This year, ClickHouse made a strong showing at FOSDEM 2025, with several team members traveling to Belgium and multiple talks covering everything from powerful new JSON data types to the challenges of fuzzing databases. If you weren’t able to attend (or just want a recap), here’s what went down in Brussels.

## What is FOSDEM?

For those unfamiliar, **FOSDEM** (Free and Open Source Developers' European Meeting) is one of the biggest open-source conferences in the world. Every year, thousands of developers, engineers, and open-source enthusiasts gather in Brussels to share knowledge, showcase projects, and engage in deep technical discussions. It's a place where open-source software meets innovation, and where deep-dive technical talks are the norm.

And this year, ClickHouse was **well represented**.

## The ClickHouse Talks

### How We Built a New Powerful JSON Data Type for ClickHouse

JSON support is a hot topic for ClickHouse users, and at FOSDEM, we shared details about the **new JSON data type** we built. This talk covered **why** we introduced it, how it **differs from traditional JSON handling**, and what kind of performance benefits users can expect. If you're working with semi-structured data at scale, this was a must-watch.

**Speaker:** Robert Schulze

**Abstract:** JSON has become the lingua franca for handling semi-structured and unstructured data in modern data systems. Whether it’s in logging and observability scenarios, real-time data streaming, mobile app storage, or machine learning pipelines, JSON’s flexible structure makes it the go-to format for capturing and transmitting data across distributed systems.

At ClickHouse, we’ve long recognized the importance of seamless JSON support. But as simple as JSON seems, leveraging it effectively at scale presents unique challenges. In this talk we will discuss how we built a new powerful JSON data type for ClickHouse with true column-oriented storage, support for dynamically changing data structure and ability to query individual JSON paths really fast.

Links related to the topic:

* [RFC: Semistructured Columns #54864](https://github.com/ClickHouse/ClickHouse/issues/54864)
* [Implement Variant data type #58047](https://github.com/ClickHouse/ClickHouse/pull/58047)
* [Implement Dynamic data type #63058](https://github.com/ClickHouse/ClickHouse/pull/63058)
* [Implement new JSON data type. #66444](https://github.com/ClickHouse/ClickHouse/pull/66444)

<iframe width="768" height="432" src="https://www.youtube.com/embed/JohjugQedSk?si=6jBEdh9MHSoAP7yA" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

### Fuzzing Databases is Difficult

Database reliability and security are always important priorities, and fuzz testing is one way to uncover hidden bugs. But as it turns out, **fuzzing databases is hard**. This talk dove into the complexities of fuzz testing, why traditional approaches don't always work well for databases, and what strategies can improve effectiveness. If you’re interested in database internals or security, this one was for you.

**Speaker:** Pedro Ferreira 

**Abstract:** After fuzzing databases for the last 3 years, I learned that simple design decisions on a fuzzer impact on the issues it can ever find. In this talk I would to address some of those decisions. As an example, I would to discuss about the design of BuzzHouse, a new database fuzzer to test ClickHouse.

Links related to the topic: 
[First iteration of Buzzhouse #71085
](https://github.com/ClickHouse/ClickHouse/pull/71085)

<iframe width="768" height="432" src="https://www.youtube.com/embed/CW4Ntdtp7lg?si=8zlyBIg8xzXiecCF" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

### ** rDNS Map: The Internet in Your Hands**

Reverse DNS (rDNS) is an often-overlooked but powerful tool for understanding the structure of the internet. This talk explored how we can **map out rDNS data** at scale and why it's useful for network monitoring, security research, and even performance optimization.

**Speaker:** Alexey Milovidov

**Abstract:** I've created an rDNS map, available at https://reversedns.space/, and I want to tell you how. It was not hard to do, but there was a lot of unusual and amusing stuff in the process.

<iframe width="768" height="432" src="https://www.youtube.com/embed/0hDOr9Pp1-4?si=ppV4bjsQUqxwz6b8" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

## Wrapping Up with a Community Dinner

Of course, no ClickHouse event is complete without some time with the community. After a full day of talks, we gathered with our community members for dinner and drinks in Brussels. It was a chance to meet users, discuss database internals, and share ideas over great food and drinks. If you were there, thanks for joining us! If not, we hope to see you at the next one. 

In the meantime, feel free to join us on [Slack](https://clickhouse.com/slack) or give the [ClickHouse repository](https://github.com/ClickHouse/clickhouse) a star.

## Some Photos

![FOSDEM_Robert.png](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Robert_7ae18824d8.png)

![FOSDEM_Pedro.png](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Pedro_281eabed62.png)

![FOSDEM_Alexey.png](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Alexey_f71f4433fb.png)

![FOSDEM_Room_Full.png](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Room_Full_42d27aab9b.png)

![FOSDEM_Dinner_Venue.jpg](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Dinner_Venue_2d60dbbefb.jpg)

![FOSDEM_Dinner_Full.jpg](https://cms.clickhouse-dev.com:1337/uploads/FOSDEM_Dinner_Full_bbae49afa6.jpg)


This year, ClickHouse made a strong showing at FOSDEM 2025, with several team members traveling to Belgium and multiple talks covering everything from powerful new JSON data types to the challenges of fuzzing databases.

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ClickHouse at FOSDEM 2025: talks, tech, and a community dinner

Today we are pleased to announce the availability of an integration allowing ClickHouse users to quickly, and simply, install and use Deepnote.


![deepnote-2.webp](/uploads/deepnote_2_3a74fdc5db.webp)

If you understand the impact already, then get started with the [documentation](https://clickhouse.com/docs/en/integrations/deepnote/). 

Our friends at Deepnote have also written an excellent, detailed, [blog post](https://deepnote.com/blog/clickhouse-cl4zs29ikocet0blrv08ishlj) on the topic.


## **What is Deepnote?**

So, what is it?

[Deepnote](https://www.deepnote.com/) is a collaborative data notebook for teams to discover and share insights. In addition to being Jupyter-compatible, it works in the cloud and provides one central place to collaborate and work on data science projects efficiently.

Deepnote provides first-class support for SQL. This means you get to query your ClickHouse database right from your notebook. Transitions between Python and SQL are seamless as there’s no need for a Python connector. With Deepnote, you get all the bells and whistles of a SQL editor right in your notebook, including formatting, autocomplete, and linting.


## **Getting Started**

If you want to explore an interactive example of querying ClickHouse from Deepnote data notebooks, click the button below to launch a template project connected to the ClickHouse playground.

[![launch-in-deepnote.svg](/uploads/launch_in_deepnote_b9ad65cb49.svg)](https://deepnote.com/launch?template=ClickHouse%20and%20Deepnotenk)

## **Sample Queries and a Challenge**

If you don’t have a dataset handy, the ClickHouse Playground is convenient for exploring ClickHouse using Deepnote. While multiple datasets are available, it is worth issuing a bit of a challenge of exploration.

First, look at the [GitHub Archive](https://ghe.clickhouse.tech/) data that we persist in Playground.

Second, explore the data after connecting your Deepnote notebook to the playground (sample queries are available at the link above).

Finally, [Tweet](https://twitter.com/clickhousedb), [Telegram](https://telegram.me/clickhouse_en), [Slack](https://join.slack.com/t/clickhousedb/shared_invite/zt-rxm3rdrk-lIUmhLC3V8WTaL0TGxsOmg), or [contact us](https://clickhouse.com/contact/) to let us know what you have created.

The best submission will receive a ClickHouse t-shirt!


## **Summary**

ClickHouse is the fastest OLAP database on the planet. But data is only as compelling, and valuable, as the insight you derive from it. We are delighted to partner to provide a new, exciting, and collaborative way of interacting with your data.


Today we are pleased to announce the availability of an integration allowing ClickHouse users to quickly, and simply, install and use Deepnote.

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ClickHouse + Deepnote: Data Notebooks & Collaborative Analytics

## We're meeting in person! Join us on March 9th 2023 in Amsterdam. [Register here](https://www.meetup.com/clickhouse-netherlands-user-group/events/291485868/).





**tl;dr – Join us virtually or June 8th in Amsterdam – [Watch Recording](https://www.youtube.com/watch?v=DP7l6Swkskw&ab_channel=ClickHouse)!**

Following the success of our day of virtual meetups, we’ve decided it is time to meet again…in person.

A significant portion of the ClickHouse team will be in Amsterdam. So we decided to take the opportunity to get together, talk tech, and share some food. Please [register now](https://www.meetup.com/clickhouse-netherlands-user-group/events/286017044/) so that we can ensure sufficient space and food. It’s not just an opportunity to network, though. You will hear stories of ClickHouse powering a performance & monitoring service and a detailed exploration of how, and why, ClickHouse is so fast.

We’ve identified a venue…

We’ve found speakers…

Our founding team will be in town…

All that is missing is you!


## **Why a Meetup?**

ClickHouse, as an open-source company, is rooted deeply in the importance of community. The opportunity to interact with our users, in physical space, is core to the identity of our team.

In addition, our history (as a company and individuals) is built on the power of conversation. Some of the stories we share, and memories we cherish, involve a small venue, lightning talks, and conversations that last until security reminds us that the booking period is over.

Our community matters. Our interaction with our community is core to who we are and who we aspire to be.


## **Agenda**

The current agenda is as follows:

17:45 Doors Open – Networking & Snacks

18:00 Welcome

18:05 Lorenzo Mangani, QXIP BV – 45 mins

18:50 Lightning Talks! (Will it be you???)

19:00 Coffee Break

19:15 Alexey Milovidov, ClickHouse – 45 mins

20:00 Beverages, Food, and Conversation

![pasted image 0.png](/uploads/pasted_image_0_33b308b535.png)

We will be gathering at the beautiful, and unique, Night venue in Venture Studio at the [Startup Village](https://www.startupvillage.nl/). A big thanks to the team there and Rabobank for hosting us.

If you have an idea for a Lightning Talk please let us know (tyler@clickhouse.com). We are finalizing them as we speak.


## **Speaker Bios:**

**Lorenzo Mangani** – CEO of Amsterdam based [QXIP BV](https://qxip.net/), Co-Founder of Uptrace & cLoki, directing a dream team specialized in active and passive large scale monitoring and ingestion of Traces, Logs, Packets and Metrics on top of ClickHouse

**Alexey Milovidov**, CTO & Co-Founder ClickHouse: Alexey Milovidov is the original creator, co-founder and CTO of ClickHouse. He started work on ClickHouse in 2009 and initiated its release to open-source in 2016.

It’s not just an opportunity to network, though. You will hear stories of ClickHouse powering a performance & monitoring service and a detailed exploration of how, and why, ClickHouse is so fast.

Tyler Hannan

Blog posts by Tyler Hannan

ClickHouse at FOSDEM 2025: talks, tech, and a community dinner

ClickHouse + Deepnote: Data Notebooks & Collaborative Analytics

Amsterdam Meetup With The ClickHouse Team – June 8th, 2022