First there was Mondrian, an open-source OLAP server then there was JPivot, a client which first spoke to Mondrian, then also to XML/A then there were more OLAP clients, and applications which wanted to use a particular client, but wanted to talk to a variety of servers and companies using a particular OLAP server that wanted to get at it from several clients. The Open Source community has been developing a taste for OLAP.
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It leaves unsolved the problem of how to construct queries to answer business questions, but application developers could solve that problem by embedding one of the off-the-shelf OLAP clients. A query language is easier to explain than an API. Second, there was a ready reference implementation, and Microsoft saw to it that there were sufficient OLAP clients to make these standards viable forums for competition and innovation. First, since the standards (OLE DB for OLAP in particular) were mainly driven by one vendor, they were not a compromise attempting to encompass the functionality of several products. These standards were more successful, for a variety of reasons.
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Meanwhile, Microsoft introduced OLE DB for OLAP (which works only between Windows clients and servers), and then XML/A (XML for Analysis, a web-services API). The standards were large and complex, and no user-interface provider stepped forward with a UI which worked with multiple back-ends. These all failed, it seems, because at some point during the committee stages, all of the OLAP server vendors concerned lost interest in releasing an implementation of the standard. First, the OLAP council's MDAPI (in two versions), then the JOLAP API emerged from Sun's Java Community Process. History is strewn with attempts to create a standard OLAP API. To understand why, it helps to understand the history of OLAP standards. However, creating a standard OLAP API for Java is a contentious issue. You can write an OLAP application in Java for one server (say Mondrian) and easily switch it to another (say Microsoft Analysis Services, accessed via XML for Analysis). olap4j has a similar programming model to JDBC, shares some of its core classes, and has many of the same advantages. In essence, olap4j is to multidimensional data what JDBC is for relational data. Olap4j is an open Java API for building OLAP applications.