TP monitors and transaction protocols

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

ACID stands for Atomicity, Consistency, Isolation & Durability which are the properties of a transaction. Atomic means that when a transaction is being executed, it is not interrupted by any other process from another transaction. Consistency means that a transaction must leave stored data in a consistent state until the whole (e.g. update) proccess has been completed. Isolation means that a transaction must not be interrupted by another transaction. Durability means that after a transaction has completed its operations, the results are reflected to the data (Ince 2005, pp.356-357).

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

Normally, a TP monitor operates in a multi-thread system (e.g. mainframe computers or distributed client/server system) because it manages the concurrent execution of the threads and processes that make up a transaction and ensure that the ACID properties are enforced. It schedules threads so the low-priority transactions are allowed a smaller share of resources than high-priority transactions such as online transactions and enable load-balancing when an operating system is being overwhelmed (Ince 2005, pp. 363-364).

3.What is difference in load balancing with traditional and transactional MOM, RPC and conversations?
First of all, we need to have some undersrandings on these terminologies.

The process of sharing the processing load in a distributed system equally among the servers in the system, which is known as load balanacing (Ince 2005, p.386).

The process of executing code on a remote computer by invoking it from another computer often known as RPC (Ince 2005, p.259).

In Enterprise Javabean, message-oriented middleware (MOM) is a software which manages the transactions that pass from a client to a server and vice versa (Ince 2005, p.364).
In Enterprise Javabean, a conversation is a potentially long-running sequence of interactions (document exchanges) between multiple web services. In many situations, the backend logic triggered as part of these conversations may be transactional because of their transactional properties (Frolund, S and Govindarajan, K, n.d.).

Load balancing is a design to evenly distribute the resources among the servers. This is a server-side operation. The rest of them occur on both the client-side and the server-side operations.

4.Why is a two-phase commit protocol better than a one-phase atomic commit protocol?
One-phase atomic commit protocol can only ensure that all or none of the operations in a distributed transaction either commit or abort is to keep sending a commit or abort. Two-phase commit protocol can handle the operations of the nested transactions that one-phase atomic commit protocol can't handle. For example, a transaction that has many sub-transactions and each sub-transaction can make a decision to abort or provisional decision to commit to a transaction. A transaction will only commit if all its sub-transactions are provisionally committed. However, commitment can still occur even if some of its sub-transactions have been aborted. In order words, the sub-transactions can be aborted without causing their parent transaction to abort. This is because the parent transaction may contain code which handles any abortion of its sub-tansactions (Ince 2005, pp.358-360).

References

Ince, D 2004, Developing distributed and e-commerce applications, 2nd edn, Harlow, Essex, UK: Addison – Wesley, pp. 259,355-389.

Frolund, S and Govindarajan, K, n.d., Transactional Conversations, Hewlett-Packard Company, viewed 17 June 2009,<http://www.w3.org/2001/03/WSWS-popa/paper50>.