Rafał Sumisławski
Senior Software Engineer at Coralogix
Lower Silesian Voivodeship, Poland
Summary
🤩
Rockstar🎓
Top SchoolRafał Sumisławski is a Senior Software Engineer from Lower Silesia with nine years of experience building high-performance Scala systems and leading engineering teams. He specializes in functional programming, JVM concurrency and performance tuning, and has led migrations to AWS and production-grade telemetry for millions of devices. Rafał is an active open-source contributor to flagship Scala projects—cats-effect, http4s and the Scala compiler—where his work improved scheduling, connection handling and core library performance. As a team lead he introduced purely functional practices, ran ~25 knowledge-sharing workshops, and routinely troubleshoots hard production issues like deadlocks, memory leaks and GC tuning. Beyond production systems, he has a demonstrated knack for squeezing latency out of real-world benchmarks (TechEmpower) and for turning complex telemetry needs into SQL-like query languages that accelerated product delivery.
9 years of coding experience
4 years of employment as a software developer
High School, High School at Liceum Ogólnokształcące nr 3 im. Adama Mickiewicza we WrocławiuEngineer’s Degree, Computer Science - Internet Engineering, 5.5 (excellent), Engineer’s Degree, Computer Science - Internet Engineering, 5.5 (excellent) at Wroclaw University of Technology
English, German, Polish
Github Skills (44)
asynchronous10
application-framework10
benchmark10
performance-monitor10
performance-analytics10
cats-effect10
typeclass10
async10
app-framework10
http10
benchmarking10
performance-measurement10
akka-http10
http4s10
scala10
Programming languages (12)
Github contributions (5)
http4s/http4s
Mar 2019 - Oct 2022
A minimal, idiomatic Scala interface for HTTP
Role in this project:
Back-end Developer
Back-end DeveloperContributions:46 reviews, 145 commits, 38 PRs in 3 years 7 months
Contributions summary:Rafał primarily contributed to the Blaze client, focusing on improving its timeout and connection management capabilities. Their work included implementing and refining the handling of connection timeouts, addressing issues related to cancellation during connection acquisition, and fixing problems with connection reuse and the handling of responses. They also updated dependencies, including the use of cats-effect, and refactored code for better readability.
idiomatichttp-clientscalafs2typelevel
typelevel/cats-effect
Mar 2019 - Oct 2022
The pure asynchronous runtime for Scala
Role in this project:Back-end Developer / Performance Engineer
Back-end Developer / Performance EngineerContributions:3 reviews, 20 commits, 11 PRs in 3 years 7 months
Contributions summary:Rafał primarily contributed to improving the performance and stability of the `cats-effect` library, which is a pure asynchronous runtime for Scala. Their work focused on optimizing the internal thread pool, specifically around scheduling Runnables and reducing lock contention within internal data structures. They introduced performance benchmarks and made fixes to address potential issues, such as NullPointerExceptions, within the fiber and monitoring infrastructure. Furthermore, the user added new features such as `parReplicateAN` and addressed an issue with fiber deregistration from monitoring.
asynchronousfunctional-programmingcontinuationtypeclassesruntime
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