AS ABAP Architecture & Instances

You inherit a server whose instance name starts with 'D', so a teammate insists it only runs dialog work. Why is that assumption wrong?

In a multi-instance SAP system, which single component decides which instance is called the 'central instance', and what are all the others called?

A colleague spins up a full SAP NetWeaver AS with the database, application, and presentation all running on one host. What is the only legitimate use for that layout?

In some CRM scenarios with a mobile frontend, SAP inserts a fourth layer into the classic three-tier stack. Where does it sit, and what is it called?

What single architectural fact turns an ordinary SAP system into what SAP calls a 'distributed system'?

Beyond running work processes, what core services does the primary application server (PAS) instance provide that make it mandatory in every AS ABAP system?

An instance directory reads DVEBMGS00. Decode the letters DVEBMGS — what service does each one stand for?

What job does the SAP message server do that no other process in the application layer handles?

You're about to pick a 3-letter SID. Which combinations are off-limits, and what's the risk if you use one?

Two SAP systems share one database (MCOD). What naming collision must you avoid, and why does it bite here specifically?

What is a standalone gateway instance — and what does it deliberately NOT have?

You keep adding application-server hosts to handle more users, but throughput stops improving. Architecturally, what's the ceiling — and why can't you remove it the same way?

Can you scale a system by running several Primary Application Server (PAS) instances, the way you add Additional Application Servers (AAS)? Why or why not?

In a (legacy) double-stack system, how were ABAP and Java kept apart yet connected — and why does modern SAP avoid this design?

You see one instance dir is `...DVEBMGS00...` and another is `...D20...`. What does each tell you about the instance's role — and which holds the system's single point of failure?

Scaling out by adding application-server hosts boosts throughput — what's the hidden cost that grows with each host?

What are the naming rules for an SAP System ID (SID)?

When is a two-tier SAP configuration (DB + app on one server) the right choice, and what do you give up versus three-tier?

What are the three layers of the SAP three-tier client/server architecture, and what does the split buy you?

'SAP system', 'instance', 'application server' — which two are synonyms, and what precisely defines an instance?

Since NW 7.2 the all-in-one 'central instance' was split up. What moved into ASCS vs PAS vs AAS, and what problem did the split solve?