Valentin Kovalenko

Server-side software engineer

Concurrency and distributed systems

`Main language`	Java
`Second language`	Rust
`Frameworks/libraries`	Netty, Protocol Buffers, JMH, Spring Framework, Hibernate ORM, JUnit, Mockito
`DBMSs`	PostgreSQL, Redis + Sentinel, Cassandra
`Distributed coordination`	ZooKeeper + Curator
`Other`	Maven, Git, Docker, basic front-end knowledge, good understanding of the Java memory model (this presentation may back the claim)

Senior Java driver engineer / Jan 2021–current

MongoDB Java Drivers
- my pull requests;
- implemented a PoC for using gRPC as a "transport" protocol for MongoDB Wire Protocol messages, the most interesting part is GrpcStream.java;
- designed and implemented a query building API for MongoDB Atlas Search;
- investigated and fixed an interesting performance degradation caused by fair locking, without blowing up the tail latency;
- implemented a large set of improvements that help avoiding connection storms; the most interesting parts are
  - implemented a mechanism of limiting the number of connections that can be established concurrently, featuring the connection hand-over mechanism to support fairness;
  - refactored read/write retries.
MongoDB Kafka Connector
- my pull requests.
MongoDB Rust Driver
- my pull requests.

Senior software engineer / Apr 2018–Nov 2020

Coverity Connect server (Tomcat, PostgreSQL, Hibernate ORM)—stores and manages issues detected by code analysis tools
- designed and developed export/import functionality (the technical side is more complex than it sounds) in pair with a colleague, designed and implemented support of this functionality in a cluster environment on my own;
- migrated from JDK 8 to JDK 11, took me about 1.5 months in case you are curious;
- cluster—identified and fixed/mitigated multiple bugs, including replication failures caused by
  - incorrect inference of the order of replicated changes, here is my related blog post;
  - broken transaction isolation due to incorrect cache strategy;
- incremental updates functionality—allows automatically updating client applications; fixed a number of bugs, optimized the process of downloading updates so that updates that are not needed or have been cached would not be downloaded.

Senior server-side developer / Jan 2016–Mar 2018
Server-side developer / Dec 2014–Dec 2015

King of Thieves server (Netty, ZooKeeper + Curator, Protocol Buffers, Redis + Sentinel, Cassandra, Docker, Ansible, Datadog, Mixpanel)
- joined this project in Aug 2015, 7 months after the global release, and supported it / developed new features until resigning;
- implemented validation and processing of in-app purchases and subscriptions, here is my related blog post;
- implemented automatic data extraction/grouping from Mixpanel and sending it to partners.
Server and client homegrown frameworks (Netty)
- developed from scratch;
- the server framework is similar to my server project, which is based on it.
Configuration server (Netty, Hibernate ORM, JGroups, PostgreSQL, Tapestry)—manages and delivers game configurations to game clients, supports having configurations for split-testing
- redeveloped;
- a couple of dozen projects started using it compared to only a single one before redevelopment.
Notifications server/client (Netty, ZooKeeper + Curator, JGroups, Protocol Buffers, PostgreSQL)—provides unified access to major push notification services, e.g., Amazon Device Messaging, Apple Push Notification Service
- refactored the design as inappropriate usage of ActiveMQ and Quartz Scheduler was causing performance bottlenecks;
- improved the throughput by an order of magnitude;
- when I resigned, this server was used by all ZeptoLab games.

Senior software programmer / Aug 2013–Dec 2014
Software programmer / Apr 2010–Aug 2013

TELUS Self Service Portal (JMS, WebLogic Server, Oracle Database)
- integrated with Oracle OpenSSO;
- availability—developed functionality allowing front-end servers to persist and later initiate asynchronous execution of tasks on back-ends. This allowed users to use some features of the portal even when back-end servers are temporarily unavailable.
Data objects, a homegrown persistence library—developed various concurrent caches.
Implemented OpenID authentication.
Data migration (Oracle Database, Oracle PL/SQL).

Implementation engineer / Jul 2008–Apr 2010

Student / Sep 2006–Jun 2009

MSc in applied mathematics and physics / Sep 2007–Jun 2009
BSc in applied mathematics and physics / Sep 2003–Jun 2007