Velihan Zelev, Developer in Sofia, Bulgaria
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Velihan Zelev

Verified Expert  in Engineering

Bio

Velly is a talented developer with around 14 years of professional experience in a variety of programming languages (mostly Java and some Python) and many frameworks and tools (e.g., Spring Boot). Velly specializes in back-end development and data processing.

Portfolio

Booking.com
Java, Spring, Dropwizard, MySQL, Perl, APIs, REST, Kubernetes, Swagger, OpenAPI...
Large construction equipment manufacturer
Java, Amazon Web Services (AWS), Python, Flink...
Ocado Technology
Docker, Python, Spring Boot, Java, Amazon Web Services (AWS), Software Design...

Experience

  • Java - 10 years
  • Spring Boot - 10 years
  • Spring - 10 years
  • Docker - 8 years
  • REST - 7 years
  • Amazon Web Services (AWS) - 6 years
  • Python - 5 years
  • Kubernetes - 1 year

Availability

Full-time

Preferred Environment

IntelliJ IDEA

The most amazing...

...development effort I've participated in was a big data platform for article and tweet processing. It stores entities to a graph database and Elasticsearch.

Work Experience

Senior Back-end Engineer

2023 - 2025
Booking.com
  • Performed deep integration of over 60 internal systems in a unified manner, irrespective of their nature, including Perl-based databases, Redis queues, Kafka streams, and REST APIs.
  • Modernized a large-scale legacy Perl-based monolithic system transparently and smoothly by migrating it to new Spring Boot applications with fail-safe measures.
  • Owned and managed development, CI/CD, monitoring, support, alerting, and adhering to service-level agreements (SLAs).
Technologies: Java, Spring, Dropwizard, MySQL, Perl, APIs, REST, Kubernetes, Swagger, OpenAPI, Enterprise, Multithreading, Redis, Back-end

Java and Python Developer (via Toptal)

2021 - 2023
Large construction equipment manufacturer
  • Built robust big data pipelines in AWS to process millions of records hourly. I actively participated in the development process, including design, implementation, testing (E2E Cucumber tests), CI/CD, and support.
  • Refactored and redesigned several services and APIs, achieving better performance of the overall pipeline.
  • Reduced AWS cost by reducing and redesigning AWS infrastructure after deep analysis and monitoring of the data pipelines with no quality loss.
  • Worked in three teams owning different parts of the big data flow, actively performing and leading significant improvements and migration on the live pipeline with zero data loss.
Technologies: Java, Amazon Web Services (AWS), Python, Flink, Amazon Simple Queue Service (SQS), Amazon Simple Notification Service (SNS), Cucumber, Apache Flink, Amazon Kinesis, NoSQL, Amazon DynamoDB, Amazon Athena, Amazon S3 (AWS S3), Spring, CI/CD Pipelines, AWS Fargate, AWS Lambda, Snowflake, Big Data, AWS IAM, Software Design, Architecture, Git, Spring Boot, Docker, REST, REST APIs, SQL, Back-end Development, APIs, Object-oriented Programming (OOP), Data Integration, Enterprise, Multithreading, Amazon EC2, Back-end

Senior Back-end Developer

2017 - 2022
Ocado Technology
  • Designed, developed, and deployed microservices and IoT device software.
  • Managed a team of 10 people and grew them. All of them upskilled a lot and even got promoted.
  • Built a new system from scratch in AWS using managed services and Spring boot.
Technologies: Docker, Python, Spring Boot, Java, Amazon Web Services (AWS), Software Design, Architecture, Git, Spring, REST, Big Data, CI/CD Pipelines, AWS Lambda, Amazon DynamoDB, SQL, Back-end Development, APIs, Object-oriented Programming (OOP), Data Integration, Enterprise, Multithreading, Back-end

Spring Back-end Developer

2019 - 2019
Toptal Client
  • Designed and delivered new features rapidly in a large-scale codebase.
  • Refactored major portions of the project for better reliability and performance.
  • Migrated the Spring version from 3 to 5, along with the modernization of the testing framework.
  • Optimized the performance at the DB and service level.
  • Designed and implemented a new multi-tenant product.
Technologies: PostgreSQL, Spring Boot, Software Design, Architecture, Git, Amazon Web Services (AWS), Spring, Docker, REST, CI/CD Pipelines, SQL, Back-end Development, APIs, Object-oriented Programming (OOP), Data Integration, Enterprise, Multithreading, Back-end

Senior Django Developer

2016 - 2017
Consumers Unified, LLC
  • Implemented a vast number of new features in several teams.
  • Optimized a lot of database queries and long-running processes.
  • Supported, owned, deployed, and tested the product.
Technologies: Python, Django, Git, Agile Software Development, MySQL, REST, SQL, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Senior Java Developer

2015 - 2017
EPAM Systems
  • Developed a non-blocking I/O application from scratch and constantly communicated with the stakeholders.
  • Headed the team members and facilitated communication with upper management.
  • Established, monitored, improved, and led the working processes.
Technologies: Non-blocking I/O, Spring, Java, Git, Linux, Spring Boot, Docker, REST, CI/CD Pipelines, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Big Data Senior Back-end Developer

2015 - 2015
Yatrus Analytics
  • Contributed to system design by building a persistence engine and creating a REST API for data consumption.
  • Developed Spark streaming application from scratch.
  • Implemented integrations with numerous data providers.
  • Trained, mentored, and guided newcomers and juniors, and managed processes.
Technologies: Spark, Hadoop, Java, Git, Linux, Spring Boot, REST, Big Data, SQL, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Senior Back-end Python Developer

2014 - 2015
Questers
  • Contributed to system design.
  • Developed a Python REST API.
  • Built a Django application.
  • Trained newcomers.
  • Managed processes.
Technologies: MySQL, Django, Python, Git, REST, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Senior Back-end Developer

2013 - 2014
Musala Soft
  • Served as backup team leader.
  • Developed a few REST and SOAP APIs.
  • Developed and supported several back ends.
  • Contributed to process management, team events, etc.
Technologies: PHP, Databases, Java, Git, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Back-end Developer

2012 - 2013
DG Net
  • Analyzed and developed requirements.
  • Managed deliveries.
  • Developed back-end sides of several web projects, including a mobile application market, webcam channels site, and Google TV platform.
  • Contributed to front-end development.
  • Designed solutions.
Technologies: PostgreSQL, Django, Python, Git, Back-end Development, APIs, Object-oriented Programming (OOP), Back-end

Junior Back-end Developor

2011 - 2012
Atia Soft
  • Developed the back-end for https://www.zesteronline.com/.
  • Learned about algorithms, HTTP models, Python, and solution design.
Technologies: PostgreSQL, Django, Python, Git, Back-end Development, Object-oriented Programming (OOP)

Experience

Spring Service Controlling Embedded Devices

I contributed to the implementation of a Spring application communicating and controlling a bunch of embedded radio devices. I also took a part into the implementation of the firmware regarding the communication to the Spring service.

Tech stack: Spring Boot, Postgres, REST, Hibernate, Python, Docker, Kubernetes

Footwear Company Project

A bunch of Spring Boot microservices orchestrated within a Spring Cloud providing functionalities for footwear production cost calculation according hundreds of different formulas.

Tech stack: Spring Boot, Spring Cloud, Oracle, Hibernate, REST

Lead Engineer

I have implemented a framework on top of Spring providing abilities with very simple configuration for the operator to create a brand new service out of an existing set of "functional blocks" (Spring beans) and connect them in a pipeline, so when a request occurs (via REST or Event depending on the handler chosen by the operator), it can go thru a bunch of functional blocks, each converting the request to a context, and then each functional block enhances the context.

Tech stack: Spring Boot, Postgres, Hibernate

DHL Address Helper

A Spring REST microservice consuming a bunch of Geo Maps APIs is explicitly required to scale very fast to respond quickly, so we used Spring and Reactor to accomplish that. We also stored results in Elasticsearch for fast queuing.

Tech stack: Spring, Reactor, Elasticsearch

Clever Observer

A prototype-phase product that crawls vehicle sales websites and merges the data from all of them. It notifies the users according their predefined filter about vehicles they want to buy.

Apache Spark, Spring REST, Elasticsearch, PostgreSQL

Nautilus

Big data platform for articles and Tweet processing in real time.

Tech: Spark, Hadoop, HDFS, OrientDB, HBase, Solr

Non-blocking IO REST Services

NDA.

Tech: Java, Spring REST, Reactor

Internal Telecom Systems

Developed and provided third-level support in a huge telecom company. We developed and supported many internal and external systems (more than 60) in a team of 10 people. I developed an Android application and the REST service for testing the network parameters of the telecom also.

Tech: SOA, Tibco, Java, PHP, REST, Android

Empters Wifi

Hot spot system with Facebook and another OAuth2 login providers.

Tech: Django, Python, OAuth2, DD-WRT

Zesteronline

Video sharing and management platform. It is used mostly by movie directors and producers to share and manage different scenes. For example, in some scene there is a redundant plane and the producer doesn't want it, so he/she can mark (draw over the video canvas) and comment the exact scene of the video and share it with the director or the team.

Tech: Django, Python, PostgreSQL, Serving video streaming - Flash, WebM, MP4

Mobile Market

Multi-platform mobile market.

Tech: Django, Python, MySQL

Google TV Application

Google TV application providing several different channels (streams). It was approved by Google (the approval process was very difficult).

Tech: Django, Python, Flash media server, FFMpeg, Java

M-Pin

Two-factor authentication platform. It is SOA and it can be installed on the client side or use as a service

Tech: Python, Tornado, NoSQL, Django, PostgreSQL

Business Process Performance Booster

Spring Boot-based platform helping different businesses to boost their internal and external processes.

Booking Customer Services Platform

• Performed deep integration with more than 60 internal systems in a unified manner regardless of their nature—Perl-based, databases, Redis queues, Kafka streams, REST APIs.
• Fully transparent, smooth, large-scale scale, and fail-safe modernization of legacy—Perl-based monolithic system by migrating to brand new Spring Boot applications.
• Full ownership and responsibility—Development, CI/CD, support, monitoring, alerting, SLA.
• Stack: Java 21, Spring Boot, Perl, Kubernetes, Grafana, micrometer.

Education

2013 - 2016

Master's Degree in Mathematics and Informatics

Sofia University - Sofia

2009 - 2013

Bachelor's Degree in Mathematics and Informatics

Sofia University - Sofia

Skills

Libraries/APIs

REST APIs, OpenAPI

Tools

Git, IntelliJ IDEA, Flink, Amazon Simple Queue Service (SQS), Amazon Simple Notification Service (SNS), Cucumber, Amazon Athena, AWS Fargate, AWS IAM, Grafana, GitLab

Languages

Python, Java, SQL, PHP, Snowflake, Perl

Frameworks

Spring, Spring Boot, Django, Apache Spark, Hadoop, Spark, Dropwizard, Swagger

Paradigms

REST, Object-oriented Programming (OOP), Microservices, Agile Software Development, Continuous Development (CD)

Platforms

Amazon Web Services (AWS), Linux, Docker, Kubernetes, AWS Lambda, Apache Flink, Amazon EC2

Storage

Amazon DynamoDB, Data Integration, PostgreSQL, MySQL, Databases, Elasticsearch, NoSQL, Amazon S3 (AWS S3), Redis

Other

CI/CD Pipelines, Software Design, Back-end Development, APIs, Enterprise, Multithreading, Back-end, Big Data, Architecture, Non-blocking I/O, Amazon Kinesis, micrometer

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