ABAPintermediate
RFC (Remote Function Call)
The protocol and function module types that enable cross-system and cross-process calls between SAP and external systems.
RFC is the plumbing underneath nearly every SAP integration scenario - BAPIs, IDocs triggered programmatically, and even many CPI/PI adapters ultimately ride on RFC connections, so interviewers expect both ABAP developers and integration consultants to understand it at a mechanical level, not just as a buzzword.
Synchronous RFC is a phone call where you wait on the line for an answer; transactional RFC is dropping a letter in the mailbox - it will arrive, but you don't wait for a reply; queued RFC is certified mail with sequence numbers so letters arrive in the order you sent them.
Key Concepts
1
A function module becomes remote-enabled by setting its processing type to "Remote-Enabled Module" in SE37, which restricts its interface (no exporting parameters passed by reference in certain contexts, all parameters must be RFC-compatible types) and registers it for external callability. RFC destinations, maintained in transaction SM59, define the connection details (target system, logon credentials, gateway) for both outbound calls from this system and are the basis for trusted/trusting system relationships used in single sign-on scenarios.
SE37SM59
2
Candidates should distinguish synchronous RFC (caller waits for response, used in real-time lookups like BAPI calls), transactional RFC / tRFC (CALL FUNCTION ... IN BACKGROUND TASK, queued and executed asynchronously with guaranteed at-least-once delivery, tracked in SM58), and queued RFC / qRFC (adds strict sequencing on top of tRFC, tracked in SMQ1/SMQ2), each suited to different reliability and ordering requirements.
CALL FUNCTION ... IN BACKGROUND TASKSM58SMQ1SMQ2
3
On the monitoring side, SM58 (tRFC monitor) and ST22/SM21 for dumps and system logs are the first stops when integrations fail, and a practical answer mentions checking the RFC user's authorizations (a very common real-world failure mode) as well as gateway security settings (secinfo/reginfo) that can silently block registered RFC server programs.
SM58ST22SM21secinforeginfo