The results of a committed transaction will be made permanent even if a failure occurs after the commitment.
Once a transaction completes successfully(commits), its changes to the state survive failures.

10. Distributed Transactions
Consists of one coordinator (initiator of the transaction) and several participating processes (remote process)‏
At commit
- Atomicity: either all nodes commit or none do
- Isolation: effects of the transaction not made visible until all nodes have made an irrevocable decision to commit or abort
ACID properties can be achieved by the two-phase commit(2PC) protocol.
There is one coordinator and multiple participants. Each of them have access to some stable storage.

11. Actors Coordinator : The processor that initiates the transaction.
- The coordinator oversees the activities of the other participants in the transaction to ensure a consistent outcome.
Participants : All the remaining processors.

12. Activity Log [2]
Each participant keeps track of updated data objects by maintaining a private work space.
Updates contain old and new value.
Each site has an activity log which is kept on the disk.
- On abort: undo of uncommitted transactions (rollback)‏
- After crash: redo of committed transactions (roll forward)‏
Needed for durability of committed transactions.

13. Shadow Paging [3] Here, transaction logs are not required.
Two directories created during the life of transaction current directory shadow directory
When transaction starts, both directories are same.
Shadow directory never changed during the transaction.
Current directory updated when write operation is performed.
When transaction commits, shadow directory is discarded and current directory is copied to the storage.

14. The Two Phase Commit Protocol
Coordinator Participant
- precommit the transaction
- send request to all participants received request message if ready
then precommit and send YES
- collect all replies else abort transaction and say NO
- if all votes are unanimous YES
then commit and send COMMIT
else abort and send ABORT receive decision
- if commit then COMMIT
- if abort then ABORT
- received response send response
The Two Phase Commit Protocol

15. What does the coordinator write to the log?
When the coordinator sends request, it writes a start-2PC record and a list containing the identities of the participants to its log. It also sends this list to each participant at the same time as the request message.
Before the coordinator sends Commit to the participants, it writes a commit record in the log.
If the coordinator sends rollback to the participants, it writes a rollback to the log.

16. What does the participant write to the log?
If a participant votes Yes, it writes a yes record to its log before sending yes to the coordinator. This log record contains the name of the coordinator and the list of the participants.
If this participant votes No, it writes a rollback record to the log and then sends the No vote to the coordinator.
After receiving Commit / Rollback, a participant writes a commit / rollback record into the log.

17. Categories of Recovery Actions [1]
Failures before a precommit
Action : Simply abort the transaction.
Equivalent to voting NO for transaction.
Failures after a precommit but before a commit
Action : Abort the transaction.
Remulticast the request message.
Failures after a commit
Action : Resend the commit message.

18. References
[1] Distributed Operating Systems & Algorithms, by Randy Chow and Theodore Johnson, 1997
[2] Operating System Principles, by Silberschatz and Galvin, Seventh edition
[3] Recovery in Parallel Database Systems, by Svein-Olaf Hvasshovd, second edition
[4]
[5]