Marek Lewandowski
Senior Software Engineer
Experience
Senior Software Engineer
Allegro
- Led a cross-functional squad (6–8 engineers + PM + designer) owning design → delivery → SLOs; shipped Go + Kotlin/Spring Boot services, Python ops tooling, Node.js utilities, and gRPC/REST contracts deployed on Kubernetes (GKE/EKS) with Istio ingress, Argo CD/Helm GitOps, and Prometheus/Grafana + OpenTelemetry dashboards.
- Built idempotent cart/checkout (Go microservices; gRPC/REST) with token-scoped retries and consistent hashing; p95 latency – 35%, auth errors – 18%; cached reads via Redis and stored transactions in PostgreSQL; Kafka outbox guaranteed once-only effects.
- Migrated legacy endpoints using Strangler Fig + canary/blue-green on Kubernetes (GKE/EKS); controlled traffic with Istio (circuit breakers, rate limiting) and promoted via Argo CD/Helm; change-failure rate <5% with weekly release cadence; Prometheus/Grafana SLOs enforced.
- Orchestrated an event-driven order pipeline on Kafka (outbox/inbox, DLQ, replay tooling) with Go/Python consumers, gRPC/REST façades, Redis for idempotency keys, and PostgreSQL for durable state; duplicate shipments → near-zero; reconciliation hours → minutes.
- Implemented PSD2/3-D Secure and acquirer failover: Go gateway + Node.js/NestJS webhook handlers; Kafka events; Prometheus/Grafana SLO alerts; SCA success +12%, soft declines –8% without conversion loss.
- Peak-readiness: k6 tests, HPA/autoscaling on Kubernetes (GKE/EKS), chaos drills; Istio fault injection verified bulkheads; served 2.1× peak TPS with zero Sev-1 incidents.
- Observability-first: OpenTelemetry traces, RED/USE metrics, Prometheus/Grafana alerting; MTTR –30%, alert noise –40%; Argo CD/Helm rollbacks gated on SLO burn rates.
- Search/listing performance: Redis edge caching, feature-flagged ranking tweaks via Node.js control plane; session→add-to-cart +0.6 pp; Next.js/React internal surfaces with selective hydration improved Core Web Vitals (LCP –300 ms; CLS within budget).
- Returns & refunds: saga orchestration in Go with idempotent compensations; durable workflows in PostgreSQL; Kafka event choreography; RMA tickets –22%.
- Standardized CI/CD: GitHub Actions → Argo CD/Helm (GitOps), ephemeral PR envs on Kubernetes, Istio traffic-splits; lead time –25%, deploy success >95%.
- Mentored three engineers; led design reviews/hiring panel; authored runbooks/post-mortems adopted by two sister teams.
- Front-of-site interfaces & tooling: React, Next.js, Angular, Vue.js for ops consoles and internal storefront modules; backend admin tools in Python (FastAPI/Django) and Node.js (NestJS).
Full Stack Developer
- Built and maintained internal enterprise systems used by over 1,000 wholesale partners, streamlining operations for apparel distribution and order processing.
- Developed and maintained full stack internal platforms using React (frontend), Node.js (Express) and Python (FastAPI) for warehouse management and partner operations, reducing manual workflows by 60%.
- Migrated legacy .NET apps to a React + Next.js frontend integrated with NestJS and Django-based APIs, improving system reliability and cutting feature delivery time by 40%.
- Architected microservices for order processing and inventory control using Python (Django, FastAPI) and Node.js, deployed on AWS ECS with Fargate, enabling auto-scaling and zero-downtime deployments.
- Implemented user dashboards and admin panels with React, pulling data from REST and GraphQL APIs built with Node.js and secured with JWT-based auth via AWS Cognito.
- Leveraged AWS Lambda, S3, and API Gateway to build a document management feature for invoices and product assets, improving upload speed and reducing infrastructure costs by 25%.
- Built a real-time inventory sync pipeline using Python, PostgreSQL, Redis, and AWS SNS/SQS, improving stock update latency by 50% across partner systems.
- Designed and integrated CI/CD pipelines using GitHub Actions for frontend (React) and backend (Node/Python), including Cypress/Jest/Pytest testing and Docker-based deployment workflows.
- Monitored application health using New Relic, CloudWatch, and custom metrics from backend services, reducing MTTR for production issues by 45%.
Freelance Software Engineer
Confidential
- Developed responsive web components using HTML5, CSS3, and JavaScript, increasing session duration by 20% on client sites.
- Built scalable REST API development in Node.js and PHP, improving throughput and latency by 35%.
- Independently implemented token-based algorithms for user session tracking and custom search indexing, which reduced data retrieval time by 30%.
- Built real-time chat features with WebSocket, boosting engagement metrics by 25%.
- Contributed to mobile development using Ionic and Cordova for iOS and Android platforms.
- Integrated Stripe and PayPal into eCommerce flow, increasing transaction completion rate by 18%.
- Participated in bi-weekly Agile sprints by estimating stories, resolving tickets, and reducing backlog incidents by 40%.
- Authored unit tests and automated browser tests using Selenium, enhancing regression coverage.
- Led internal training sessions and authored documentation for onboarding new junior engineers.
Industries Experience
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Experienced in Retail (8.5 years) and Information Technology (2.5 years).
Business Areas Experience
The graph below provides a cumulative view of the freelancer's experience across multiple business areas, calculated from completed and active engagements. It highlights the areas where the freelancer has most frequently contributed to planning, execution, and delivery of business outcomes.
Experienced in Information Technology (10 years), Operations (7.5 years), Product Development (3.5 years), and Quality Assurance (2.5 years).
Summary
Versatile, results-driven Senior Software Engineer with 10+ years of experience architecting and delivering scalable, secure full stack applications. Highly proficient in React, Node.js, Python, and AWS. Known for solving complex technical problems, optimizing performance, and fostering cross-functional collaboration from idea to launch.
Skills
Frontend (Apps & Internal Tools): React 18, Next.js (Ssr/isr), Typescript, Vite/webpack, Redux Toolkit/zustand, React Query, Formik/react Hook Form, Tailwindcss, Css Modules/scss, Storybook, Shadcn/ui, Radix Ui, Headless Ui, Charting (Recharts, D3, Visx), React Flow, I18n (React-i18next), Accessibility (Wcag/aria), Core Web Vitals (Lcp/cls/inp), Code-splitting/lazy Loading
Backend & Apis: Python (Fastapi, Django), Node.js (Nestjs/express), Go; Rest & Graphql (Apollo/graphene), Grpc, Websockets/sse, Schema Validation (Pydantic/zod), Rate Limiting, Idempotency
Cloud & Devops: Aws (Ecs/fargate, Lambda, S3, Api Gateway, Cloudfront, Cognito), Gcp (Gke, Gcs, Bigquery), Docker, Kubernetes, Terraform/iac, Github Actions, Argo Cd (Gitops), Helm, Feature Flags (Unleash/flagsmith)
Data & Messaging: Postgresql, Mysql, Redis, Kafka/rabbitmq (Dlq/retries/outbox), Etl/elt, Dbt Basics, Indexing/partitioning, Performance Tuning
Observability & Reliability: Opentelemetry, Prometheus/grafana, Elk/opensearch, Slos/slis, Red/use, Canary/blue-green, Chaos/drills, Incident Response (Mttr↓)
Security & Compliance: Oauth2/oidc, Jwt/sso/2fa, Hmac Signatures, Tls, Secrets Management, Least Privilege, Gdpr; Hipaa-aligned Handling For Intake/marketing Systems
Architecture & Patterns: Microservices, Event-driven Design, Cqrs, Saga, Strangler Fig, Api Gateway, Circuit Breakers/bulkheads, Ddd-inspired Boundaries
Delivery & Ways Of Working: Agile/scrum/kanban, Pr Workflows, Trunk-based/gitflow, Preview Envs, Runbooks/post-mortems, Mentoring/hiring, Stakeholder Communication
Domains: E-commerce (Checkout/payments/fulfillment), Platform Engineering, Analytics/data Platforms, Healthcare Intake/marketing
Languages
Education
National Louis University
Bachelor of Computing in Computer Science · Computer Science · Poland
Profile
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