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Presented by Dilouar Hossain

dilouarbd@gmail.com

Two phase commitWhat is Two phase commit ?Describe the Two phase commitHow to work two phase commit Problem Example

In transaction processing, databases, and computer networking, the two-phase commit protocol (2PC) is a type of atomic commitment protocol

(ACP).

What is Two phase commit ?

Describe the Two phase commit

I believe most of people know what 2PC (two-phase commit protocol) is and how to use it in Java or most of modern languages. Basically, it is used to make sure the transactions are in sync when you have 2 or more DBs.Assume I've two DBs (A and B) using 2PC in two different locations. Before A and B are ready to commit a transaction, both DBs will report back to the transaction manager saying they are ready to commit. So, when the transaction manager is acknowledged, it will send a signal back to A and B telling them to go ahead.

How to work two phase commit The steps performed in the two phases are as follows ....

Phase 1: Prepare PhaseAfter each slave has locally completed its transaction, it sends a “DONE” message to the controlling site. When the controlling site has received “DONE” message from all slaves, it sends a “Prepare” message to the slaves.The slaves vote on whether they still want to commit or not. If a slave wants to commit, it sends a “Ready” message.A slave that does not want to commit sends a “Not Ready” message. This may happen when the slave has conflicting concurrent transactions or there is a timeout.

Phase 2: Commit/Abort PhaseAfter the controlling site has received “Ready” message from all the slaves −

The controlling site sends a “Global Commit” message to the slaves.The slaves apply the transaction and send a “Commit ACK” message to the controlling site.When the controlling site receives “Commit ACK” message from all the slaves, it considers the transaction as committed.

After the controlling site has received the first “Not Ready” message from any slave −

The controlling site sends a “Global Abort” message to the slaves.The slaves abort the transaction and send a “Abort ACK” message to the controlling site.When the controlling site receives “Abort ACK” message from all the slaves, it considers the transaction as aborted.

The greatest disadvantage of the two-phase commit protocol is that it is a blocking protocol. If the

coordinator fails permanently, some cohorts will never resolve their transactions: After a cohort has

sent an agreement message to the coordinator, it will block until a commit or rollback is received.

Two phase commit Disadvantages

Example

Clients want all-or-nothing transactionsTransfer either happens or not at all

Bank A Bank B

Transfer $1000From A:$3000To B:$2000

client

Strawman solution

Bank A Bank B

Transfer $1000From A:$3000To B:$2000

client

Transactioncoordinator

Failures in a distributed systemConsistency requires agreement among

multiple serversIs transaction X committed?Have all servers applied update X to a replica?

Achieving agreement w/ failures is hardImpossible to distinguish host vs. network

failuresThis class:

all-or-nothing atomicity in distributed systems

Correctness firstclient transaction

coordinatorbank A bank B

start

result

prepare

prepare

rB

rA

outcomeoutcome

If rA==yes && rB==yes outcome = “commit”else outcome = “abort”

B commits uponreceiving “commit”

Handling timeout on A/BTC times out waiting for A (or B)’s “yes/no”

responseCan TC unilaterally decide to commit? Can TC unilaterally decide to abort?

Sinfonia (SOSP’07)

A Case study of 2P commit in real systems

What problem is Sinfonia addressing?Targeted uses

systems or infrastructural apps within a data center

Sinfonia: a shared data serviceSpan multiple nodesReplicated with consistency guarantees

Goal: reduce development efforts for system programmers

Sinfonia uses 2P commit

prepare

commit

action1

action2

actions…

Traditional transactions: general but expensiveBEGIN txIf (a > 0 && b== 0) b = a * afor (i = 0; i < a; i++) b += iEND tx

Mini-transaction: less general but efficientBEGIN txIf (a == 3000 && b==2000) { a=2000 b=3000}END tx

Prepare & exec

commit

Traditional transactions

Mini- transactions

coordinator coordinator