Role interview guide
Backend developer interview questions and what to expect
Backend developer interviews usually focus on APIs, databases and how a service behaves under load or failure, alongside coding in the role's main language (often Java, Python, Node.js, Go or C#). Candidates commonly report questions on SQL and indexing, caching, concurrency and authentication, and for experienced roles, a system design round and stories about production incidents.
What candidates commonly report; it varies by employer, level and country. Last reviewed 18 October 2026.
The usual stages
- 1Recruiter or HR screen
- 2Online coding test or take-home: often a coding problem or a small API to build
- 3Coding interview in the role's main language
- 4Backend fundamentals: databases, APIs, caching, concurrency and your framework
- 5System design for experienced roles: design a service and discuss scale and failure
- 6Hiring manager or behavioural round
What interviewers check
- Clean API design: resources, status codes, versioning and errors
- Database judgement: schema design, indexes, transactions and query cost
- Knowing what happens under load: caching, queues, timeouts and retries
- Security basics: authentication, authorisation and validating input
- Debugging from logs and metrics, and owning production issues
- Real depth in one language and framework
Behavioral questions
Tell me about a production incident you were part of. What was your role in fixing it?
A good answer covers
- What users saw and how the team found out
- What you checked and how you narrowed down the cause
- The fix and the follow-up: alerts, tests or design changes
Describe a time you sped up a slow API or query. How did you find the problem?
A good answer covers
- How slow it was and who it affected
- How you found the cause: query plans, profiling, tracing
- What you changed, with before-and-after numbers
Tell me about a time you had to change a database schema or API that other teams depended on.
A good answer covers
- Why the change was needed
- How you kept things working during the change: versioning, backward compatibility, migrating in steps
- How you told the other teams, and what you'd do differently
Tell me about a technical shortcut you took to meet a deadline. Did you go back and fix it?
A good answer covers
- The shortcut and why it seemed worth it at the time
- What it cost later, honestly
- How you paid it back, or how you'd decide differently now
Questions about the job
What makes a REST API well designed? Talk me through one you've built.
A good answer covers
- Clear resource names, the right HTTP methods and status codes
- Consistent error responses, pagination and versioning
- Authentication, rate limits and documentation
Explain database indexes. How do they speed up reads, and what do they cost?
A good answer covers
- An index as a sorted structure (often a B-tree) that avoids scanning every row
- Composite indexes and why column order matters
- The cost on writes and storage, and checking a query plan to confirm an index is used
What are transactions and ACID, and when might you accept a weaker isolation level?
A good answer covers
- Atomicity, consistency, isolation and durability in plain words
- Problems isolation prevents, such as dirty reads and lost updates
- The trade-off between strict isolation and throughput, with an example
When would you choose a relational database, and when a NoSQL store?
A good answer covers
- Relational for structured data, joins and strong consistency
- Document, key-value or wide-column stores for flexible shapes or very high write rates
- Letting the access patterns decide, with an example from your work
How would you add caching to a read-heavy service, and how do you stop it serving stale data?
A good answer covers
- What to cache and where: in memory, a shared cache such as Redis, a CDN
- Expiry times, clearing entries on writes, and cache-aside versus write-through
- What happens if the cache goes down or many requests miss at once
Explain the difference between authentication and authorisation, and how token-based login works.
A good answer covers
- Who you are versus what you're allowed to do
- Issuing a signed token, sending it with each request and verifying it on the server
- Expiry and refresh tokens, and storing passwords with a slow, salted hash
Your service calls a payment provider that sometimes times out. How do you make this safe?
A good answer covers
- Timeouts, and retries with backoff rather than retrying blindly
- Idempotency keys so a retry can't charge the customer twice
- Recording each payment's state and reconciling uncertain ones later
Talk me through how you'd design a notification service that sends email, SMS and push messages.
A good answer covers
- Requirements: volume, priority, user preferences and quiet hours
- A queue between the API and the senders, with retries and a dead-letter queue
- Rate limits per provider, avoiding duplicates, and tracking delivery
Two requests update the same record at the same moment and one change is lost. Why, and how would you prevent it?
A good answer covers
- The read-modify-write race and why it happens
- Fixes: row locks, optimistic locking with a version number, single atomic updates
- How you'd reproduce it in a test
When would you put a message queue between two services instead of a direct API call?
A good answer covers
- Moving slow or unreliable work out of the user's request
- Absorbing spikes and retrying failures
- The costs: delayed consistency, duplicate messages, harder debugging
Explain how your main framework handles a request, from reaching the server to sending the response.
A good answer covers
- Routing, middleware, the handler and the response
- Where validation, authentication and error handling sit
- How it serves many requests at once: threads, an event loop or worker processes
Motivation and fit
Why backend development rather than frontend or full-stack work?
A good answer covers
- What you enjoy: data, reliability, scale, the logic behind the product
- An example from your work or projects that shows it
- What you'd like to learn next
What would you want to know about our systems and team before your first week?
A good answer covers
- Questions about the stack, on-call, and how code is reviewed and shipped
- A question that shows you've looked at the product
- What a good first three months looks like to them
Tips
- Know your main language deeply: its concurrency model, how memory is managed, and the usual libraries for web work.
- In any design question, ask about traffic, data size and how stale data is allowed to be before drawing boxes.
- Bring up failure without being asked: timeouts, retries, duplicates and what happens when a dependency is down.
- Practise reading a query plan and explaining why an index is or isn't being used.
- Have one incident story ready with a timeline, your part in it and what changed afterwards.
Practise it out loud
A mock interview asks what backend developer interviews usually test, follows up on your answers and gives you a report. The warm-up is free, needs no account and checks five of these questions as you answer.
About this guide
Written by Orbit from what candidates and interviewers commonly describe for this kind of job. The questions are Orbit's own practice questions, not questions taken from any employer's interviews. Processes change, so check the job post and what your recruiter tells you.