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A shadow (SHD) system can be used during system maintenance (or update) parallel to the existing ABAP system.

A shadow system uses the same system-ID as the original system, but has a separate instance (shadow instance, own instance number) and a separate repository (shadow repository).

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The shadow repository contains selected tables: basis tables, some customizing tables, but no application tables. The advantage is that the shadow instance is working on the shadow repository to update these tables to the new software level (target release) already during uptime processing so that the downtime processing is faster.

The shadow system has the advantage that the downtime is reduced, but you have to consider additional resources: memory on the application server for running the additional instance, and space on the database for the additional tables of the shadow repository. (Remember that the repository is typically only a very small part of the database content, and it is independent on the total DB size.)

The work on the shadow repository makes it necessary to prohibit changes on the original repository. That is why transport and development of ABAP objects are forbidden once the creation of the shadow repository started.

Remember the two possibilities above to create the shadow house: “copy your house, or create from blueprint”?

For system maintenance, this is equivalent to the difference between update and upgrade:

An upgrade

• is a release change (like SAP ERP 5.0 to 6.0)
• creates the shadow repository from DVD

An update

• is an implementation of EHP or SPs (like SAP ERP 6.0 EHP3 -> EHP7)
• creates the shadow repository from the existing repository

Single System Mode:

If you choose single system mode in the preconfiguration mode (see first screenshot above), this means:

• An update (implementing SPs or EHPs) does not create or use a shadow system at all

This maintenance type is sometimes also referred to as “transport like”

• An upgrade (release change) creates a shadow repository (from DVD) and a shadow instance,

but the original instance is stopped as long as the shadow instance works on the shadow repository;

only a single system (instance) is running at a time -> “Single System Mode”

Single System Mode is sometimes also referred to as resource-minimized mode, as only few additional resources are required. On the other hand, the Standard and the Advanced Mode are both referred to as downtime-minimized mode, as only they offer downtime minimization. After handling the pre configuration mode dialog (first screenshot above), the next dialog “Parameters for procedure” allows setting the number of processes, and it lists the mode as resource or downtime minimized (e.g. “The tool uses strategy Downtime-minimized.”).