Two-Phase Commit Protocol

Overview

The two phase commit protocol is a distributed algorithm which lets all sites in a distributed system agree to commit or rollback a transaction based upon consensus of all participating sites. The two phase commit strategy is designed to ensure that either all the databases are updated or none of them, so that the databases remain synchronized. The protocol achieves its goal even in many cases of temporary system.

Assumptions

In two phase commit protocol there is one node which is act as a coordinator and all other participating node are known as cohorts. The assumptions made in this protocol are listed below:

1. Each cohort should have its own stable storage.
2. To provide atomicity and durability each participating node should maintain a log before performing and operation. This type of logging called write-ahead logging.
3. In case of system failure of any of the cohort the storage can be recovered and information from the log file can still be retrieve.
4. No participating node can fail permanently.
5. In case of permanent failure of any cohort or complete lose of storage then the data cannot be recovered.

Basic Algorithm

As the name suggests the two phase commit protocol involves two phases. First phase is "Commit Request" phase second phase is "Commit" phase.

• Commit Request Phase:
  
  1. To commit the transaction, the coordinator sends a request asking for "ready for commit?" to each cohorts.
     
     
  2. The coordinator waits until it has received a reply from all cohorts to "vote" on the request.
     
     
  3. Each participant votes by sending a massage back to the coordinator, as follows:
     
     • It vote YES if it is prepared to commit
     • It may vote NO for any reason, usually because it cannot prepare the transaction due to a local failure.
     • It may delay voting indefinitely, for example, because its system was busy with other work because it failed.
• Commit Phase:
  
  1. If the coordinator receives YES response from all cohorts, it decides to commit. The transaction is now officially committed. Otherwise, it either receives a NO response or give up waiting for some participants, so it decides to abort.
  2. The coordinator sends its decision to all participants (i.e. COMMIT or ABORT).
     
  3. Participants acknowledge receipt of commit or about by replying DONE.

Disadvantages

The two phase commit protocol is a blocking protocol; hence it has some disadvantages which are listed below:

1. A cohort locks the required resources while it is waiting for a message from coordinator. Other processes competing for resource will have to wait for the locks to be released.
2. A single node will continue to wait even if all other sites have failed.
3. If the coordinator fails permanently, some cohorts will never resolve their transactions, causing resources to be locked up forever.
   

Me @ HCL (January-22-2007 To June-07-2010)

Today I got back home from office. But it was not like any ordinary days. Today I had my relieving letter with me. It was the end of 3 years, 3 month and 17 days stint with HCL Technologies Ltd.

I still remember the day (19th January 2007) when I first reached to New Delhi railway station from my home town Kolkata with a hope to do well in my professional career. I was still 6 months away from getting my MCA degree but I came to this place because HCL provided all its campus recruits an opportunity to complete their academic project.

It was 22nd January 2007, A-10/11, Sector - 3. Noida, HCL Technologies Ltd Corporate Office, we all queued up and were waiting for our turn to enter into the building to get offer letter. I did not know any of them except few of my college friends who also came with me. They all came from different part from India, but one thing was common among us. We all share a common DREAM to excel in the field of Information Technology. HCL gave wings to our dreams!! During my training period many of us became good friends. I shall always owe HCL for the same.

Once the training was over and I submitted my project papers to my university and I came back to Noida to join one of HCL’s offshore development facilities at Noida, where I met with some of my great colleagues, friends and mentors. It was a huge learning curve for me. Without them I would have been never achieved what I am today. I shall always be grateful for their support, guidance, friendship and encouragement.

But things are always not remain the same. Few months back I realized I need to give new heights to my dreams which probably no longer became possible within HCL. So, I started looking for new opportunity outside. When dream is powered by effort it converts to reality. Finally the day had come when I have to say my last goodbye to all.

Today is 7th June 2010. My journey with HCL is now over but I shall always cherish the great memories I have gained here.