- Explain the advantages and disadvantages of two-tier architectures when examined under the following topics: scalability, maintainability, reliability, availability, extensibility, performance, manageability, and security.
- Explain the advantages and disadvantages of three-tier architectures when examined under the following topics: scalability, maintainability, reliability, availability, extensibility, performance, manageability, and security
- Explain the advantages and disadvantages of multi-tier architectures when examined under the following topics: scalability, maintainability, reliability, availability, extensibility, performance, manageability, and security.
Two Tiers:
First a brief description of the situation, a two tier application its mainly a client application which has everything in one layer and connects to a repository (a database if you prefer), this is also known as Client-server architecture. (this is just a brief introduction, you could go client-server architecture to get more detail.
Lets ask first, if all the code is in one place (the client... like a swing application) the client will have all the power force and everything depends on its machine, is it an scalable solution? what will happen if you add new clients? the network overload will be a problem, because these type of application uses it a lot.
In the example the work flow will be as follows:
1. Executes a SQL to get all the current flights (between some date range)
2. shows the flights, and select the one using the ticket information
3. Executes a SQL to get the customers on that flight
4. Shows the list and select the passenger
5. Executes a SQL to verify the status of the ticket.
6. and so on.
The DB connections are very expensive and very slow, so, if you put new customers your application will have serious problems in bandwidth, overload, etc. So your application is not easily scalable. But... if you have only 3 clients (maybe three counters) this will be a very fast solution, the client application will do all the work and will be very effective. The troubles will come if you want to export this to Internet, because the clients will not have access to your internal network, you will need to spread the user and password for the db, etc.
Is this application maintainable? Think of this, if all the code is in the client will be easy to find an error? I don't think so.
What about reliability? the reliability states: "the ability of a system or component to perform its required functions under stated conditions for a specified period of time." so will the client-server be reliability? its hard to figure it out. If the system could be easily break down (because the code is in the client side) someone could change its reliability changing a part of the code. If the network configuration change the client might have problems to establish the connection to the DB, so the reliability will be affected, but as the definition states, everything depends on the conditions not the architecture.
The system will have a high availability? because all the code will run on the client this configuration will not react to a fail over problems, because everything is in one machine if that machine fails the system will not work, so the availability will decrease with this architecture.
Extensibility, mmm... I think no, its hard to maintain and extend a Client-server application, just because everything is in one place.
Performance, ok, here is the big question, the performance in a Client-server application relies on how it will work, if it has several calculations everything depends on the client, there's no way to distribute the work between several machines, all the work will be running in one PC, but... the client-server applications runs fast if you do some tricks. In the example above the performance could be an issue in here, just because it has a lot of DB queries dependencies.
Manageability, think if you are able to change some parameters and your application will run fast or will have a better reliability, the answer will be no, just because everything runs in the client and you will have absolutely no control over it.
In general I would say that increasing the tiers will ensure extensibility, maintainability, security but will decrease performance.
Remember, tiers refers to the layers in which your system could split off, if your application have a lot of classes but works in a single machine which connects to a DB then it's a client-server application.
Three Tiers
In the three tiers your application will have some logic in the client (usually the presentation and some validations), the business tier will have the 80% of the application logic (lets say this will have most of the business code) and the DB which could have some Stored Procedures in it.
What will be different from the previous approach? you will have a lot of things running in one machine, so its easier to maintain, you will increase security because the client will not have the user/password pair to access to the DB, and will be more extensible than the client-server application. But what will you gain or lose? the answer is not easy, I would say It's relative, If the application is a calculation centric then you might lose performance (because the server will do a lot of work for several clients at the same time) or you could gain performance if the application is mainly a db application, just because you could place the DB server near (referring to the network topology not the physical distance) to the business server in order to avoid the network overload.
In comparison with the Client-server you will have:
- better scalability, you could increase the hardware in the business machine and could have more clients.
- better maintainability, everything is in one place so you can replace a component and fix a bug quickly
- better reliability, because you have a better control of the environment in which the business code runs.
- better availability, if the business machine has problems you can easily replace it by a new machine a your client will be running in a blink of an eye
- better extensibility, you could add new functionality in a better way
- decrease or increase in performance, this depends on the application, if its a db application its 90% to get a better performance.
- Better manageability, its easy to control the environment in the business tier, so you can change configuration settings, improve connections etc
- Better security (I think this does not need further explanation...)
N-Tier
I think N-tier and 3-tier are pretty much the same thing, if you have a good security in 3-tier you will have even better security with N-tier and this applies to all the statements. (Even to the performance thing, a bad decision could lead to a poor performance if you choose N-Tier over the others and you don't pay attention on the specific requirements)
Why will you prefer N-Tier over 3-tier, you could distribute the work in several machines, so if you are working in a collaborative application (I loan application which will need a CRM, ERP etc then will be distribute its responsabilities over several machines and will have a better performance because of it)
In JEE you could easily use some patterns like Service Locator to change a N-Tier to a 3-Tier application (or, at least, behave as a 3-tier application), just changing the uses of the EJBs from Remote to Local. So if I need a tiered application I will choose N-tier over 3-tier every day of the week (and twice in Sunday)
(Please let me know if you need further explanation and I will add more information on N-Tiers)
