Design a URL Shortener
Design a URL shortening service similar to TinyURL that allows users to convert long URLs into short, shareable links and manage their shortened URLs.
Asked at:
Amazon
Whoop
Meta
Netflix
A URL shortener is a web service that converts long, unwieldy URLs into short, shareable links and tracks basic engagement. Think of TinyURL or Bitly: you paste a long URL, get a compact code like https://sho.rt/Ab3Cd, and anyone hitting that short link is redirected to the original address. Interviewers ask this because it looks simple but exercises core distributed systems skills: globally unique ID generation without collisions, extreme read scaling (redirects), low latency at the edge, write-heavy analytics capture, abuse prevention, and solid data modeling. It’s a perfect canvas to see if you can define crisp requirements, estimate scale, choose the right storage and caching strategy, and make pragmatic trade-offs around availability, consistency, and cost.
Hello Interview Problem Breakdown
Design Bit.ly
System design answer key for designing a URL shortener like Bit.ly, built by FAANG managers and staff engineers.
Common Functional Requirements
Most candidates end up covering this set of core functionalities
Users should be able to create a short link from a long URL.
Users should be able to use a short link to be redirected to the original URL with low latency.
Users should be able to manage their links (view, disable/delete, optionally update destination) via an authenticated dashboard.
Users should be able to see basic analytics for each link (total clicks, time series, referrers, country/region).
Common Deep Dives
Common follow-up questions interviewers like to ask for this question
Code generation is the heart of correctness and availability. At scale, a naive global counter or random string generator can become a bottleneck or produce collisions. Interviewers look for collision-avoidance, horizontal scalability, and multi-region considerations. - Consider base62/base64 encoding of monotonic IDs (e.g., Snowflake-style time+shard+sequence) or pre-allocated ID ranges per shard to avoid a single, contended counter. - If you offer custom aliases, reserve namespaces and enforce uniqueness via strongly consistent checks only on the affected key, not a full-table scan; you could use a lightweight conditional write. - For multi-region writes, add a region or shard prefix in the code space to guarantee uniqueness without cross-region coordination, and ensure idempotency if clients retry create requests.
Redirects dominate traffic and are extremely read-heavy. To meet user expectations (clicks feel instantaneous), you must serve the mapping from the edge or a nearby cache and minimize cold-path penalties. - Put the short-code→URL mapping in a regional Redis cache (or edge cache/CDN) with cache-aside and short TTL; add negative caching for not-found to protect the datastore. - Use anycast DNS or a global load balancer to route to the nearest POP/region; return compact 301/302 responses and avoid synchronous work on the hot path. - Plan cache invalidation for disabled/updated links (e.g., versioning or soft TTL with background refresh) so state changes propagate quickly without hammering the database.
Analytics are valuable but can easily bloat tail latency if handled synchronously. A clean separation between user-facing redirects and back-end analytics pipelines is a classic system design interview expectation. - Emit a fire-and-forget event (e.g., to Kafka) containing link ID, timestamp, client hints, and referrer; perform aggregation offline with stream processors and batch stores. - Use at-least-once ingestion with idempotent deduplication keys to tolerate retries; compute rollups (minute/hour/day) to power dashboards without scanning raw events. - If you need real-time counters, maintain approximate counters (e.g., Redis with periodic flush) and reconcile later for accuracy, keeping the redirect path fast and resilient.
Public URL shorteners attract abuse. Interviewers want to see rate limiting, content safety controls, and operational safeguards that protect users and infrastructure. - Enforce distributed rate limits per IP/user/API key and apply WAF rules; throttle create and resolve endpoints separately to protect against traffic spikes. - Integrate blocklists/safe-browsing checks at creation time, support rapid takedown/disable, and propagate state to caches quickly; consider link expiration/TTL settings. - Monitor for high error/redirect rates by domain or AS, quarantine suspicious links, and require CAPTCHA or auth upgrades when signals trip thresholds.
Relevant Patterns
Relevant patterns that you should know for this question
Redirect traffic is overwhelmingly read-heavy, often orders of magnitude higher than writes. Meeting global low-latency SLAs requires aggressive caching (Redis/edge), geo-distribution, and cache-invalidation strategies.
Short code creation and analytics event ingestion are write paths that must scale horizontally. You need write-friendly data models, append-only event streams, and techniques like sharded ID generation to avoid bottlenecks.
Naive global counters or uniqueness checks can create hotspots. Handling contention via sharding, pre-allocation, conditional writes, and region-scoped namespaces is essential to keep code generation fast and reliable.
Relevant Technologies
Relevant technologies that could be used to solve this question
Similar Problems to Practice
Related problems to practice for this question
It is the same class of system: short-code generation, low-latency redirects, heavy caching, and an analytics pipeline for clicks and referrers.
Both ingest high-volume click events, require durable event pipelines, and compute time-windowed aggregates without blocking user-facing requests.
Abuse prevention in a public URL shortener needs per-tenant and per-IP throttling at scale, mirroring the design of distributed rate limiters.
Red Flags to Avoid
Common mistakes that can sink candidates in an interview
Using a single auto-increment database counter for code generation
This creates a hot primary key, contention, and a single point of failure. Under load or across regions it will throttle writes and hurt availability.
Serving redirects directly from the primary datastore without caching
Ignoring edge/Redis caching will miss latency targets and explode costs at scale. It also increases blast radius during incidents and makes hotspots more likely.
Performing synchronous analytics writes on the redirect path
Coupling I/O heavy logging to the user request raises tail latency and failure correlation. A backlog or partial outage in analytics would break redirects.
Question Timeline
See when this question was last asked and where, including any notes left by other candidates.
Late September, 2025
Apple
Senior
Early September, 2025
Amazon
Senior
Mid July, 2025
Amazon
Senior
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