An SAP system consists of SAP instances. An SAP instance is a group of processes that are started and stopped at the same time.

    In SAP NetWeaver 7.0, there are the following instances:
                ? Central instance
                ? Central services instance
                ? Database instance
                ? Dialog instance

    All instances, except the database instance, are identified by an instance number, which is a two-digit number between 00 and 97 that is unique on a host. Instances can reside on one host, or they can be distributed on several hosts.


Central Instance

    Every SAP system must have one central instance , which includes:
    ? Usage type AS ABAP
             Dispatcher -- dw.sap_DVEBMGS
             Work processes (dialog, batch, spool, or update[2],backgrup jobs) --

 ps -mp $process_id to check out what kind of work process

dw.sap_DVEBMGS forked by Dispatcher, same command name, RZ10,SM50
             Gateway -- gwrd
             Internet Communication Manager (ICM) --icman
             Internet Graphics Service (IGS) -- ig.sap_DVEBMGS,igsmux_mt,igspw_mt
   ? Usage type AS Java
             Java dispatcher

/usr/sap/xxx/exe/jlaunch pf=xxx -DSAPINFO=xxx_dispatcher 

java dispatcher has been replaced by ICMAN from netweaver 7.1

             Server processes 

NW 6.40 and NW 7.00 has Jcontrol and Jlaunch processes.

NW versions above 7.00 i.e. 7.10, NW CE has Jstart process.

/usr/sap//J/exe/jstart xxxx

             Software Deployment Manager (SDM) and JSPM (Java Support Package Manager)

jlaunch pf=/usr/sap/xx -DSAPINFO=xxxx_sdm -nodeId=2 -file=/usr/sap/xxx/SDM/program/config/sdm_jstartup.properties -syncSem=xxx -nodeName=sdm xxx 

 SDM is replaced with JSPM and directory SDM is altogether removed

             Internet Communication Manager (ICM) --icman
             Internet Graphics Service (IGS) ig.sap_J,igsmux_mt,igspw_mt


  When the central services are located on a separate instance, the configuration of the central instance is the same as the configuration of the dialog instance.
   If the central services are not installed as a separate instance, the message server and enqueue server are part of the central instance.


Central Services Instance

    The central services instance – Java central services instance (SCS) or ABAP central services instance (ASCS) –), forms the basis of communication and synchronization for the Java or ABAP clusters.
    The ASCS can only be installed for a high availability system with AS ABAP.
A central services instance consists of the message server and the enqueue server:
    ? Message server
           Only one message server can run on each AS Java or AS ABAP usage type. The message server handles the communication between the dialog instances and also supplies information to the SAP Web dispatcher about load balancing. 

  ms.sap_ASCS -- for abap

  ms.sap_SCS  --- for java
  ms.sap_DVEBMGS  --- for abap none cluster(single node)


    ? Enqueue server
    The enqueue server contains a lock table that handles logical database locks plus infrastructure locks set by Java server process. The enqueue server also synchronizes data in a Java cluster. In usage type AS ABAP, the enqueue server handles only locks on data objects.
  en.sap_ASCS -- for abap

  en.sap_SCS  --- for java
 
Database Instance
     The database instance is a mandatory installation component for the installation of an SAP system. AS Java and AS ABAP have separate database schemes. When AS ABAP with AS Java (also known as Java Add-In) is installed, the AS ABAP and the AS Java database schemes are installed in the same database. It is not recommended to use separate databases for the AS Java and AS ABAP scheme.

      The AS Java scheme, named SAPDB, holds the data stored by AS Java and all
deployed application objects. The ABAP scheme is named SAP


Dialog Instance
    Dialog instances are optional and can be installed on separate hosts. Dialog instances include:
        ? Usage type AS ABAP

                  Dispatcher

                  Work Processes 
                  Gateway 
                  Internet Communication Manager (ICM) icman
                  Internet Graphics Service (IGS) 

       ? Usage type AS Java
                  Java dispatcher
                  Server processes
                  Internet Graphics Service (IGS)

    When you add a dialog instance, it belongs to the software cluster. The central instance initiates an update of the dialog instance so the dialog instance has access to the deployed applications on that cluster (this also includes usage types such as EP or PI).
A cluster always needs a load balancing solution, for example, SAP Web dispatcher.