Lesson 1 of 5
System, instance and client
9 min
Three words cause more confusion in SAP than almost any others: system, instance and client. Administrators, testers and consultants use them every day, often loosely. This lesson pins them down, because every later topic in this course (work processes, load balancing, the database, front ends) builds on them.
The system and its SID
An SAP system is one complete installation: one database, one repository of programs and dictionary objects, and one or more instances that run the work. Each system has a system ID (SID) of exactly three characters, starting with a letter, using uppercase letters and digits. SAP reserves some combinations (such as SAP and SYS), the SID must be unique in your landscape, and it cannot be changed after installation, so naming conventions are worth agreeing early.
In this course, Contoso Motor Co.'s new SAP S/4HANA landscape uses the SIDs CMD (development), CMQ (quality assurance) and CMP (production). The old ECC 6.0 production system keeps its own SID until it is retired.
Instances and instance numbers
An instance is an administrative unit: a set of processes on one host that share memory and are started and stopped together. Each instance has a two-digit instance number, which must be unique on its host and from which many network ports are derived. A typical ABAP-based system has several kinds of instance:
| Instance | What it contains | Typical count |
|---|---|---|
| ABAP central services (ASCS) | Message server and enqueue server: the services that exist once per system | One (plus a replication instance for high availability) |
| Enqueue replication server (ERS) | A replica of the lock table so locks survive a central services failover | One in high-availability setups |
| Primary application server (PAS) | Dispatcher, work processes, gateway and ICM; the first application server installed | One |
| Additional application servers (AAS) | Further application server instances for capacity | Zero or more |
| Database instance | The SAP HANA database for the system | One (with replicas for high availability) |
Instance types of an ABAP-based SAP S/4HANA system.
A small development system may run everything on one host. Contoso's production system CMP runs central services on a high-availability pair, a primary and three additional application servers, and an SAP HANA database with a replica, all inside the RISE with SAP private cloud that SAP operates.
Clients
A client is a three-digit number you enter at logon. SAP describes a client as a self-contained unit in commercial, organizational and technical terms, with its own user master records, application data and client-specific customizing. Programs and dictionary objects are shared by all clients of a system. Client 000 is SAP's reference client and is not used for business work. Contoso's quality system CMQ has test client 100, where integration and acceptance testing run, and training client 200.
| Term | Identified by | Seen by users as | Contoso example |
|---|---|---|---|
| System | Three-character SID | An entry in SAP Logon or a launchpad URL | CMQ, the quality assurance system |
| Instance | Two-digit instance number on a host | Usually hidden behind a logon group | Application server instance 00 on one CMQ host |
| Client | Three-digit client number | The client field at logon, or sap-client in a URL | Test client 100 in CMQ |
What would you do?
A Contoso tester says, "I tested the fix on instance 100 and it passed." What should Priya ask?
- A system is one installation with one database and a three-character SID.
- An instance is a group of processes on one host with a two-digit instance number.
- A client is a three-digit, self-contained data and user unit inside a system.
- Programs are shared by all clients of a system; client 000 is SAP's reference client.
- Always record SID and client in test evidence.
Check your understanding · not graded
Lesson check
1. How many characters does an SAP system ID (SID) have?
2. Which identifier is two digits and tied to a host?
3. What do all clients of one system share?
4. Which instance contains the message server and enqueue server?