Alexander Reelsen

Understanding Lucene

Sun, 01 Mar 2026 00:00:00 +0000

Introduction into Apache Lucene, with a strong focus on performance and practical use-cases helping you to keep in mind what is important.

Solar power monitoring with Elasticsearch and ES|QL

Tue, 07 Oct 2025 00:00:00 +0000

TLDR; This post explains how I used ES|QL with my three years of data of my photovoltaic installation and how easy it was to write the queries and create a dashboard.

Two years ago I used Jupyter and DuckDB for the same task, but time to try something different. If you want to know more about the setup and polling the data, read that previous blog post.

Importing CSV data into Elasticsearch

I have a cronjob that downloads the latest data via API in JSON format and automatically creates two CSV files - one with daily data and one with five minute intervals. As you guessed right, the big file has about 350k entries, so we’re slightly short as a big data use case - maybe if someone sponsors me a few thousand more installations to monitor 😂

Programming an Elgato Streamdeck with Java - Part 3

Sun, 06 Apr 2025 00:00:00 +0000

TLDR; This is a multi part series of using your Elgato Streamdeck with Java. This is the third part, implementing DND, Elgato Airlight handling and managing blinds.

You can look a the first part dealing with USB communiation or the second part dealing with event handling and state

Controlling DND mode

DND under osx is a tricky thing. There used to be CLI tools like do-not-disturb for node (which included a small binary), or desktop tools like Muzzle or AutoDND. At some point each tool worked or stopped working, depending on the osx version. So just running one of these binaries was not an option.

Programming an Elgato Streamdeck with Java - Part 2

Thu, 27 Feb 2025 00:00:00 +0000

TLDR; This is a multi part series of using your Elgato Streamdeck with Java. This is the second part, which deals with event handling and creating a stateful application on top of a stateless StreamDeck.

You can read part 1 here.

In this post we will implement a stateful application, even though the StreamDeck does not know any state, but we want an application that has subscreens and back buttons, so we need to put that into our Java app.

Programming an Elgato Streamdeck with Java - Part 1

Tue, 11 Feb 2025 00:00:00 +0000

TLDR; This is a multi part series of using your Elgato Streamdeck with Java. This is the first part, which deals with the initial USB communication.

I’ve owned an Elgato Stream Deck+ for quite some time, but never really used it. At some point I tried to create a node.js + typescript project to programmatically control it, because there are some nice libraries available. There were some issues and I quickly got frustrated for an evening project so I turned to my favorite programming language to solve this: Java.

Shorts: Rolling up s3 logs using Bun

Sun, 02 Feb 2025 00:00:00 +0000

TLDR; This is a much shorter blog post than usual, as I just summarize things I tested in a few hours over the weekend. My plan is to get back into some writing rhythm over this year and come up with more but shorter posts.

I am hosting this blog in a S3 bucket. I used various systems for logging and getting statistics over the years. Over time all the JavaScript based solutions get added to an ad blocker and so it is hard to figure out the amount of visits. My current one is beam analytics, that is ad blocked, so I don’t have any accurate numbers, as I suppose that most tech savvy folks have an ad blocker enabled.

Shorts: Converting & compressing a CSV file

Tue, 28 Jan 2025 00:00:00 +0000

I started reading a DuckDB post about better compression more recent parquet versions and wondered if I could how compression works with own dataset.

My testing dataset was quite small, so it does not represent any substantial. I have collected a little more than two years of PV data in five minute intervals, around 240k entries. Each entry consists of

Evolving Search at a Marketplace

Fri, 01 Nov 2024 00:00:00 +0000

Kaufland owns and runs an online marketplace, spanning several countries and languages, searching tens of millions of products, millions of times per day. Elasticsearch has been the product search engine at Kaufland e-commerce for more than a decade, surviving the move from data centers into the cloud and the internal shift from a Monolith to Microservices. Our team owns search from the microservices, maintaining the whole cluster up to improving relevance. We’ll cover the journey of continuous improvement over the last years, finding surprising performance improvements and how we plan to evolve the search in the next years.

From Keyword Search to Data Science

Fri, 01 Nov 2024 00:00:00 +0000

Elasticsearch serves as the product search engine on the Kaufland Marketplace, powering search across several european storefronts and serving millions of queries and updates a day. While keyword based search has served us well for over a decade, we’re working hard on the next evolution of search. How can we keep the speed of a keyword based search, while improving the search experience for the user? Scouring the internet for interesting approaches, we teamed up with our Data Science team for query rewriting and query expansion topics and came up with a few interesting query preprocessing steps that involve scripted queries and even ESQL. Let’s take a look!

Tracking my meeting time with Hammerspoon

Tue, 01 Oct 2024 00:00:00 +0000

TLDR; If you have a lot of meetings as an IC (individual contributor), you’re often feeling less productive. Sometimes it’s true, sometimes those meetings are necessary. I still would like to know, how much time I really spent in meetings during my working hours. This blog post shows how to use Hammerspoon to figure this out.

What is a meeting?

This is already hard to define. Is your monthly all hands a meeting? Probably. Is your daily a meeting? Totally. Are regular 1:1’s with team members a meeting? Absolutely. Is the spontaneous production outage where you gather with everyone a meeting? I’d say so! Your manager calling? Meeting! You typing furiously? Maybe. You watching YouTube? Not so much! You reading email? Nah.

Node process permissions: An introduction

Tue, 23 Jul 2024 00:00:00 +0000

TLDR; This post will talk about node.js process based permissions feature behind the --experimental-permission command line flag. We’ll also cover a little bit of history and why I think that such a feature is so important in an execution environment.

Note: This feature is still marked as Stability 1 - experimental and in active development, so there are no guarantees of BWC and it’s possible that new features are still being added. This article does not cover module based permissions as those are marked for removal.

Indexing german Wikipedia articles for fun and vector search

Thu, 07 Dec 2023 00:00:00 +0000

TLDR; This article covers extracting and indexing german Wikipedia articles into Elasticsearch by generating vector embeddings using the Deep Java Library and a Hugging Face model.

Disclaimer: This is my first time venturing into vector search with Hugging Face models, it’s likely I have made a ton of mistakes. Feel free to notify me and point out every little one of them, allowing me to learn.

But why?

My former colleague Karel published a wonderful demo/presentation how to get started with semantic search and vector embeddings creation. Take your time and check it out over here. There is also another wonderful blog post in addition using Elasticsearch from Karel. Thank you!

Goodbye DevRel… (for now)

Tue, 28 Nov 2023 00:00:00 +0000

After almost four years of developer advocacy as my main job, I am going back to software engineering. This blog post is my venting rant about what I think is going wrong with developer advocacy/devrel/you-name-it at the moment and why it’s not for me at its current state.

I think there is a major mismatch between company expectations what developer relations are able to achieve and deliver and what happens in the real world, especially if your product is closed source. Not being able to share progress, no exposed roadmap, no way to discuss with developers it is more complex for advocates to find topics to talk about and also lead technical people into some kind of interest funnel that is not purely sales or leads driven. Transparency is required for advocates to go to work.

A year of solar & power monitoring using Jupyter and DuckDB

Tue, 14 Nov 2023 00:00:00 +0000

TLDR; This blog post takes a look at one year of collected solar cell/power consumption data using Crystal, shell scripts, Jupyter, matplotlib and a little bit of DuckDB.

If you just care for the graphs, jump to this part.

Minor note: I do use kw/h and kwh interchangeably - I know strictly speaking it is not 😀.

Polling the API

I am having an AlphaESS solar setup (inverter, battery, car charger). There already is a nice app, also used to control the different charger settings like using only solar power, solar + battery or regular charging and seeing realtime data. Luckily over the last year there was an API added under open.alphaess.com as well as a GitHub repository for discussion.

Search Search Evolution

Fri, 01 Sep 2023 00:00:00 +0000

This talk is an introduction into search and focuses on the recent search trends in the last years. Search is not a solved problem. We start with the basics like relevance, extending over to learning to rank and vector search and will - of course - also cover LLMs and what might come in the future.

Create a custom Tailwind CSS build with Gradle in your Java project (2023 edition)

Thu, 13 Jul 2023 00:00:00 +0000

TLDR; This post will show how to create a custom Tailwind CSS build in a Gradle based Java project. This post is an update to the post made in 2021, as the steps outlined in that one are completely unneeded.

As of late December 2021 Tailwind features a CLI install, which is a node.js distribution packaged together with the tailwind code. All you need to do is to install that binary and no need to fiddle with any node or npm setup.

The ultimate workshop-as-code approach

Tue, 20 Jun 2023 00:00:00 +0000

TLDR; This blog post describes my process of creating, preparing and delivering a workshop, including content creation and how to tackle all of this from an engineering perspective.

Intro

I was asked at my previous employer to create a workshop allowing prospects could take a look at the platform and gather some first experience. The workshop is optimized for engineers to be maintained and held.

Can’t we just use Google Slides or keynote?

This will likely be the first question, once you outline a different approach to running the workshop.

Mirror Mirror What Am I Typing Next

Sat, 01 Apr 2023 00:00:00 +0000

A practical introduction into auto suggest. A good search engine implementation shows relevant results to the user, but also helps getting there, as fast as possible. Often this is done using search as you type or suggest functionality, offering possible results while a user is typing. This talk will cover the underlying data structures and algorithms to explain implementations behind a fast search as you type functionality using radix trees, finite state automatons and dive into advanced topics like ranking and boosting using concrete Java code for explanations and live demos.

New Generation of Data Stores

Sat, 01 Apr 2023 00:00:00 +0000

Storing data is part of every application. The landscape has shifted dramatically in the last years, because of the cloud providers/hyperscalers. The race of storing ever-growing data at a cheaper price is bottomless and resulting in a seismic shift using hyper scale infrastructure in modern data stores, that are only running in cloud environments. The most visible result of this is the common mention of splitting storage and computing, but that is only part of it. This talk covers concepts in new data stores to reduce costs while staying competitive from a performance and pricing standpoint as well as the implications for developers.

Using Hammerspoon to enable lighting for meetings

Wed, 01 Feb 2023 00:00:00 +0000

TLDR; I recently got my hands on some Elgato gear (holy cow, are those expensive at recommended retail prices!). Time to use Hammerspoon to automatically enable them, once a camera turns on. The code applies also to other lights, if you have a way to switch those lights with a command line tool.

Just to quickly repeat: Hammerspoon is an awesome automation framework for OSX. It’s one of the first things I install when setting up a new MacBook. You can read an older post about my setup from 2016 or a little bit newer post about properly supporting DnD mode