A SAP instance is all the components created by the SAPinst program who all share the same database.  There are three mandatory sub-instances;  the Database Instance (DB), the Central Instance (CI), and the Dialog Instance (DI) aka Application Server.  This is the minimum configuration of any SAP instance.  There can be multiple DIs but only one DB and only one Active CI which means that a copy of the CI can exist for High Availability but only one of the CIs can be active at any given time.  You will hear about SAP being multi-tiered and that term is referancing these three layers.  Sometimes you might see “other SAP tiers” like ITS but that really isn’t a separate tier, it used to be an additional piece of software working with IIS, and now ITS is part of Basis/WAS so it is part of the CI Tier. These layers, plus other SAP written software, are also known as the SAP Business Framework.

You can probably guess at what the DB contains.  There are any number of tables in a SAP instance, from 21,000 to 38,000.  Many have four or five character names that are usually abbreviations of things like USR for user, MANDT for client, etc.  You could guess that USR is short for user.  But MANDT?  So, we add one more variable to the equation – those four or five character names are abbreviations of German words.  Needless to say, trying to look at SAP from a typical DBA prospective is almost impossible.  Fortunately, SAP supplies the tools for you to manage all these tables.

The Central Instance is a lump term for all the SAP executables, the installed OS file structure, and anything that is placed on the OS to support and communicate with the SAP instance.  It “talks” to the database, handles requests made by the application server(s), and sends back the information.  Other software products often call this the middleware layer.