Question 1
Design the system behind the main workflow in this posting. What has to be fast, and what can wait?
Approach. Users and API first. Name one bottleneck and the piece you will not build in the interview.
Backend engineer interview questions from a job description
See the API, data, and reliability questions a backend engineer job description is likely to turn into, grouped by the requirement they test.
Matched in your browser from the requirements in the posting and Luna’s interview-question hub. These are practice prompts, not a claim that this wording will be asked.
Requirements in the posting
Backend Engineer
Own backend services for a multi-tenant product.
You will:
- Design and evolve REST APIs that clients can retry safely
- Build services with clear boundaries, and keep the hot path fast when a dependency fails
- Model data in Postgres, write SQL, and ship schema changes without a long freeze
- Talk through system design: latency, failure, and what you will not build yet
- Work in Python or Node and own the service after it is in production
We care how you explain a tradeoff to another team.
Own backend services for a multi-tenant product.
You will:
- Design and evolve REST APIs that clients can retry safely
- Build services with clear boundaries, and keep the hot path fast when a dependency fails
- Model data in Postgres, write SQL, and ship schema changes without a long freeze
- Talk through system design: latency, failure, and what you will not build yet
- Work in Python or Node and own the service after it is in production
We care how you explain a tradeoff to another team.
Likely questions
System design
System design
Talk through system design: latency, failure, and what you will not build yet
Question 2
How would you estimate capacity before picking a database?
Approach. Use rough numbers, label every assumption, and say which number would change the design.
System design
Backend services
Backend Engineer
Question 3
Where would you draw the service boundary for this product, and what would you keep in one process for now?
Approach. Start from the request that must be fast, then name the split you would postpone.
Question 4
A dependency you call is slow or down. What does this product do?
Approach. Say the user-visible fallback and the failure you will not hide.
Coding
SQL and data modeling
Model data in Postgres, write SQL, and ship schema changes without a long freeze
Question 5
What tables would you need for the core object in this posting, and what is the grain of each?
Approach. Name the entity, the key, and one query you must keep fast.
Question 6
Write the shape of a query this team would run every day. Where would it get slow?
Approach. Say the join, the filter, and the index or rewrite you would try first.
Coding
Python
Work in Python or Node and own the service after it is in production
Question 7
How would you structure a Python module for the core task in this posting so someone else can change it?
Approach. Name the function boundary and the test you would write before adding libraries.
Question 8
Tell me about a time a Python script or service was correct and still too slow.
Approach. Say what you measured, the change, and the cost you left in place.
System design
APIs
Design and evolve REST APIs that clients can retry safely
Question 9
Design the API a client of this product would call first. What is the request, the success, and the error?
Approach. One resource, the idempotency story, and what the user sees on a timeout.
Question 10
How would you change this API without breaking clients that are already shipped?
Approach. Say what you would version or add, and the client you refuse to strand.
What to prepare
- Narrate one design in sixty seconds before you draw any boxes.
- Draw one service boundary and say what you would not split out yet.
- Practice one join out loud: the grain of the result, then the filters.
- Solve one small task in Python while saying the cost of the obvious version.
- Write one endpoint on paper: method, body, error, and what a retry should do.
- Mark the three requirements you cannot yet support with a story.
- Say each answer out loud once, in under two minutes.
Backend postings interview you on boundaries. An API bullet becomes “what is the request, the error, and the retry?” A database bullet becomes grain, indexes, and a migration. A reliability bullet becomes what the user sees when a dependency is down. If the posting says microservices, they will ask what you would refuse to split.
Read for the data that must be correct versus the data that can be stale. That single distinction predicts the design round better than the cloud logo list.
Prepare one endpoint you can specify from memory and one incident where the user-visible behavior was the point. The sample is a services role. Highlighted requirements map to the questions beside them.
Sample job description
The posting below is already in the tool. The questions are the ones its requirements point to. Replace it with a posting you are actually interviewing for.
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