Appendix

Elastic Cloud Server (ECS): ECS provides secure, scalable, on-demand compute resources, enabling you to flexibly deploy applications and workloads.

Elastic IP (EIP): EIP provides static public IP addresses and scalable bandwidths that enable your cloud resources to communicate with the Internet. You can easily bind an EIP to an ECS, BMS, virtual IP address, NAT gateway, or load balancer, enabling immediate Internet access.
Virtual Private Cloud (VPC): VPC allows you to isolate online resources with virtual private networks. VPC enables your cloud resources to securely communicate with each other, the internet, and on-premises networks.
Security Group: A security group is a collection of access control rules for ECSs that have the same security protection requirements and are mutually trusted within a VPC. After a security group is created, you can create different access rules for the security group, these rules will apply to any ECS that the security group contains.

SAP S/4HANA: SAP S/4HANA is an enterprise resource planning (ERP) business suite based on the SAP HANA in-memory database and provides consumer-grade user experience through SAP Fiori.
PAS stands for the primary application server.
AAS stands for the additional application server.
ABAP Central Services (ASCS) is the core SAP application service. It consists of following servers:
Message Server: works as a load balancer. All user requests are first processed by the message server and then distributed to each SAP application server.

Enqueue Server: manages lock table. To prevent different operations from modifying a record at the same time, the table is locked to ensure data consistency.
The differences between PAS and AAS: The PAS contains the ASCS, but an AAS does not. In a system, there is only one PAS, but there can be multiple AASs. The number depends on the service requirements. If any problem occurs in the ASCS, the entire SAP system breaks down. Therefore, adopt the HA architecture for the ASCS.