Payment Gateway Integration and Subscription Management Building: a developer's diary of technical implementation and professional development

Abstract

This thesis is a diary-style account of the author’s internship experience as a full-stack developer at a small Finnish startup from February to April 2025. Conducted remotely, the internship aimed to enhance the author’s skills in full-stack development, self-management in a distributed work environment, and technical documentation. The main focus of the project was the integration of a payment gateway and implementation of subscription management features within amultilingual web platform, using the Next.js framework.

The scope of the thesis is limited to an eight-week observation period, during which the author documented daily tasks, technical challenges, and progress. Weekly reflections were used to explore selected topics in greater depth. The reflection and analysis in this thesis are based on the author's practical experiences during the internship, focusing on the connection between daily development work and overall professional growth. Practical work included implementing payment workflows, handling user subscriptions, and integrating APIs, while the diary served as a structured tool for self-assessment and progress tracking.

The internship resulted in significant improvements in the author’s technical competencies, particularly in payment integration, backend–frontend interaction, and user-centered design. Self management skills were strengthened in the remote work context, and the reflective journaling method facilitated continuous learning and critical evaluation. These outcomes contributed to professional growth and clarified the author’s career direction. The study highlights the value of reflective practice in bridging theoretical understanding with practical software development.