Data Structure of SAP Systems
A client is a unit within an SAP system that is self-contained in terms of business, organization, and data. A client is characterized by its own business-data environment, as well as master, transaction, and user data. This data in a client is known as client-specific data. All types of client-specific data depend on each other in various ways. For example, business application data is checked for consistency against the Customizing settings for the client when the data is entered and rejected.
When an SAP system is prepared for use, customizing is done for each client. The Customizing settings are used to define the organizational structures of customers, such as distribution channels and company codes. The Customizing settings are also used to set the parameters for SAP transactions so that they reflect customer-specific processes. In addition to the client-specific customizing settings, there are settings that need to be made only once for all clients in the SAP system. These cross-client Customizing settings include the public holiday calendar, for example, and settings for the system change option.
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Repository
The repository is where objects are accessed using the ABAP Workbench. The repository is a cross-client store containing all Dictionary objects (tables, data elements, and domains) and all ABAP programs, menus, and screens. If we develop or change any repository object in one client, an SAP system uses the same form of the object in every other client.
Repository objects are grouped together to form packages. Packages are containers for semantically-linked development objects (programs, tables, and so on). The ABAP Repository contains packages from all software components.
Three-System Landscape
In a three-system landscape, you can set up several SAP systems on one license. However, you may use only one of these systems as a production system. Each three-system landscape contains a working client and other clients. To facilitate the consistency of Customizing settings, three working clients can have the same identification number. In practice, however, the development, test, and productive clients sometimes have different numbers.
Corresponding changes of program coding and/or Customizing data could be critical and may need to be tested beforehand. The changes also demand detailed logging of transport requests.
In addition, ABAP Repository objects are client independent, which means that the changes made in one client immediately affect all other clients of the same SAP system. Customizing settings must not be configured directly in the productive systems in most cases. Special client settings can be set up to prevent any changes in Customizing settings.
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Process Involved in Three-System Landscape
In a three-system landscape, you perform the required customization of the SAP standard processes in the development system. You also develop programs on the development system. All Customizing settings and changes (developments, corrections, or modifications, if required) to the repository are then transferred to the quality assurance system (or “test system” for short) to be checked without influencing production.
After testing all objects and settings imported into the test system, you can transfer them to one or more production systems.
The systems in a three-system landscape must have unique three-character descriptions, such as Development (DEV), Quality Assurance (QAS), and Production (PRD).
The system identifier (SID) is always three characters long and starts with a letter, but it can consist of letters and numbers. SAP reserves some of the SIDs. The SID SAP cannot be used to install an SAP system. For further details, check the installation guide of the SAP software that you would like to install.