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Brandon Braner
All case studies

Fitness / Consumer Apps

Replatforming a fitness app from Django to FastAPI — data, web, and mobile

A fitness app company brought me in to lead a full replatform: strategize their data, move the backend from Django to FastAPI, and rewrite the web and mobile apps against the new API — all on a limited timeline, with a data migration that had to be handled properly. An engagement of leadership, strategy, and software architecture.

Client
O2X
Industry
Fitness / Consumer Apps
My role
Leadership, Strategy & Architecture
Duration
8 months
Django → FastAPI
Backend replatformed end to end
3 fronts
API, web app, and mobile app — one coordinated plan
Verified
Data migrated with integrity checks at every step
On time
Shipped inside a fixed, limited window

Situation

This was a replatforming engagement that required leadership, strategy, and software architecture across three fronts: backend, web app, and mobile app.

A fitness app company had outgrown its Django backend. The product had momentum, but the platform was holding it back — and they knew the fix wasn’t a patch, it was a replatform. They hired me to lead it: strategize their data, move the backend to FastAPI, and get the front-end web app and mobile app rewritten against the new API.

The constraints were real. The timeline was limited, the data behind the product — users, workouts, history — couldn’t be put at risk, and three surfaces had to land together: backend, web, and mobile.

Approach

This was an engagement of leadership, strategy, and software architecture in equal measure.

Data strategy first. Before any code moved, we mapped the data: what the product actually needed, how the model should look on the other side, and how to migrate it safely. The migration plan was rehearsed and instrumented — integrity checks at every step, with a rollback path — because a data migration you can’t verify is a data migration you can’t trust.

Contract-first architecture. I designed the FastAPI backend around the product’s real access patterns and locked the API contract early. That one decision let the web and mobile rewrites run in parallel against a stable spec instead of chasing a moving backend.

Leadership across three fronts. With a fixed window, the job was as much sequencing as engineering: scoping ruthlessly, coordinating the backend, web, and mobile tracks, and staging the cutover so each piece proved itself before the next depended on it.

Outcome

  • The backend moved from Django to FastAPI, architected for the product’s next stage rather than its past.
  • The data migration landed cleanly — planned, rehearsed, and verified, with the integrity checks to prove it.
  • The web app and mobile app shipped rewritten against the new API, delivered as one coordinated release rather than a big-bang gamble.
  • All of it inside the limited timeline the engagement started with — because the strategy work up front is what made the delivery math hold.

Stack & tools

Django → FastAPI migration Data strategy & migration API design Web app rewrite Mobile app rewrite