Exercise 11: TP monitors and transaction protocols

1. Give a description in your own words of the ACID properties of a transaction
The ACID model is one of the oldest and most important concepts of data theory. It sets forward 4 goals that every database management system must strive to achieve: Atomicity, Consistency, Isolation, and Durability. Any database that fails to meet any of these 4 goals should not be considered reliable.

Transactions are sequences of operations or smaller sub tasks to be executed all together successfully or else the entire transaction is void.

Atomicity: requires that all operation of a transaction be completed; if not, the transaction is completed aborted. If a transaction has 4 steps, all 4 steps must be successfully completed; otherwise, the entire transaction is aborted. So in other words, a transaction is treated as a single, indivisible, logical unit of work.

Consistency: indicates the permanence of consistent state. If for some reason a transaction is executed that violates data consistency rules, the entire transaction must be rolled back and the data restored to a state of consistent with those data rules. An example of this is within databases that a string is trying to write to a integer field in the database. This is not sticking to the rules of the database and is therefore breaking the state of the data, only an integer value can be stored in this field which would then take this to the next consistent state.

Isolation: means that the data used during the execution of a transaction cannot be used by a second transaction until the first one is completed. This property is particularly useful in multiuser database environments because several users can access and update the database at the same time.

Durability: ensures that any transaction committed successfully will not be lost, undone, even in the event of a system failure. A good example of this is say you make a payment on a banking website and you receive the receipt however this is never passed through to the database because of a server failure during the time of the transaction. This would be the ACID "D" rule.

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

A TP or Transaction Processing Monitor is a control program that manages the transfer of data between multiple local and remote machines and the programs that serve them.

In a client/server environment how a TP monitor might be used to stop an operating system from being overwhelmed is to be placed between multiple machines and balance the load between these machines so that the operating systems are never overwhelmed with too much work.