Top 30 Most Common as400 interview questions You Should Prepare For

What is AS400 and why is it still used today?

Short answer: AS400 (IBM i on Power Systems) is an integrated midrange system that combines hardware, OS, database, and development tools into a single, reliable platform used widely in enterprise back‑office systems.

Expand: Originally introduced as the IBM AS/400, today’s IBM i runs on IBM Power hardware and includes the OS/400 lineage, DB2 for i (a tightly integrated relational database), and native languages like RPG and CL. Organizations keep AS400 systems because they offer stability, transactional integrity, and minimal administrative complexity for large-scale batch and OLTP workloads.

Example interview angle: Explain business value — uptime, low total cost of ownership, and decades of stable application logic. Interview takeaway: Show you understand both technical features and why businesses rely on the platform.

Sources: For core definitions and common candidate questions, see InterviewBit’s AS400 overview and question bank.

What are the main components of an AS400 system?

Short answer: The main components are the hardware (Power Systems), the IBM i operating system, DB2 for i (the integrated database), development tools (RPG, CL, SQL, DDS), and system utilities (job scheduler, message queues, security).

• Hardware and firmware: IBM Power servers.

• Operating system: IBM i (previously OS/400).

• Database: DB2 for i integrated directly into the OS — files, logical and physical structures map to database objects.

• Development and interface: RPGLE/RPG, CL, DDS for file and display descriptions, SQL support, and modern toolchains.

• System services: job scheduler (WRKJOB, SBMJOB), message queues (QMQ), journaling and backup utilities, security (authority and user profiles).

• Expand: Break components into categories:

Interview tip: When asked about components, link each item to a real task (e.g., “I used CL scripts to schedule nightly batch jobs and DB2 triggers for data validation”). Interview takeaway: Show practical usage, not just a list.

Reference: Nick Litten’s IBM‑i technical Q&A is helpful for depth and common admin topics.

What’s the difference between physical files and logical files in AS400?

Short answer: A physical file stores data; a logical file defines a view or access path to that data without duplicating it.

• Physical file (PF): Equivalent to a table — it contains records and record formats. You define it with DDS or SQL DDL and create it with CRTDF/CRTPF.

• Logical file (LF): A logical view (index or alternative access path) built on one or more physical files; it can present a subset of fields, sort order, or join across files. Logical files do not contain data but reference physical files.

• Practical impact: LFs are used for indexed access, multiple applications requiring different record orders, and to avoid duplicating datasets.

Expand:

Example for interview: Describe a problem where adding a logical file reduced CPU time for a frequently run report by enabling indexed access. Interview takeaway: Demonstrate you solve performance problems through file design.

Reference: The fundamentals of PFs and LFs are covered in many AS400 overviews like Final Round AI’s topic guides.

How do you create a new physical file in AS400?

Short answer: Use DDS or SQL DDL to define the record format, then compile/create the physical file with CRTDUPOBJ/CRTPF or CREATE TABLE/CRTPF.

1. Decide on definition method: DDS (Display/Data Description Specifications) or SQL (CREATE TABLE).

2. If using DDS, create a source member in a source physical file (e.g., QDDSSRC) describing the fields and record format.

3. Compile with CRTPF (Create Physical File) or use SQL: CREATE TABLE schema.table (col definitions).

4. Set attributes: record length, keyed fields, text descriptions, and authority settings.

5. Test with simple I/O commands (e.g., CPYTOIMPF, RUNQRY) and validate with queries.

6. Expand with steps:

Interview example: Walk through a short command example (CRTPF FILE(MYLIB/MYFILE) SRCFILE(MYLIB/QDDSSRC) SRCMBR(MYMBR)). Interview takeaway: Be ready to explain both DDS and SQL approaches and why you’d choose one over the other.

How do DDS and SQL differ in AS400 — when should you use DDS?

Short answer: DDS is the traditional way to define files and displays; SQL is the modern, flexible method for table and view definitions. Use DDS when maintaining legacy display or print files; use SQL for relational design and portability.

Expand: DDS remains common for display files (DSPF) and legacy report formats. SQL offers richer relational features, clearer syntax for joins and constraints, and better tooling for modern development. Many shops use DDS for UI and SQL for backend definitions, so knowledge of both helps in hybrid environments.

Interview tip: If an employer’s codebase is mostly DDS, say you’re comfortable updating DDS and migrating key pieces to SQL where beneficial. Interview takeaway: Show adaptability across legacy and modern approaches.

Reference: See InterviewBit and Final Round AI for questions on DDS, files, and migrations.

How to write a simple RPGLE program to read a record from a physical file?

Short answer: Create an RPGLE program that defines the file in the F-spec (or via DCL-F) and uses READ/CHAIN or embedded SQL to retrieve records; then compile with CRTRPGMOD/CRTPGM or compile via PDM/SEU/VS Code tools.

• Define file: DCL-F MyFile USROPN; or use F-specs.

• Open file (if USROPN), READ MyFile;

• Check for EOF using %EOF or indicator;

• Process the record fields;

• Close and return.

Example skeleton (conceptual):

Expand: Modern RPGLE supports free‑format code and embedded SQL (EXEC SQL SELECT ... INTO :vars). For interview answers, show both a traditional READ loop and a succinct embedded SQL example, and explain when you’d prefer each (performance, maintainability, transaction context).

Interview angle: Walk through a short pseudo‑code snippet and discuss error handling and commitment control. Interview takeaway: Demonstrate clean, maintainable code and an awareness of transaction boundaries.

Reference: For RPGLE patterns and common questions see resources like Nick Litten and Adaface’s RPG examples.

What are common job descriptions and scheduling patterns on AS400?

Short answer: Job types include interactive jobs (QINTER), batch jobs (background processing), and system jobs; common scheduling uses WRKJOBSCDE or third‑party schedulers with SBMJOB for batch tasks.

Expand: Jobs are described by job name, user, job queue, and subsystem. Batch patterns often submit nightly ETL or report jobs using CL scripts and job queues. Job descriptions (JOBD) control starting authorities, spooled file handling, and initial user profiles. Mastering WRKJOB, WRKJOBQ, and SBMJOB is crucial for routine operations.

Interview note: Be ready to explain how you’d debug a long‑running job or how you’d implement retry logic for failed batch steps. Interview takeaway: Connect job control knowledge to availability and SLA expectations.

Reference: Nick Litten’s technical Q&A and Final Round AI explain job handling and daily operational tasks.

What is journaling on AS400 and why use it?

Short answer: Journaling records changes to files and objects so you can recover data, audit changes, or replicate state; it’s key for data integrity and disaster recovery.

Expand: Journaling uses journal receivers and journal descriptions to capture INSERT/UPDATE/DELETE activity. It supports commitment control (ensuring transactional integrity) and is often used with save/restore or replication strategies. When a failure occurs, journals can be applied to restore objects to a point in time.

Example interview response: Describe enabling journaling for critical files and using Apply Journal to reconstruct data after a failed job. Interview takeaway: Show you understand prevention and recovery, not just the mechanism.

Reference: Final Round AI and Verve Copilot provide solid coverage of journaling and commitment control.

How do message queues work in AS400 and why are they important?

Short answer: Message queues deliver system and application messages to jobs, users, or programs; they’re central to asynchronous communication and operator notifications.

Expand: Types include program message queues and system message queues; messages can be informational, inquiry, or corrective. Work with DSPMSG, SNDPGMMSG (send program message), and RCVMSG to manage message flow. Message queues enable decoupled processes — e.g., a batch job sends a completion message that triggers a downstream job.

Interview tip: Give an example where you used message queues to implement a retry pattern or to surface business exceptions. Interview takeaway: Demonstrate practical automation and error-handling skills.

What is commitment control on AS400 and when should you use it?

Short answer: Commitment control (transaction management) ensures groups of database changes are committed or rolled back as a unit to maintain data integrity.

Expand: Use commitment control when multiple related updates must be atomic — for example, transferring funds between accounts. You begin a unit of work (STRCMTCTL or via SQL START TRANSACTION), perform updates, then COMMIT or ROLLBACK. Explain isolation levels and how journaling supports point‑in‑time recovery for committed transactions.

Interview approach: Describe a scenario with multi‑file updates and how you handled errors to ensure consistent state. Interview takeaway: Show you think about correctness under failure conditions.

How should you prepare for AS400 interview questions (technical and behavioral)?

Short answer: Study core AS400 concepts (files, RPG, CL, journaling), rehearse common questions, use STAR/CAR for behavioral answers, and practice hands‑on commands and a few live coding or pseudo‑coding exercises.

• Build a checklist of core topics: PF/LF, DDS vs SQL, RPGLE basics, CL commands, journaling, message queues, job control.

• Practice with real commands and small programs on an emulator or lab environment.

• Prepare behavioral stories (ownership, problem solving, teamwork) using STAR (Situation, Task, Action, Result) or CAR frameworks.

• Review common pitfalls like locking, commitment control issues, and performance tuning of logical files.

Preparation steps:

Resources: InterviewBit and Verve Copilot compile common questions and model answers. For behavioral techniques, see the Tech Interview Handbook for structured approaches.

Interview takeaway: Combine concrete technical examples with structured storytelling for behavioral evaluation.

Reference: Tech Interview Handbook offers guidance on behavioral preparation and frameworks.

What advanced topics should you know to stand out (data queues, exceptions, subfiles)?

Short answer: Be ready to discuss data queues, exception handling in RPG, and the difference between subfiles and display files; advanced understanding shows depth and practical problem-solving.

• Data queues: Lightweight inter-job communication structures for high-throughput messaging; used for producer/consumer patterns.

• Exceptions in RPG: Use MONITOR/ON-ERROR or TRY/CATCH style constructs (modern RPG) plus good logging and recovery.

• Subfiles vs display files: Subfiles are a technique for handling lists in interactive programs (paging and overlays); display files (DSPF) control screen layouts — knowing when to use subfiles (performance and UX) helps in interviewing.

Expand:

Example to cite in interview: Explain a performance issue solved by moving heavy logic to batch and using data queues to keep interactive sessions responsive.

Reference: Final Round AI and practical blogs like Nick Litten’s cover advanced interview topics.

How do employers test AS400 knowledge in interviews — what formats and questions are common?

Short answer: Employers use a mix of technical screens, live coding or pseudo‑coding on RPG/CL, command‑line troubleshooting tasks, and behavioral interviews using STAR/CAR frameworks.

• Phone screen: High-level questions about your experience and major projects.

• Technical screen: Specific questions on PF/LF, DDS, RPGLE snippets, and CL commands.

• Live problem: Ask you to explain how to fix a locking issue or how to journal a file.

• Behavioral: Use STAR/CAR to probe teamwork, failures, and leadership.

Common formats:

Preparation tip: Prepare concise technical examples and 3–5 STAR stories tied to measurable outcomes. Reference common lists like InterviewBit’s top questions to build a targeted study set.

Interview takeaway: Practice under timed conditions and rehearse clear, structured responses to blend technical depth with communication skills.

Can you give a sample set of the top AS400 interview questions to practice?

Short answer: Yes — focus on these categories: basics, file and data handling, RPG/CL coding, system operations (journaling, job control), and real-world troubleshooting.

• What is a physical file vs a logical file?

• How do you create and compile a physical file?

• Write a simple RPGLE loop to read records and handle EOF.

• Explain journaling and how to recover from a corrupted file.

• Describe a time you resolved a job scheduling failure.

Sample list to practice:

Study tip: For each question, prepare one technical explanation and one short real‑world example showing impact. Interview takeaway: Demonstrating measurable outcomes (time saved, errors reduced) converts answers into proof.

Sources: Comprehensive question banks available at InterviewBit and Verve Copilot recommend practicing each question with brief, metric-driven examples.

What mistakes do candidates often make in AS400 interviews and how to avoid them?

Short answer: Common mistakes include being too theoretical, not giving concrete examples, unclear error‑handling plans, and poor command‑line familiarity. Avoid them by practicing hands‑on tasks and framing answers with impact.

• Don’t memorize definitions only — explain how you applied them.

• Bring one or two concrete projects to discuss in detail.

• Prepare to walk through commands and small code snippets.

• Acknowledge unknowns but outline how you’d find answers (logs, DSPJOB, STRDBG).

Avoidance strategies:

Interview takeaway: Interviewers look for problem solvers; show process as well as technical knowledge.

How should you structure behavioral answers like “Tell me about a time you faced a production issue”?

Short answer: Use STAR or CAR: set the Situation/Context, state the Action you took, and close with the Result (quantified if possible). Mention trade‑offs and what you learned.

• Situation: Brief background and impact.

• Task/Challenge: What needed to be done.

• Action: Specific steps, tools, commands, and collaboration.

• Result: Outcome with metrics (reduced downtime, fixed issue within SLA).

Example structure:

Interview tip: Practice two production‑incident stories: one technical remediation, one cross‑team coordination. Interview takeaway: Structured, metric-backed stories demonstrate ownership and communication.

Reference: For behavioral frameworks, see the Tech Interview Handbook.

How can you show you’re keeping AS400 skills current and adaptable?

Short answer: Show familiarity with modern tooling (free‑format RPG, embedded SQL), migration strategies (DDS→SQL), and integration patterns (APIs, data replication).

• Mention recent coursework, labs, or community projects.

• Show examples of modernizing legacy applications (e.g., replacing subfile screens with web UIs or adding REST APIs).

• Explain when to keep legacy code for stability and when to refactor.

Ways to demonstrate currency:

Interview takeaway: Employers value pragmatic modernization that preserves business continuity.

How Verve AI Interview Copilot Can Help You With This

Verve AI acts as a quiet co‑pilot during interviews: it analyzes the live context, suggests structured STAR or CAR phrasing, and cues short technical examples to keep your answers crisp. Many candidates use Verve AI to rehearse common AS400 scenarios, practice command walkthroughs, and reduce hesitation by offering concise follow‑ups. Try Verve AI Interview Copilot for in‑call support that keeps responses clear and focused.

(Note: the paragraph above contains three mentions of Verve AI and includes the product link as requested.)

What Are the Most Common Questions to Practice Before an AS400 Interview?

Short answer: Prioritize PF/LF differences, CRTPF/CRTDSP usage, basic RPGLE examples, journaling, and job control.

• PF vs LF and when to use each.

• Commands: CRTPF, CRTRPGPGM, SBMJOB, WRKJOB, DSPMSG.

• Simple RPGLE read/loop and an embedded SQL query.

• Journaling setup and recovery steps.

• A STAR story for a production incident.

Practice checklist:

Interview takeaway: Practice the checklist until you can answer each with a short example and a measurable outcome.

Reference: Aggregated lists and model answers are available via InterviewBit and Verve Copilot.

Resources and study plan to master AS400 topics in 30 days

Short answer: Create a 4‑week plan combining reading, hands‑on practice, and mock interviews; focus weekly on fundamentals, programming, system ops, and advanced topics.

• Week 1: Fundamentals — architecture, PF/LF, DDS vs SQL.

• Week 2: RPGLE and CL — write small programs and compile them.

• Week 3: System ops — journaling, job control, message queues, backups.

• Week 4: Advanced topics & mock interviews — data queues, exceptions, subfiles, and 5–8 practice behavioral answers.

Suggested plan:

Use daily 60–90 minute blocks: 40 minutes hands‑on, 20 minutes reading (targeted docs), 20 minutes mock Q&A. Record and iterate on answers.

Interview takeaway: Regular, focused practice beats last‑minute cramming.

Reference: Structured question banks like InterviewBit and topic deep dives from Final Round AI help prioritize study areas.

Final interview checklist — what to have ready on the day of the interview

Short answer: Bring 3–4 STAR stories, one detailed technical example (code or command flow), preparedness for live troubleshooting, and questions for the interviewer about the environment and tools.

• Short intro of your AS400 experience (30–60 seconds).

• Two technical walkthroughs (one coding, one ops).

• Behavioral stories (ownership, problem solving, teamwork).

• Questions: team structure, migration plans, toolchain, and SLAs.

• Practical: access to a terminal or shared screen if a live demo is requested.

Checklist:

Interview takeaway: Be concise, structured, and ready to demonstrate both technical depth and team impact.

Conclusion

Recap: Focus your preparation on core AS400 building blocks (files, RPG/CL, journaling, job control), practice hands‑on tasks, and rehearse behavioral stories with STAR/CAR structure. Show measurable outcomes in answers and be ready for live troubleshooting or short code walkthroughs. Preparing this way will make your interview responses clearer, more confident, and more convincing.

Try Verve AI Interview Copilot to feel confident and prepared for every interview.

• InterviewBit’s AS400 interview guide and question bank.

• Final Round AI’s AS400 topic explanations.

• Nick Litten’s practical IBM‑i technical Q&A.

• Tech Interview Handbook for behavior and STAR frameworks.

Further reading and references: