System Architecture & Instances

In a dynpro, what runs at process before output (PBO) vs process after input (PAI)?

Where does the enqueue server keep its locks?

Where does the message server run, how many exist in a system, and what does it do?

SM66 vs SM50 vs SM51 — what does each show about work processes and instances?

Today, what's the only real difference between the primary application server (PAS) and additional application servers?

What is the ABAP platform's relationship to SAP NetWeaver and to S/4HANA?

What exactly makes up one 'SAP system', and how independent is it from other systems in the landscape?

What are the rules for a System ID (SID), and why can't you name a system SAP or SYS?

What's the difference between a technical instance and a dialog instance, and which one do end users connect to?

What is the valid range for an instance number, and what are the two instances in the simplest possible SAP system?

An instance has two names — what are name 1 and name 2, and where is each used?

Why must an instance number be unique per server even across different SAP systems?

What distinguishes one-level, two-level, and three-level hardware configurations, and which is production-grade?

HANA only runs on Linux (SLES/RHEL) — what does that force about a two-level configuration, and what's the production recommendation?

What five types of work process run on a dialog instance?

If a single work process crashes vs the dispatcher crashes, what's the impact on the instance?

What are the minimum work-process counts, per dialog instance vs system-wide?

If the message server fails, what happens to users already logged on vs users trying to connect?

How can the dispatcher and every type of work process be the same executable?

ABAP is neither fully compiled nor interpreted — what is an 'ABAP load' and where does it live?

What happens when a work process exceeds its extended-memory quota or the shared pool is exhausted?

Why is the central services instance a single point of failure, and how is it protected in production?

Why are application servers considered easily replaceable while the database is not?

What is an application server made of, and how does it run ABAP?

Why are dialog instances less critical than central services, and what's the production recommendation?

What role does the SAP Web Dispatcher play, and what are its HA requirements?

What problem does the enqueue replication server (ERS) solve?

What does the gateway (GW) process enable, and how many run per instance?

What does the Internet Communication Manager (ICM) do, and how many run per instance?