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Design a serverless platform like AWS lambda

PremiumCloud infrastructureMedium

  • Serverless
  • Cold starts
  • Concurrency limits

Quick answer

This cloud infrastructure interview question asks you to design a serverless platform like AWS lambda. Aim for a design that runs reliably and securely in the cloud at a sensible cost, then explain the services, regions, deployment and trade-offs. Interviewers want to see how you balance reliability, security and cost.

The problem

Design a serverless platform like AWS Lambda: customers upload function code, and the platform runs it for each request or event, scaling from zero to thousands of copies and billing per millisecond. The hard parts are cold starts, isolating untrusted code on shared hosts and enforcing concurrency limits fairly.

A function has code, a runtime, a memory size from 128 MB to 10 GB (CPU grows with memory) and a timeout of up to 15 minutes. An execution environment handles one request at a time and is reused for later requests while it stays warm.

What must a serverless platform like AWS lambda do?

  • Invoke: Synchronous HTTP invocation that returns the result, and asynchronous invocation that queues the event and retries on failure.
  • Event triggers: Poll queues and receive storage events, invoking the function in batches.
5 more requirementsUnlock to see all

How well must it do it?

  • Isolation: Code from different customers never shares a kernel; each environment runs in its own microVM.
  • Cold start: Platform overhead under 250 ms at p99 for small functions, on top of the customer's own init code; warm invoke overhead under 10 ms.
3 more requirementsUnlock to see all

What scale should you design for?

Use these numbers in your estimates.

The scale numbers unlock with this questionUnlock

Key concepts

  • Serverless
  • Cold starts
  • Concurrency limits
  • microVM isolation
  • Provisioned concurrency
  • Function versions

How you’ll practise

6 short steps · save and come back any time
  1. Requirements
  2. Environments and topology
  3. Deployment pipeline
  4. Reliability and disaster recovery
  5. Observability and cost
  6. Wrap-up

Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.

What do interviewers look for?

In any cloud infrastructure design round.

  • Clarifies availability, security and cost targets
  • Chooses managed services with reasons
  • Plans regions, zones and failover
  • Covers identity, secrets and network boundaries
  • Explains deployment, rollback and observability
  • Keeps an eye on cost as usage grows

Questions people ask about this interview question

Which design type is this question?
It is a cloud infrastructure question. You design a serverless platform like AWS lambda and explain your choices the way you would in an interview.
Is this question asked at entry level?
It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
What should I cover first?
Start with the workloads, the availability target and the security and cost limits. Agree on scope with the interviewer before you design anything.
Can I practise it for free?
It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.

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Updated · Written by the SevyDevy team

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