A Programmable Logic Controller (PLC) is an industrial controller that checks the state of input devices and decides how to control the state of output devices based on a written program.
Get ready for your next PLC interview with our large database of frequently asked questions and their answers in the automation industry. Gain the knowledge and confidence to impress your potential employers.
Preparing for a technical interview as a controller programmer can feel daunting. You need to demonstrate your skills in a wide range of languages and frameworks like C#, Java, PHP, and .NET. I’ve interviewed hundreds of candidates over my career, and I’ve discovered the 20 most common and effective controller programming interview questions. Mastering these will help you stand out and land the job.
The Basics
1. What is the role of a controller in a typical MVC framework?
The controller acts as an intermediary between the model and the view. It handles user requests and input, manipulates data through the model, and determines which view to render. The controller is responsible for the overall flow of the application, maintaining separation of concerns.
2. How does a controller interact with the database? What strategies help optimize this?
Controllers communicate with databases through models and ORM mappers. Optimization strategies include minimizing queries through eager loading, caching frequently accessed data, indexing columns, batch processing requests, and using robust ORMs to simplify complex SQL. The goal is to reduce load on the database.
3, What is inversion of control and how does it relate to controllers?
Inversion of control inverts the flow of control compared to procedural code. Instead of calling dependencies directly, they are injected via an external framework. This promotes loose coupling in controllers by allowing dependencies like services to be injected rather than hard-coded.
Handling Requests
4. Explain how routing works and how requests are directed to controllers.
Routing uses pattern matching to map incoming requests to controllers. Routes consist of a URL pattern, controller and action. The routing engine parses the request URL and matches it to the appropriate route to handle the request.
5. How do you handle concurrent requests in a controller?
Use asynchronous programming and multi-threading. Async allows for non-blocking operations so tasks can execute in parallel. Multi-threading creates threads to handle concurrent requests simultaneously. This improves performance for high traffic.
6. What techniques help keep controllers thin to avoid bloating?
Apply the single responsibility principle, delegate business logic to services, use command/query separation, and employ presenters or decorators for view logic. This keeps controllers focused on their core task of handling requests.
Best Practices
7. How can you ensure controllers remain clean and maintainable over time?
Follow single responsibility principle, minimize repetition through abstraction, use descriptive naming conventions, implement thorough testing, continuously refactor, and keep controllers thin by offloading logic to services. This promotes reuse and simplifies maintenance.
8. What are some common security issues to watch for when working with controllers?
Top threats include injection attacks, cross-site scripting, insecure direct object references, cross-site request forgery, and unvalidated redirects/forwards. Mitigate these by validating inputs, using secure session management, and implementing robust error handling.
9. How should exceptions be handled within a controller?
Use try/catch blocks. Place risky code within ‘try’ and handle resulting exceptions gracefully in ‘catch’ by logging errors or displaying user-friendly messages. This prevents app crashes.
Testing and Debugging
10. How would you unit test a controller and what tools help?
Use mocking frameworks like Moq to test controller methods in isolation. Mock dependencies like services instead of instantiating them to focus on unit under test. For integration testing, leverage frameworks like Jest to simulate requests.
11. What techniques help debug issues in controllers?
Isolate the problem method, use breakpoints and variable inspection, implement logging to trace execution flow, leverage unit tests to replicate bugs, and add tests to prevent regressions after fixing issues. This facilitates rapid diagnosis and resolution.
12. How do you ensure data persistence across requests?
Main options are sessions, cookies, database storage and flash messages. Sessions and cookies store data between requests while databases are more secure and reliable. Flash messages temporarily persist data between redirects.
Interacting with Views
13. What are some ways to pass data from controllers to views?
Viewbag, ViewData, and TempData in ASP.NET MVC. Viewmodels also allow passing strongly typed objects. These approaches maintain separation of concerns by keeping logic out of views.
14. How would you implement complex UI interactions using a controller?
Have controller listen for events from view like button clicks. Perform logic on models in response, validating data and updating models. Send feedback to view to update UI based on model changes. This encapsulates complexity.
15. When is it useful to create custom controller actions?
When the default CRUD operations don’t meet specialized needs. For example, if complex business logic is required like applying dynamic pricing rules based on user data. Custom actions allow tailoring controllers to domain requirements.
Advanced Concepts
16. How are controllers used in a microservices architecture?
Controllers manage communication between services. They handle request routing, validation, and response formatting to decouple services. This isolation enables independent development and deployment of services.
17. When would you create nested or namespaced controllers?
For related resources like nested e-commerce store and product controllers. Namespacing helps differentiate controllers for separate app areas like admin vs user sections. This provides clarity.
18. How do you determine when to use services vs adding logic to controllers?
Use services for reusable, complex or unrelated business logic. This promotes thin controllers and DRY code. Services can be consumed across multiple controllers.
19. What role do controllers play in REST APIs?
Controllers handle CRUD operations in REST APIs. Requests to get, create, update or delete resources are routed to controller actions. They provide an interface to the underlying models.
20. How are dependencies managed in controllers?
Use dependency injection to inject services into controllers rather than instantiating internally. This loose coupling makes controllers easier to test and maintain.
Controller programming interviews vary, but mastering these common questions demonstrates your knowledge across multiple languages and frameworks. Keep your responses clear and concise. Provide specific examples from past projects when possible to showcase your technical experience and development skills. With preparation, you can emphasize your capabilities and stand out from other candidates.
What is an HMI and how does it interface with a PLC?
An HMI (Human-Machine Interface) is a device that provides a graphical interface for interacting with a PLC. It can show data from the PLC and give the PLC instructions based on what the user types in.
What are some common PLC manufacturers?
Some common PLC manufacturers include Siemens, Allen-Bradley (Rockwell Automation), Mitsubishi, Schneider Electric, and Omron.
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How do you answer a controller interview question?
To answer this question, tell them about a tool that you recently learned and how you picked it up. During your controller interview, you may face questions that are more general in nature. They seek to measure your soft skills, and your answers will say a lot about your personality, work ethic, and communication skills.
What questions are asked in a microcontroller interview?
The answer will depend on the specific type of controller being discussed. Microcontroller Interview Questions about working with controllers can also include inquiries about peripheral interface buses (PIBs) and power supply management.
How do you interview a financial controller?
Companies hiring financial controllers look for individuals with specific qualities such as self-motivation and skills in strategic planning and communication. Your first opportunity to demonstrate these qualities is during your interview. You can practice these common controller interview questions to impress the hiring manager.
What are your responsibilities as a controller?
One way to answer this question is to name your core responsibilities as a controller: ensuring proper accounting practices and setting up financial reporting controls. You can discuss a past financial project and how you coordinated with the accounting department to properly document financial transactions.
