Types of OLAP
Like everything else in technology, there's not just a single way to solve the technology problem. There are three basic approaches to OLAP servers: MOLAP, ROLAP, and HOLAP.
Multidimensional Online Analytical Processing (MOLAP) is the most standard approach to OLAP solutions. It uses a multidimensional database which directly stores the information contained in the various cubes. This is the best performing solution when using SQL Server Analysis services.
Relational Online Analytical Processing (ROLAP) provides the same solution but uses a relational database for storage of the data. This approach translates native OLAP queries, written in a language called multidimensional expressions (MDX) into the appropriate SQL statements. This is primarily done to prevent the need for another copy of the data. The data created directly by the online transaction processing (OLTP) applications are used. The primary disadvantage to this solution is that it does not, generally speaking, perform as well as a MOLAP database.
Hybrid Online Analytical Processing (HOLAP) is a hybrid approach to the solution where the aggregated totals are stored in a multidimensional database while the detail data is stored in the relational database. This is the balance between the data efficiency of the ROLAP model and the performance of the MOLAP model.
If you're developing applications, you ultimately do not care which model is being used because your code will work against any kind of OLAP model (unless you're creating the cubes yourself). It is only a concern when a large scale project is being rolled out and you must decide which solution will meet its unique needs.
A matter of perspective
OLAP solutions provide a new perspective on the reams of data that are being created, and provide ways to transform that data into information and useful answers to important questions.
