# Paweł Pacana (@mostlyobvio.us)

Profile: https://sifa.id/p/mostlyobvio.us
Headline: Staff Engineer digging large-scale legacy Ruby/Rails apps

## About

Staff Engineer with over 15 years of professional experience designing, building, and maintaining complex web applications. I specialize in large-scale Ruby on Rails systems, with particular expertise in navigating and improving legacy codebases.

Throughout my career I have consulted and collaborated with teams across many domains including insurance, accounting, fintech, e-commerce, book publishing and event management — helping organizations improve their software architecture and delivery practices.

For nearly a decade I've been contributing to the vision and leading development of RailsEventStore, an open-source toolkit that enables Ruby engineers to adopt event-driven architecture and event sourcing patterns.I am also committed to strengthening the Ruby community. I organise a monthly Ruby meetup and the annual wroclove.rb conference. I regularly share my lessons through blogging and speaking.

## Experience

- **Ruby and Rails Consultant • RailsEventStore Leader • RailsArchitectMasterclass Trainer at Arkency** (2010 – 2025)
  Collaborated with engineers, client teams, and business stakeholders across startups and enterprises to deliver quality software solutions. 
  
  Shaped the vision, led development and maintenance of arkency's strategic open-source project, RailsEventStore. Organized several internal RailsEventStore coding retreats to share knowledge and build team culture. 
  
  Supported colleagues across projects and assisted users through community support channels. Regular speaker at Ruby meetups. Instructor for Arkademy and Rails Architect Masterclass on Domain-Driven Design, Event Sourcing, CQRS and Event-Driven Architecture.
- **Senior Software Engineer at Quiltt (quiltt.io) at Arkency** (2025 – 2025)
  Worked independently, reporting directly to the CTO.
  
  Focused on stabilising and optimising the platform.
  
  Cleared technical debt blocking Rails upgrades, reduced locking contention in core processes and eliminated OOM events through memory optimisation.
  
  Diagnosed subtle bugs including cache leaks and a faulty third-party API adapters impacting data quality from financial institutions.
- **Staff Software Engineer at Lemonade (lemonade.com) at Arkency** (2023 – 2025)
  Collaborated with multiple customer teams, reporting directly to Engineering Director. 
  
  Designed an isolated, always-on verification system in production to validate a new customer onboarding flow — enabling safe iteration and a seamless switch to the new one with no impact on the underwriting outcomes.
  
  Drove platform modernization across a large Rails monolith without service interruptions — upgrading Ruby and Rails versions, eliminating dead code and unused dependencies.
  
  Owned the monolith migration in an organization-wide shift from Grafana to Datadog across ecosystem of services. Optimized CI pipelines and production Docker images, cutting build times and speeding up deployment cycles. 
  
  Consolidated multiple background queue systems into SQS, standardizing async job processing in the platform.
  
- **Staff Software Engineer at Trezy (trezy.io) at Arkency** (2021 – 2022)
  Worked as part of a tight-knit engineering team with direct CTO reporting, taking ownership of key technical decisions.
  
  Enhanced a double-entry bookkeeping system by implementing batch processing, significantly reducing background job execution time and improving system throughput. 
  
  Designed and and implemented a front-end mapping tool in Elm, allowing accountants to visually define rules for categorizing transactions against their chart of accounts. 
  
  Architected read-optimized data models to power cashflow statements and financial reports across multiple time granularities (daily, monthly, quarterly), optimizing for low-latency rendering and scalable query performance. 
  
  Integrated open banking APIs for transaction retrieval, troubleshooting specification gaps and documentation inconsistencies across providers.
- **Senior Software Engineer at Bleach London (bleachlondon.com) at Arkency** (2020 – 2021)
  Mentored the team on RailsEventStore best practices after they had independently selected and adopted it over competing solutions. 
  
  Integrated a headless Shopify storefront with third-party logistics and Salesforce document flow. 
  
  Built an analytics facade to selectively surface data in optimized formats for business intelligence consumption.
- **Senior Software Engineer at Titleleaf (titleleaf.com) at Arkency** (2018 – 2019)
  Designed a hierarchical configuration system enabling white-label deployments for multiple publishers. 
  
  Automated multi-tenant deployments with Terraform, SaltStack, and Chef Solo, improving provisioning consistency and operational efficiency. 
  
  Engaged directly with customer support to troubleshoot issues, deepen domain expertise and translate feedback into system improvements.
- **Senior Software Engineer at Heycater (heycater.com) at Arkency** (2016 – 2017)
  Accelerated deal closures by improving operations workflows and reducing manual bottlenecks. 
  
  Introduced event-driven process managers to enable instant matching between caterers and customers, improving responsiveness and reducing delays.
- **Software Engineer at PONS (pons.com) at Arkency** (2010 – 2014)
  Introduced infrastructure-as-code to automate VM provisioning on a pool of private, dedicated servers.
  
  Partnered with mobile developers to design and integrate in-app purchase APIs, enabling seamless transaction flows across platforms.
- **Software Engineer at Starware** (2009 – 2010)
  Responsible for development of an e-learning platform for high-school students and improving internal CMS product with Ruby on Rails.
- **Contract worker for Google in their Summer of Code program at Google** (2008 – 2008)
  My project was to research and implement new  data backend based on distributed version control system (Mercurial) for the MoinMoin Wiki. I worked with Radomir Dopieralski as my project mentor.
- **Software Developer at Comarch** (2007 – 2008)
  Responsible for web application architecture design and development using ASP.NET 2.0 and ASP.NET AJAX. Delivered a new e-commerce frontend featuring WYSIWYG configuration, integrated with the company's flagship ERP platform.
- **System Administrator at Wroclaw University of Science and Technology** (2006 – 2008)
  Responsible for preparing and improving Windows and Linux installations on lab workstations, at scale.
  
  Maintaining and keeping up-to-date directory services — for students and teachers of the faculty.
  
  Ensuring software and hardware reliability, improving and monitoring system and network security.
- **Senior Software Engineer at Billetto (billetto.dk) at Arkency** (2013 – 2020)
  Designed a star-schema data warehouse fed by domain events to enable cross-system reporting. 
  
  Led an incremental, non-disrupting migration of a public API to a more efficient serialization library using sampled production traffic for validation in place of missing test coverage. 
  
  Applied lessons from high-demand, resource-constrained traffic to design better model boundaries and shorter event streams. Contributed to an in-house event store that helped shape the design of RailsEventStore. 
  
  Later led its replacement through a zero-downtime online migration, incrementally moving millions of event rows without service interruption. 
  
  Gained firsthand experience scaling a product across market segments: from independent organizers to managed accounts to enterprise clients—understanding the unique demands of each.

## Education

- **Wrocław University of Science and Technology** — MSc (2004 – 2009)

## Skills

- Databases
- Data Modeling
- Terraform
- Nix
- DevOps
- Impact Mapping
- Code Refactoring
- PostgreSQL
- Sidekiq
- Docker
- MySQL
- Amazon Web Services (AWS)
- Kubernetes
- TypeScript
- ELM
- SQL
- JavaScript
- Ruby on Rails
- Event Sourcing
- Domain-Driven Design (DDD)
- Mercurial
- Ruby
- Git
- Python
- Linux
- Test-Driven Development
- SaltStack
- Performance Improvement
- Network Administration
- Stimulus
- Event Storming
- React.js
- Process Managers
- Hotwire
- Chef.io
- Cascading Style Sheets (CSS)
- File Systems
- Snowflake
- RailsEventStore
- Event-Driven Archicture
- Debugging

## Volunteering

- Co-organizer at DRUG czyli Dolnośląska Grupa Użytkowników Ruby
- Co-organizer at wroclove.rb
- Mentor at Hackfest Poznań

## Languages

- Polish (native)
- English (professional_working)

## Other profiles

- website: https://mostlyobvio.us
- github: https://github.com/mostlyobvious