Design YouTube view counting
PremiumData pipelineMedium
- View counting
- Hot keys
- Deduplication
Quick answer
This data pipeline interview question asks you to design YouTube view counting. Aim for a design that moves data from sources to consumers reliably and on time, then explain ingestion, processing, storage and trade-offs. Interviewers want to see how you handle volume, late or bad data and failures along the way.
The problem
Design YouTube view counting: the pipeline that turns player events into the view count shown under every video, for 4 billion plays a day. Counts should look live, must not include bots or replays, and feed creator payouts. The hard parts are deduplication, viral hot videos and fast count reads.
The view count is public and also drives creator revenue, so inflated counts are a trust and money problem, while a frozen count on a viral video looks broken.
What must the system do?
- Count views: Keep a total view count per video using the rule above.
- Show counts: Show the count on watch pages, search results and channel pages.
How well must it do it?
- Freshness: A valid view shows in the public count within 2 minutes at p95.
- Read speed: Count reads answer in under 20 ms at p99.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- View counting
- Hot keys
- Deduplication
- Bot filtering
- Stream processing
- Kafka
How you’ll practise
- Requirements
- Data volume
- Ingestion
- Storage and processing
- Serving and freshness
- 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 data pipeline design round.
- Clarifies sources, consumers and freshness needs
- Chooses batch, streaming or both with reasons
- Handles late, duplicate and malformed records
- Makes each stage safe to retry
- Explains storage formats and partitioning
- Plans monitoring, backfills and data quality checks
Questions people ask about this interview question
- Which design type is this question?
- It is a data pipeline question. You design YouTube view counting 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 data sources, the consumers and how fresh the data must be. 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.
Related questions
Updated · Written by the SevyDevy team