Freelance Backend Engineer · Kotlin · Java · Spring Boot · AWS

Backends that stay calm under load.

I build Kotlin and Java backends with Spring Boot on AWS – AI-assisted for speed, human for architecture, review and accountability. With more than 10 years of experience.

Book an intro call See success stories

Christoph SensYour direct point of contact

OrderEvents.kt

// Large payloads? They go to S3 automatically.
val extendedClient =
    SqsExtendedClient(
        sqsClient,
        SqsExtendedClientConfig(
            payloadStore = S3BackedPayloadStore(
                s3Client,
                bucketName = "order-payloads",
            ),
        ),
    )

extendedClient.sendMessage(SendMessageRequest {
    queueUrl = ordersQueue
    messageBody = largePayload
})

sqsoverflow · Open SourceMaven Central

10+: years in software development
70,000: market data records aggregated in under a minute in a load test
Kotlin & Java: with Spring Boot on AWS
4 Libraries: open source on Maven Central

01 — Services

From first draft to stable operation.

I step in wherever your team needs support – as developer, architect or both.

Backend development

Robust services in Kotlin and Java: clean APIs, well-designed data models, tests that actually protect something.

→ Spring Boot services & REST APIs
→ Kotlin and Java 25
→ Test automation

Modernization

Replacing legacy systems step by step without putting operations at risk – most recently a monolith in the energy sector: in a load test, the new aggregation processed 70,000 market data records in under a minute.

→ Step-by-step monolith replacement
→ Hexagonal architecture
→ Java and Spring Boot upgrades

AWS architecture

Event-driven systems that scale and stay affordable – with messaging, storage and infrastructure as code.

→ SQS, SNS & S3
→ Event-driven Architecture
→ Architecture reviews

AI-assisted development

I use AI where it adds speed – and keep architecture, code review and accountability firmly in my own hands.

→ Faster to working software
→ Every line reviewed by a human
→ AI workflows for your team

All services in detail →

02 — Open Source & Standards

Code and standards you can check before you hire me.

All projects on GitHub

I work in the open: on my own libraries on Maven Central and on the standards they build on – from the official JSONPath test suite (RFC 9535) to Jakarta JSON Processing.

Eclipse · Jakarta EE

parsson

Fix for a bug open since 2020 in Eclipse Parsson, an implementation of Jakarta JSON Processing – merged.

RFC 9535 · JSONPath

compliance test suite

New test cases for the official test suite of the JSONPath standard – merged.

io.github.treetrail

treetrail

RFC 9535 JSONPath for Java. Version 1.0 passes every case of the official test suite.

Jakarta EE

jsonp-api

Proposal: JSONPath as part of Jakarta JSON Processing – with implementations and a TCK.

com.christoph-sens

*overflow

s3overflow, sqsoverflow and snsoverflow – extended clients for aws-sdk-kotlin: large SQS and SNS messages move to S3 automatically.

03 — How I work

AI for speed. A human for accountability.

01
Intro call
We clarify goals, starting point and scope – concrete and without obligation.
02
Analysis & architecture
I review code and infrastructure and propose a clear path forward.
03
Delivery
In short iterations, with visible results and thorough review.
04
Handover
Documented and easy to follow, so your team can carry on independently.

04 — Success Stories

Results from real projects.

All success stories

Kotlin · Java · Spring Boot · AWS

Replacing a monolith without putting operations at risk

An energy supplier is replacing an unmaintainable PHP monolith for market communication step by step. In a load test, the new aggregation processed 70,000 market data records in under a minute.

A tariff change in e-mobility – on time for the new year

At the turn of the year, a car maker’s charging tariffs change. Hardly anyone wanted to take on the change in the billing backend – I took it on as a new team member.