Prove your system design skills
System design ability is almost impossible to read from a CV and hard to judge in one interview. Lemma records how you reason through designs over several live sessions, rated by practitioners who build these systems.
Preparing for an interview? System design interview questions, junior to senior
What system design competence looks like
A competent system designer starts with requirements, not boxes. They pin down what the system must do, how much traffic and data it will see in rough terms, and what consistency and latency the users actually need. Every component they add has a reason they can state in one sentence.
They talk in trade-offs. They know what a cache buys and what it costs in staleness, when a queue decouples services and when it hides problems, and how a design behaves when a dependency fails. They change their design when a reviewer exposes a weakness, rather than defending the first sketch.
Designing a system live
60 min · Learning · with Priya R.
Priya R. rated the session
Practitioner · after the call
Clear reasoning on requirements and estimates. Talked through trade-offs first.
Requirements and estimates
Clarifying functional needs, rough traffic and storage estimates, and the constraints that shape the design.
Data storage choices
Relational, document and key-value stores, partitioning, replication and the trade-offs between them.
Caching
Where to cache, how to invalidate and what staleness the product can tolerate.
Queues and async work
Decoupling services, retries, idempotency and handling poison messages without losing data.
Consistency and availability
Reasoning about failure, network partitions and the consistency guarantees users actually need.
Observability and failure
Metrics, logs, traces, timeouts and graceful degradation when parts of the system fail.
What counts as evidence of system design skills?
Certificates in system design show an exam was passed. Lemma records how you work: sessions where you talked through a design or an incident, an adaptive test, and configuration in repositories you link. Every entry says where it came from. See the evidence methodology for how each source is weighed.
System design
Elif Y. · SCI recomputed 9 Oct
Evidence · 4 entries
- Rated 5.0
Designing a system live
Live session · 9 Oct
- Taught · 1.5x
Teaching a design pattern
Live session · 2 Oct
- Level 7.2
Scenario questions on trade-offs
Adaptive assessment · 24 Sep
- Verified
Design docs or a service on GitHub
GitHub connector · 11 Sep
Designing a system live
A practitioner gives you a product brief and plays the role of a reviewer. You gather requirements, sketch a design and defend trade-offs, and they rate how you reason.
Teaching a design pattern
Explaining a pattern such as a write-ahead log or a fan-out feed to another engineer shows depth. Teaching sessions count for more than learning ones in your record.
Scenario questions on trade-offs
Adaptive questions describe realistic situations, such as a hot partition or a cache stampede, and ask for the best response. Difficulty rises or falls with each answer.
Design docs or a service on GitHub
Connect a repository with a service you designed or an architecture write-up. Lemma checks that it exists and that your account has access to it.
How is a system design skills test run on Lemma?
The system design test is adaptive, 15 to 25 questions on failure modes, permissions and cost rather than service names, and it stops once your level holds steady. With sessions and linked work it sets your system design SCI, where recent evidence carries more weight.
One celebrity profile takes most cache traffic and its node overloads. What helps most?
- AAdd a database index
- BLower the cache TTL to one second
- CMove to one bigger node
- DReplicate the hot key across nodes
Learn the building blocks
The AI tutor quizzes you on caching, replication, queues and consistency, and asks you to explain trade-offs in your own words. Drills add only a small, capped amount to your practice score.
Run design sessions with peers
Book live sessions where a practitioner gives you a brief and challenges your design. Several sessions with different reviewers build a stronger record than one long interview could.
Take the scenario assessment
Answer adaptive scenario questions on storage, scaling and failure handling. Each question has a time limit, you cannot go back, and questions do not repeat, so the result reflects your judgement.
System design skills: common questions
How do I practise for a system design interview?
How can senior engineers prove system design ability?
What do reviewers look for in a system design session?
Is system design only for backend engineers?
Skills related to system design
Browse the skills directoryCloud architecture
Show how you design cloud systems that scale, fail gracefully and stay affordable.
Evidence for cloud architectureAWS
Evidence that you can design, secure and run workloads on AWS, not just name services.
Evidence for AWSDevOps
Evidence of how you build pipelines, ship safely and keep systems observable.
Evidence for DevOpsGo
Prove you write clear, concurrent Go services, with evidence from live code and connected repositories.
Evidence for Go
Start your system design evidence record.
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