# Design an application platform like Heroku: system design interview question

> A shared platform for untrusted applications: source builds, safe releases, HTTP routes, and tenant isolation.

Source: https://sysround.com/questions/design-application-platform-like-heroku
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a managed platform that turns application source code into web services. Users can create, update, and remove applications. The platform serves many tenants."

## What to clarify first

- Applications and capacity
- Traffic and read:write mix
- Source builds
- Latency
- Application behavior
- Tenant boundaries
- Order and consistency
- Availability
- Durability and release history
- Region and exclusions

## Scope of a strong answer

Core requirements:

- Create applications and build executable releases from source code.
- Deploy releases, restore previous releases, change instance counts, and delete applications.
- Route HTTP requests to healthy application instances.
- Isolate tenant code, secrets, networks, and resource use.

Usually out of scope:

- Custom domains
- Automatic scale changes
- Managed databases and persistent application disks
- Functions that start for each request
- Source code editors
- Multiple regions
- Billing and payment collection

## What interviewers probe

### Application changes

A deployment spans builds, artifacts, instances, and routes. Retries and late results can restore an old release or recreate a deleted application.

### Traffic during an update

The platform must change releases without sending requests to failed or incomplete instances. Routers can observe changes at different times.

### Tenant boundaries

Tenants control executable code in both builds and services. Resource limits alone do not protect hosts, credentials, or private networks.

### Deployment demand

Builds consume minutes of compute and produce large images. A burst can exhaust workers or delay every tenant's releases.

## Common mistakes

- Treats a successful image build as a completed deployment.
- Builds a custom scheduler before defining application state and release behavior.
- Runs untrusted build scripts beside control services.
- Uses ordinary containers as the entire tenant security boundary.
- Makes application requests depend on the control database.
- Ignores late build results, deployment retries, and deletion races.

## The bar by level

- **Senior:** Defines durable deployment states and safe retries. Enforces the release order contract and handles deletion races. Sizes build concurrency and protects tenant boundaries.
- **Staff+:** Defines invariants across deployment work, routes, and resource ownership. Analyzes host compromise, tenant fairness, zone failures, and capacity costs. Explains recovery without weakening those invariants.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=application-hosting-platform


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.