A data access layer (DAL) is a specific part of each computer program that facilitates access between the program and any type of persistent storage. Programs are any application that runs on your computer; Persistent storage refers to any permanent storage location on your computer, such as hard drives. As programs run, they are constantly writing and reading information to and from the hard drive. The role of the data access layer is to ensure that any program running on the system can get the information it needs as soon as it needs it.
Computer programs use a data access layer to access information on the hard drive.
As an intermediary or seller in a business relationship, the data access layer sits between the program and the storage device. Every program running on the computer that needs to access information on the hard drive has data access functional layers. Instead of the main body of the program communicating directly with the persistent storage location, it delegates responsibility to the data access layer, which then acts on behalf of the program to perform the task. Its only “job” is to shuffle the information back and forth, freeing up the rest of the program to fulfill its other responsibilities.
A typical example of a functional data access layer involves a program that requires external information to operate; that is, information that the program does not have intuitively. For example, if a program tries to calculate changes in a company’s profit margin over the last 10 years, it is unlikely that this information exists within the actual program. However, it will exist within a company’s financial records database. When the program realizes that it requires “Financial Statement X” or “Earnings Report Y”, it tells the data access layer to look up this information. The data access layer then extracts the necessary information from the appropriate database or table, returning it to the program so it can continue to operate.
Some programs that use a data access layer depend on the database; this means that they are designed to work with a specific type of database, which limits their portability. Others are database agnostic, providing the ability to work with a wider range of database software. While it may seem that dependent data access layer programs are less useful, this is not necessarily the case. Because they are programmed to work with one type of database, they are likely to be optimized more efficiently, providing faster performance while working with the intended type of database system.
