Backend Engineer - AI platform in London

500M+ downloads. 77M+ monthly users. A decade of building and we’re still accelerating. Flo is the world’s #1 health & fitness app worldwide on a mission to build a better future for female health. Backed by a $200M investment led by General Atlantic, we became the first product of our kind to reach a $1B valuation in 2024 and we’re not slowing down. With 6M paid subscribers and the highest-rated experience in the App Store’s health category, we’ve spent 10 years earning trust at scale. Now, we’re building the next generation of digital health AI-powered, privacy-first, clinically backed to help our users know their body better.

Overview

Join the AI Platform team as we build the backbone of Flo’s personalization. We are looking for a Backend Engineer to own the User Profile service — a high-scale system (20k RPS, User Profile 2.0), bridging the gap between raw user data and advanced ML feature stores. You will operate in a hybrid model: building core capabilities for AI/ML data representations while governing an inner-source ecosystem that allows other teams to contribute safely. If you are passionate about high-concurrency systems and enabling AI at scale, we want to hear from you!

Responsibilities

• Develop new features for the User Profile service, the central platform that powers personalization, experimentation, and AI features including Flo Conversational AI.
• Build User Profile 2.0 capabilities including historical data arrays, AI-ready attribute representations, and integration with ML feature stores.
• Design and implement data pipelines that deliver user attributes to AI/ML systems for personalized content generation and predictive models.
• Enable inner-source contributions: establish contribution guidelines, quality gates, and architectural standards for product teams.
• Review and approve pull requests from contributing teams, providing mentorship and ensuring code quality.
• Maintain architectural oversight of core components while enabling safe, autonomous contributions from product teams.
• Build and maintain attribute governance frameworks, cataloguing systems, and documentation standards.
• Ensure compliance with healthcare regulations (HIPAA, SOC 2, GDPR) and implement privacy-first design patterns.
• Participate in on-call rotation and drive incident response for this critical platform service.
• Work in a cross-functional setup alongside other engineering teams (Backend, ML, QA), product, and analytics teams.

Your Experience

Must have:

• 7+ years of professional experience building distributed backend systems at scale.
• Fluency in Scala, Java, or Python with willingness to work primarily in Scala (we’ll support your ramp-up if needed).
• Deep experience with AWS services, particularly DynamoDB, EKS, and serverless architectures.
• Strong understanding of observability logging, metrics, tracing, and alerting and how to use it to drive operational excellence.
• Experience building high-throughput, low-latency APIs serving millions of users.
• Solid understanding of databases (SQL and NoSQL) and experience designing APIs that prioritise reliability and performance.
• Experience with code review, mentorship, and establishing engineering standards across teams.
• Track record of writing clean, documented, version-controlled code.
• Ownership mentality you take initiative, don’t wait to be told, and see problems through to resolution.
• Strong communication skills you can collaborate effectively with multiple teams and explain technical decisions clearly.

Nice to have:

• Experience with ML infrastructure, feature stores, or data platforms that serve AI/ML workloads.
• Familiarity with data pipelines and integration patterns for machine learning systems.
• Experience with inner-source or open-source contribution models and governance.
• Experience with platform/infrastructure teams supporting multiple product teams.
• Healthcare compliance framework experience (HIPAA, SOC 2, GDPR).
• Feature flagging, A/B testing, or experimentation platform experience.
• Experience in the MedTech or HealthTech domain.
• Experience at a Tier-1 product company or related experience working within a product organisation.

How we work

We’re a mission-led, product-driven team. We move fast, stay focused and take ownership from brief to build to impact. Debate is encouraged. Decisions are shared. We care about craft, ship with purpose, and always raise the bar. You’ll be working with people who take their work seriously, not themselves. It takes commitment, resilience, and the drive to keep going when things get tough. Because better health outcomes are worth it.

What You’ll Get

We support impact with meaningful reward. Here’s what that looks like:

• Competitive salary and annual reviews.
• Opportunity to participate in Flo’s performance incentive scheme.
• Paid holiday, sick leave, and female health leave.
• Enhanced parental leave and pay for maternity, paternity, same-sex and adoptive parents.
• Accelerated professional growth through world-changing work and learning support.
• Flexible office + home working, up to 2 months a year working abroad.
• 5-week fully paid sabbatical at 5-year Floversary.
• Flo Premium for friends & family, plus more health, pension and wellbeing perks.

Diversity, equity and inclusion

Our strength is in our differences. At Flo, hiring is based on merit, skill and what you bring to the role nothing else. We’re proud to be an equal opportunity employer, and we welcome applicants from all backgrounds, communities and identities.