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Design a File Storage Service: System Design Practice

Design a File Storage Service as a system design interview question: Users upload files from any device; files sync across their devices and can be shared.

How do you answer "Design a File Storage Service" in a system design interview?

Split each file into chunks of a few megabytes and name every chunk by its content hash. Store the chunks in a blob store and each file's ordered list of hashes, with a version number, in a metadata database. A client uploads only chunks the server lacks, commits the new list, and a notification service tells the user's other devices to pull the changed chunks.

  • Level: Advanced
  • Scored out of: 100
  • Cost: Free to read; a free account gets the review

Users upload files from any device; files sync across their devices and can be shared.

The scale

50 million users with 10 GB each on average; files up to a few GB.

What a strong answer covers

  1. Requirements (20 points). States the functional requirements in scope and the non-functional ones that drive the design (scale, latency, availability, consistency), and leaves out what is out of scope.
  2. Scale estimates (15 points). Turns the stated scale into requests per second (average and peak), storage and bandwidth, with the arithmetic shown, and uses the numbers to make a decision.
  3. API and data model (20 points). Defines the main API calls with their inputs and outputs, and a data model whose keys and store fit the access patterns.
  4. Architecture (25 points). Chooses components that fit the requirements and explains how a request moves through them, for the main read and write paths.
  5. Bottlenecks and trade-offs (20 points). Names the real bottlenecks and failure points at this scale and how the design handles them, weighing the alternatives (consistency vs availability, push vs pull, cost).

How the practice works

Write your design in sections — functional and non-functional requirements, scale estimates, the API, the data model, the request flow and a deep dive on the bottlenecks — and pick the components to draw the architecture. A review scores it on the five areas above in under a minute, then you can defend it in an AI mock interview.

The reference design

Design a File Storage Service (Dropbox): the walkthrough, with animated figures.

Common questions

How do you answer "Design a File Storage Service" in a system design interview?

Split each file into chunks of a few megabytes and name every chunk by its content hash. Store the chunks in a blob store and each file's ordered list of hashes, with a version number, in a metadata database. A client uploads only chunks the server lacks, commits the new list, and a notification service tells the user's other devices to pull the changed chunks.

What scale is "Design a File Storage Service" designed for here?

50 million users with 10 GB each on average; files up to a few GB. Your estimates turn that into requests per second, storage and bandwidth before you pick components.

How is a design scored?

Out of 100 on a fixed rubric: requirements (20), scale estimates (15), api and data model (20), architecture (25), bottlenecks and trade-offs (20). The review names what the design covered and what it missed, then points to the reference design.

Is "Design a File Storage Service" a good first system design problem?

It is rated advanced. Start with the beginner problems — the URL shortener and the rate limiter — and the System Design notes on load balancing, caching and sharding first.

Do I need an account?

Not to read the problem or the reference design. Getting your design reviewed needs a free account.

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