The application layer is often interwoven with both the presentation layer and the services layer. All three layers, including the database engine, almost always run on the same computer. One-tier applications are easy to design and write, especially with today's modern software construction tools. The database can be stored either on one of the PCs or on a file server. In this case, the PC running the application has its own copy of the database engine - only the data is shared, not the database logic.
Conflicts can occur because all the database work is performed in each client.
So, if the program needs to list all client addresses in the Idaho area, all the information needed to resolve the query to the database must be transferred over the network. And for some complex queries, a significant amount of data must be examined. If two or more clients attempt to retrieve the same information at the same time, it can result in what is known as record locking. Instead, it runs on the server computer that contains the database itself. A method is used to communicate between the application logic layer and the database service.