Senior Software Engineer, Backend

Flex is a growth-stage, NYC headquartered FinTech company that is creating the best rent payment experience. It’s hard to believe that it’s 2025 and paying rent on time is expensive, inflexible, and difficult. We’re here to change that! Flex enables our users to pay rent throughout the month on a schedule that better fits their finances and budget. Our mission is to empower as many renters as possible with flexibility over their most significant recurring expense. After deliberately keeping a stealth profile as we built up unprecedented investor support and an enthusiastic user base, we are looking for motivated individuals to help us keep our mission growing. Will you be a part of the team? About the job Flex is hiring Senior Software Engineer, Backend roles across four engineering teams: Consumer, Partner Integrations, Core Platform, and Risk. When you apply, you will automatically be considered for all open Senior Backend positions, and your placement will be determined based on your experience and expertise assessed during the interview process. As a Senior Backend Engineer at Flex, you will play a key role in designing, building, and scaling backend services and APIs that power our products, partner integrations, and critical risk management platforms. You’ll collaborate closely with cross-functional teams to deliver robust, fault-tolerant systems that impact renters and partners at scale. You will own the end-to-end lifecycle of your services- from requirements and architecture through implementation, testing, deployment, and monitoring- and help elevate the team by driving technical best practices and mentoring junior engineers.

Our Engineering Teams Consumer Engineering: Builds the APIs and backend systems powering Flex’s main consumer-facing products. This team focuses on delivering scalable services for a seamless rent payment experience through our mobile and web apps.

Partner Integrations Engineering: Owns the backend services and integrations that connect Flex to financial partners and payment networks, ensuring reliable, secure payment flows and partner-facing experiences.

Core Platform Engineering: Maintains the backend systems and infrastructure supporting all business features, including Payments, Billing, Identity, and Partner Integrations, ensuring Flex’s foundation is robust and scalable.

Risk Platform Engineering: Develops the platforms and APIs that mitigate credit and fraud risk, enhancing customer accessibility while safeguarding against misuse through advanced rules engines, machine learning, and event data systems. A minimum of 4 years of experience with Java is required for this position. What You'll Do Design, develop, test, and scale backend services and APIs that power Flex’s consumer products, partner integrations, risk management systems, and core platform services. Build and operate robust, fault-tolerant distributed systems with multiple external dependencies. Implement service APIs for web and mobile apps, simplifying complex rent payment processing. Work closely with Product, Design, Infrastructure, Sales, Partner, and Customer Success to deliver features and integrations that delight customers and partners. Translate feature requirements into technical specifications and lead the architecture and implementation of scalable solutions. Drive the full lifecycle of services and APIs you build, including deployment, monitoring, and optimization for high availability and performance. Contribute to evolving Flex’s service-oriented architecture, REST APIs, message queues, and scalable systems design. Develop risk management systems such as rules engines, ML-powered fraud detection, and high-scale event ingestion pipelines. Mentor junior engineers and help elevate the team’s engineering practices through code reviews, architecture discussions, and technical leadership. Ensure services are observable and reliable using monitoring tools and industry best practices. Collaborate on CI/CD improvements and automation to increase code quality and delivery velocity. Key Qualifications Strong computer science fundamentals with a proven track record of building scalable backend systems. 5–6+ years of professional software engineering experience with a focus on backend development. 4+ years of hands-on Java experience, with deep knowledge of the Java ecosystem. Proficient in Java frameworks and tools used at Flex: Spring (core/web/boot), Gradle, JUnit, JVM tuning and performance optimization. Experience operating services in distributed, service-oriented architectures at scale. Familiarity with REST API design, mobile API best practices, and message-driven architectures. Skilled at building robust, fault-tolerant systems and integrating with multiple external dependencies. Experience designing and maintaining data pipelines and working with big data platforms (bonus: Snowflake). Hands-on experience with AWS (EKS, Aurora RDS, Elasticache, DynamoDB), containerization (Docker/Kubernetes), and Infrastructure as Code (CDK/Terraform). Skilled in observability and monitoring tools (e.g., DataDog) to ensure high availability. Ability to write high-quality code that is testable, maintainable, scalable, and secure, using industry best practices throughout the SDLC (unit testing, build automation, CI/CD). Excellent communicator, comfortable working with distributed teams and cross-functional stakeholders. Proven ability to mentor junior engineers and raise the technical bar through best practices. Risk Platform candidates: Experience in risk engineering, rules engine architecture, and implementing risk/fraud/credit policies in production systems is highly valued. The salary compensation range for this role will be commensurate with the candidate's experience and Flex's internal leveling guidelines and benchmarks. For working locations in NY/NJ/CA, the base salary pay range will be $197,000-$213,000 For all other states, the base salary pay range will be $177,000-$192,000 #LI-Remote

Life at Flex:

We understand that it takes a diverse team of highly intelligent, curious, determined, empathetic, and self aware people to grow a successful company. Our HQ is located in New York City, but we have employees located throughout the US, Australia, Canada and South America. We are growing quickly, but deliberately, with a focus on building an inclusive culture. Our dynamic team has incredible perspectives to share, just as we know you do, and we take great pride in being an equal opportunity workplace.

We offer many employee benefits. For full time, U.S. based employees we offer:

Competitive pay 100% company-paid medical, dental, and vision 401(k) + company equity Unlimited paid time off + 13 company paid holidays Parental leave Flex Cares Program Free Flex subscription For full time non-US employees, we offer Competitive Pay Company Equity Unlimited PTO