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Davor Kljajic

@davorkljajic

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Backend Developer at MARGINS building NestJS services and AI integrations for a social networking platform.

Croatia

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At MARGINS, I developed and maintained production backend services for a social networking platform, including REST APIs, business logic, authentication, and database integrations.

I implemented integrations with external platforms and services, and developed functionality that combines data from multiple sources into unified user profiles.

I contributed to AI/LLM-based backend features using LangChain, LangGraph, and OpenAI, and worked with Python and FastAPI on AI development.

Previously, I developed enterprise, mobile, and frontend application services at GAUSS D.O.O. I also worked on blockchain applications and networks at BEYONDI, and write Medium articles about backend development, databases, and blockchain technologies.

Experience

Work history, roles, and key accomplishments

Backend Developer

Margins

Jan 2025 - Oct 2026 (1 year 9 months)

Developed and maintained production backend services using NestJS, TypeScript, PostgreSQL, and TypeORM. Implemented REST APIs, integrations, and AI/LLM-based features.

NestJS TypeScript PostgreSQL TypeORM REST APIs AWS SQS S3 AWS Secrets Manager Lang Chain LangGraph OpenAI Jest Supertest JWT Stripe Firebase Google API Sendgrid Mailchimp Sharp PDF Processing Docker Git

Backend Developer

Gauss D.O.O

Jan 2020 - Jan 2025 (5 years)

Developed backend services and APIs for enterprise, mobile, and frontend applications. Worked directly with clients to clarify requirements and translate business needs into backend functionality.

NestJS PostgreSQL MongoDB SQL GCP Google Cloud Functions REST APIs NodeJS TypeScript

Web Developer

Beyondi

Jan 2018 - Jan 2019 (1 year)

Developed web services and APIs for social networking projects. Worked on multiple blockchain projects, creating blockchain applications and networks, and integrated IoT with blockchain solutions.

Blockchain IoT Web Services APIs JavaScript NodeJS

User Interface Developer

Beyondi

Jan 2017 - Jan 2018 (1 year)

Focused on UX/UI design for social networks.

UX UI Design Social Networks

Education

Degrees, certifications, and relevant coursework

Blockchain Hackathon in Stuttgart

Blockchain Hackathon, Blockchain

2019 - 2019

Activities and societies: Designed and implemented a use case combining Blockchain, IoT, and Arduino sensors to detect unauthorized access and critical temperature changes and securely notify authorized participants in real time.

Participated in the Blockchain Hackathon Stuttgart 2019, collaborating in an international team to develop a blockchain-based solution for secure transportation of dangerous goods.