Design synchronization for water molecule assembly

Q: Design synchronization for water molecule assembly

This question evaluates concurrency and synchronization skills, including thread coordination, use of locks/semaphores/condition variables, and reasoning about safety and liveness properties in concurrent systems.

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This is a medium difficulty System Design question, commonly asked during HR Screen rounds at Tesla.

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This question is commonly asked for Software Engineer candidates at Tesla during technical interviews.

Question

Concurrency: Build H2O from concurrent H and O actions

Context

You are given two kinds of threads that repeatedly call the following functions:

hydrogen(releaseHydrogen)
oxygen(releaseOxygen)

Each function receives a callable action:

releaseHydrogen() emits/prints "H"
releaseOxygen() emits/prints "O"

Goal

Design synchronization so that the program outputs a valid sequence of water molecules. For every molecule, exactly two H actions and one O action must be released together (i.e., the three actions for a molecule proceed as a group; groups do not interleave).

Constraints

Do not busy-wait. Use semaphores, locks, and/or condition variables.
Safety: no extra atoms are emitted; every molecule releases exactly two H and one O.
Liveness: no starvation assuming both atom types arrive infinitely often.
Scalability: handle an unbounded stream of atoms without unbounded resource growth.

Deliverables

Pseudocode for hydrogen and oxygen functions and any supporting synchronization primitives.
A brief explanation of correctness (safety, liveness, scalability).

Design synchronization for water molecule assembly

Quick Overview

Design synchronization for water molecule assembly

Concurrency: Build H2O from concurrent H and O actions

Context

Goal

Constraints

Deliverables

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