I built a real-time voting system handling race conditions with MongoDB

Building on MongoDB
1

For a pîtch competition attended by over 500 participants to vote for their best teams, I designed a custom voting system that could handle hundreds of simultaneous votes without losing data.

Key highlights:

  • Real-time updates with Server-Sent Events

  • Atomic vote counting using MongoDB’s $inc

  • Prevented duplicate votes with atomic check-and-set

  • Ensured only one team presents at a time using partial unique indexes

  • Handled 1,700+ votes across 5 teams with sub-200ms latency

The full article walks through the architecture, challenges, and solutions:

Read the full article on medium

3 Likes
2

Impressive work :clap: Designing a system that reliably handles real-time voting at that scale is no small task.

I especially like the choice of Server-Sent Events for real-time updates—simple, efficient, and a great fit for one-way live data streams. Using MongoDB’s atomic $inc and check-and-set logic to prevent duplicate votes shows solid attention to data integrity under concurrency.

The partial unique index approach to enforce a single active presenting team is a clever solution as well—clean and enforceable at the database level instead of relying only on application logic.

Handling 1,700+ votes with sub-200ms latency in a live event environment is a strong result.

2 Likes
3

Uhhh, this is really cool, @Ahmed_Idani. I just read through your article and loved how you broke down the architecture and the challenges you ran into. Building a real-time voting system that can handle that many simultaneous users cleanly takes some solid engineering, and this is a great reference for anyone working on something similar.

Thanks for sharing this on the MongoDB Community Hub. How did the competition end up going?

1 Like
4

The competition itself went really well! Voting stayed stable throughout the sessions, results updated in real time without inconsistencies, and we didn’t encounter any race-condition issues despite the traffic spikes. It was definitely a great stress test for the design.

Thanks again for taking the time to read and for the kind words — happy to dive deeper if there’s interest.

1 Like