2PC Protocol

The two phase commit protocol is a distributed algorithm which lets all sites in a distributed system agree to commit a transaction. The protocol results in either all nodes committing the transaction or aborting, even in the case of site failures and message losses.

It reduces the vulnerability of one-phase commit protocols. The steps performed in the two phases are as follows –

Phase 1- Prepare Phase

• After 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 Phase

• After 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.

Purpose of 2PC Protocol

The purpose of 2PC protocol is to ensure conflict serializability. Consider any transaction. The point in the schedule where the transaction has obtained its final lock is called the lock point of the transaction. Now, transaction can be ordered according to their lock points. This ordering is a serializability ordering for the transactions.