Topic 8 - Exercise 11

1. Give a description in your own words of the ACID properties of a transaction

ACID is an acronym for atomicity, consistency, isolation and durability. Each of those 4 qualities are necessary for the transaction to be designed correctly. 

Atomicity means that a transaction must exhibit "all or nothing" behavior.
Consistency - All data needs to be "correct" after running a transaction in a database. 
Isolation - Means that transactions can run at the same time although it appears that there is no concurrency.
Durability - A durable transaction will not impact the state of the data if the transaction ends abnormally.

2. Describe a TP monitor environment. How can a TP monitor stop an OS being overwhelmed?

Short for transaction processing monitor, a program that monitors a transaction as it passes from one stage in a process to another. The TP monitor's purpose is to ensure that the transaction processes completely or, if an error occurs, to take appropriate actions.

TP monitors are especially important in three-tier architectures that employ load balancing because a transaction may be forwarded to any of several servers. In fact, many TP monitors handle all the load balancing operations, forwarding transactions to different servers based on their availability.

References:

http://databases.about.com/od/specificproducts/a/acid.htm accessed on 22/05/2010
http://www.webopedia.com/TERM/T/TP_monitor.html accessed on 22/05/2010