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.

Request 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
99.9%: availability: an overloaded legacy system meets its SLA again
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
Specification
With your team I clarify goals, interfaces and acceptance criteria, and define the architecture.
02
Implementation
AI writes code, tests and boilerplate to that specification – in small steps that can be checked.
03
Review
I review every change before it reaches your repository. The accountability for it is mine.
04
Tests & handover
Automated tests secure the result. Documented, so your team can carry on independently.

Your code stays your code.

Your rules applyYou decide which AI tools are allowed in the project. I work within your policies.
Nothing without approvalWithout your consent, neither code nor data goes to an AI service.
Without AI, tooIf AI is not wanted in the project, I work without it. Architecture, review and accountability stay the same.

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.