Two-Tier-Architecture

By its very nature, a two-tier application is not scalable beyond a certain point. There are several reasons for this. One reason is the number of connections a database can maintain concurrently. Imagine that one million users try to access the database at the same time…need I say more? There is no way to effectively manage the connections to the database when the connections are being created on the user’s machine (as opposed to being able to pool database connections). Another reason why a two-tier application is not scalable beyond a certain point is application functionality in relationship to the business process that the application supports. Take a situation where a business process changes and the program has to be altered. The company may have to roll the upgraded application out to 30,000 users, which is usually too cost prohibitive to do. Scalability does not have to just reflect whether an application can support a growing number of users but also how expensive it is to support them.

Then there is the concurrency issue; that is, what happens when two or more users try to access the same record in the same database at the same time to make changes to it? Usually one or more users are blocked from making changes, which can cause the application to temporarily hang. In a two-tier application, this can be both a positive and a negative aspect of the application. The positive aspect is that one user cannot alter a record that another user is modifying. The negative aspect is that it can cause the second user’s query to wait if there is a lock on the record they want to read. If the application is programmed correctly, the lock should not last for more than a few milliseconds, but on some database platforms, if the user who placed the lock is prematurely disconnected from the database, the result is a lock that cannot be removed except by the database administrator or by the database after a certain period of time. This has the potential to cause numerous problems. This particular issue is never a problem with a three-tier application, but other, more complicated issues appear with regard to this aspect of the database.