ASCS, ERS, PAS, AAS in SAP system

What are ASCS, ERS, PAS, and AAS in the SAP system:

ASCS:ABAP Central services instance (Message server)

PAS: Primary application server instance (Dispatcher,dialog, batch, spool, or update)

ERS:Enqueue replication server instance (lock table)

AAS:Additional Application Server

Let's take a look at how a SAPGUI data query works:

1. SAPGUI initiates SOCKET to connect to the SAP server.

2. It is received by Message server in ASCS.

3. Message server continues to assign connections to Dispatcher server in PAS.

4. Dispatcher server found a Work processes (dialog) in PAS.

5. Then the user manipulates the data in the ABAP layer, and if there is any data to lock the table, the content is put in the ERS.

HA installation features:

ASCS is very small and is responsible for handling network connections. To put it on storage, you can switch between hosts.

PAS is relatively large. If you want to put it on storage, you can switch between hosts.

ERS is very small, can form a cluster, two machines are put on, do not have to switch, so that the user's data can be maintained all the time.

More architectural patterns are used in SAP production systems:

High-Availability System multi-machine hot backup highly available system mode: allows the process on a host to switch to a good host to provide continuous service when a host fails.

This HA mode has always been adopted by PRD production system.