Serverless Computing Interview Questions and Answers: Concepts, Design, Limits and Cost

Key takeaways

• Serverless means no server management, not no servers.
• Functions are short-lived and stateless; keep state in external services.
• Know cold starts, timeouts, concurrency and payload limits, and check current quotas in provider documentation.
• Security centres on least-privilege roles and secret handling.
• Explain trade-offs: speed and scaling against cost at steady load, debugging and vendor coupling.

Core concepts

1. What is serverless computing?

A model where the provider manages servers, scaling and patching. You deploy code or use managed services and pay for what you use.

2. What is Function as a Service (FaaS)?

Running individual functions in response to events, such as AWS Lambda, Azure Functions or Google Cloud Functions.

3. What are serverless examples beyond functions?

Managed databases, queues, object storage, API gateways, workflow services and container platforms that scale to zero.

4. What are the benefits?

No server administration, automatic scaling, pay-per-use billing and fast delivery for event-driven work.

5. What are the drawbacks?

Cold starts, execution limits, harder local debugging, provider coupling and unpredictable cost under heavy constant load.

Functions and events

6. What is an event source or trigger?

Something that starts a function: an HTTP request, file upload, queue message, schedule or database change.

7. What is a cold start?

The extra delay when the platform creates a new execution environment. It is affected by runtime, package size and initialisation code.

8. How do you reduce cold starts?

Keep packages small, initialise heavy objects outside the handler, choose lighter runtimes and use provisioned or pre-warmed capacity where the platform offers it.

9. Why must functions be stateless?

Instances are created and removed at any time, so local memory or disk cannot be relied on. Store state in a database, cache or object store.

10. What is idempotency and why does it matter?

An operation is idempotent when repeating it has the same effect. Event delivery can repeat, so functions must handle duplicates safely.

11. How do you handle failures?

Use retries with backoff, dead-letter queues for failed messages, timeouts and alarms.

12. What are concurrency limits?

Caps on how many function instances run at once. Reserve capacity for important functions and protect downstream systems from floods.

Architecture

13. Describe a typical serverless web API.

A client calls an API gateway, which triggers a function that reads and writes a managed database, with authentication, logging and throttling added.

14. How do you process file uploads serverlessly?

A file lands in object storage, which triggers a function to validate, transform or scan it and store the result.

15. When would you use a queue between services?

To smooth spikes, decouple producers from consumers and allow retries without losing messages.

16. What is a workflow or orchestration service for?

Coordinating multi-step processes with state, branching, retries and waits, instead of chaining functions by hand.

17. Serverless versus containers: how do you choose?

Serverless suits spiky, event-driven work and small teams. Containers suit steady load, long-running processes, special runtimes and more control.

18. When should you avoid serverless?

For long-running jobs beyond limits, very steady high-volume workloads where reserved capacity is cheaper, or latency-critical paths where cold starts hurt.

Security and operations

19. How do you secure functions?

Give each function a least-privilege role, keep secrets in a secrets manager, validate input, limit network access and keep dependencies patched.

20. What is a common serverless security mistake?

Overly broad permissions on a function role, so one bug exposes many resources.

21. How do you monitor serverless applications?

Use structured logs, metrics, distributed tracing and alarms on errors, duration, throttles and queue depth.

22. How do you test and debug locally?

Use unit tests for logic, local emulators for events, and integration tests in a real cloud sandbox, because emulators differ from production.

23. How do you manage deployments?

Use infrastructure as code, versioned functions with aliases, gradual traffic shifting and automated rollbacks.

24. How is serverless billed?

Typically by requests and by execution time and memory, plus charges for connected services such as gateways, storage and data transfer. Check the current pricing on the provider's site.

25. How do you control cost?

Right-size memory, avoid unnecessary invocations, set budgets and alarms, cache results and review idle or looping functions.

Common mistakes

• Storing state inside the function.
• Ignoring retries and duplicate events.
• Giving functions administrator permissions.
• Using serverless for every workload without checking cost and limits.

Next steps

Practise with the AWS training or the Python course, which is commonly used for functions. Related reading: cloud architecture interview questions, Python for cloud computing questions and AWS interview questions.