Mainframes continue to be a critical technology for many large enterprises. As a result mainframe developers remain in high demand. However, the specialized skills required also make mainframe roles challenging to recruit for.
This article explores the key mainframe developer interview questions that employers frequently ask, along with guidance on how to demonstrate your expertise. With thorough preparation, you can showcase your mainframe capabilities and land your next career opportunity.
Mainframe Basics
These foundational questions assess your overall understanding of mainframe technology
What are the key attributes that distinguish mainframes from other systems?
Highlight the mainframe’s superior processing power, scalability, security features, and ultra-high availability. Emphasize the mainframe’s ability to handle massive transaction volumes reliably.
What are some typical mainframe applications?
Common mainframe workloads include transaction processing (e.g. financial transactions), batch processing of large datasets, enterprise resource planning (ERP), customer relationship management (CRM), and database management systems.
What are the benefits of the mainframe’s centralized architecture?
Benefits include simplicity of management, security, economies of scale, and lower TCO for workloads with high demand. The centralized architecture also minimizes latency.
What key considerations are important when migrating applications to the mainframe?
Highlight the need to determine workload suitability, assess integration requirements, evaluate processing capacity, accommodate proprietary languages like COBOL, account for operational changes, and provide staff training.
Programming Languages
Strong programming language skills are mandatory. Expect detailed questions on languages like COBOL, PL/I, Assembler, C/C++, Java, and more.
Explain your experience programming in COBOL on the mainframe.
Highlight COBOL program structures, syntax, data types, operations, and specialized functions you have used. Outline complex COBOL programs or modules you have developed.
How are modern COBOL dialects different from earlier versions?
Newer releases add object-oriented features like user-defined types, inheritance, and dynamic memory allocation. They include integrated language support for JSON, XML, Unicode, and web services.
What are some differences between COBOL and Java?
COBOL is imperative, procedural, statically typed, and mainframe-based. Java is object-oriented, platform independent, and dynamically linked. Java uses automatic memory management while COBOL requires explicit allocation/freeing.
When might you choose Assembler over COBOL for mainframe programming?
Assembler would be used for low-level operating system services, device drivers, performance-sensitive code segments, or codes requiring direct hardware access. COBOL is better suited for higher-level business application logic.
What are pros and cons of PL/I versus COBOL or Assembler?
Pros include rich data types, exception handling, generics, OOP, and portability across platforms. Cons are lower performance than Assembler and less mainframe-focused than COBOL.
Testing
Testing forms a significant portion of mainframe development. Be ready for in-depth testing questions:
What mainframe testing methods are you familiar with?
Highlight experience with unit testing, integration testing, system testing, acceptance testing, load testing, regression testing, and user interface testing. Know when and why each method is applicable.
How do stub testing and driver testing differ in mainframe development?
Stubs simulate dependent modules that a module under test calls. Drivers simulate modules that call the module under test. Stubs isolate and Driver exercise functionality.
What automated testing tools have you used on the mainframe?
Common tools include CA Intertest, Compuware File-AID, Compuware Xpediter, Compuware Strobe, and BMC Developer Studio. Discuss your experience with specific tools.
How does testing differ between batch and online mainframe applications?
Batch testing focuses on input/output validation and relies more on static testing. Online testing validates user workflows and emphasizes dynamic testing.
What best practices should be followed for unit testing mainframe code?
Recommendations include structuring code to increase testability, maximizing automated tests, integrating with CI/CD pipelines, testing boundary cases, keeping tests independent, using stubs/drivers, and matching test case coverage to code risks.
Debugging and Performance Tuning
Many questions will focus on resolving issues and optimizing performance:
What common methods do you use to debug mainframe applications?
Discuss using interactive debuggers, logs, snapshots, trace analysis, memory dumps, repro steps, core dumps, and source debuggers like Compuware Xpediter.
What steps would you take to performance tune a transaction-heavy mainframe application?
Highlight using monitoring tools to identify bottlenecks, reviewingjobs and queries, checking database access paths, analyzing resource consumption, tuning at the code level, optimizing I/O, and leveraging caching mechanisms.
You suspect a CICS transaction dump indicates a memory leak. How would you investigate further?
Recommend using trace analysis and debugging tools to analyze memory allocation trends. Review code segments using EXEC CICS GETMAIN/FREEMAIN for leaks. Inject faults to induce failures and observe memory usage before/after.
How can poor performing SQL cause problems in IMS or CICS regions on the mainframe?
It can degrade transaction response times, trigger timeouts, use excessive CPU/memory, cause locking issues, and result in poor end-user experience. Proactively optimizing SQL is key.
How would you troubleshoot a batch mainframe job that terminates abnormally?
Suggest reviewing JCL for errors, checking job logs, validating dataset space allocation, analyzing dumps, isolating failures via job steps, and leveraging debugging tools to pinpoint code defects.
Mainframe Concepts
Employers want to validate your breadth of mainframe expertise. Expect questions probing your knowledge of key systems and architectures:
Explain the architecture and key capabilities of a CICS region.
Cover the basics like program isolation, transaction management, security, shared resources, scaling capabilities, and the ability to interface with terminals, programs, and data.
What are some common uses cases for IMS on the mainframe?
IMS excels at high-volume transaction processing and industrial-strength database management for mission-critical applications. Large-scale OLTP applications often utilize IMS.
How does VSAM differ from sequential datasets on the mainframe?
VSAM provides direct access and indexing capabilities not available with sequential files. However, sequential is simpler and lower overhead.
What are the components of a mainframe TP network using SNA?
Cover mainframe terminals, cluster controllers, communications controllers, modems, transmission lines/circuits, NCP packet switches, and SNA protocols.
What advantages does JES3 offer over JES2 for job scheduling?
Highlight JES3’s ability to leverage multiple processors for I/O handling, greater throughput capabilities, and increased reliability via independent control of functions.
Administration and Operations
Many employers will assess your operational mainframe knowledge:
What key performance indicators would you monitor for a mainframe system?
Top indicators include CPU utilization, paging rates, I/O rates, workload throughput, response times, and queue lengths. These provide insight into bottlenecks.
What steps are involved in a mainframe IPL?
Steps include running hardware diagnostics, initializing hardware components, locating the nucleus, loading the nucleus into storage, extending the system, and initiating scheduling and memory management.
As a mainframe system programmer, how would you optimize disk space usage?
Recommend periodic reporting on utilization trends, archiving/removing old files, compressing datasets, identifying wasted space, consolidating free space, and tuning SPACE parameters.
You need to install a new mainframe OS version. What considerations are important?
Consider workload testing, migration planning, staff training, application compatibility validation, fallback strategies, and coordinating cutover. Communicate plans clearly with stakeholders.
What mainframe system programmer duties have you performed in past roles?
Provide examples of activities like applying maintenance, managing storage, administering security profiles, automating operations, maintaining libraries, configuring networks, and troubleshooting outages.
By mastering these common mainframe interview questions, you can convincingly demonstrate specialized expertise that positions you for success in your next mainframe developer role. Use these questions for practice to highlight your technical capabilities as well as your passion for the platform.
How are TYPE 1 and TYPE 2 indexes different?
TYPE 2 index is available for DB2V4. In this type, the data pages get locked, but the index pages are not locked. Therefore, TYPE 2 index are faster.
2 State the difference between JES2 and JES3?
Because of JES3, the steps are given their data sets before the job is even scheduled to run. While in JES2, this allocation takes place just before the execution of each step.
