How would you manage 30 million lines of PHP? You can't rewrite it - you'd be fired long before you delivered. That was the problem facing Facebook around 2011, and their answer was to add a type system, giving birth to the Hack language. But adding types to PHP turned out to be the easy part. The hard part was building a type checker that developers used to "edit, save, refresh" would actually tolerate. So how do you type-check tens of millions of lines of code fast enough that nobody notices?

Julien Verlaguet co-designed Hack at Facebook, and has spent the fifteen years since chasing a better way to build incremental systems. We start with the internals of Hack's language server - forked OCaml workers, a shared immutable heap, lock-free dependency tracking, and the months spent hunting non-deterministic bugs. From there we follow the thread to Skip, a language designed around tracking mutability in the type system, with stops at why Haskell's monads and "reactive Java" didn't fit, functors vs type classes, and Skip's bang operator for working with immutable objects. Then it's on to the Skip reactive framework: eager vs lazy collections, a garbage collector without long pauses, a reactive SQL database that didn't find a market, TypeScript bindings, and Skipper, an AI coding agent built on a sound version of TypeScript.

If you're interested in compilers, language design, or building real-time systems that stay fast as they grow, Julien has plenty of hard-won lessons to share.

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SkipLabs: https://skiplabs.io/

Skip Framework Docs: https://skiplabs.io/docs

Skip on GitHub: https://github.com/SkipLabs/skip

Skiplang: https://skiplang.com/

Skipper Launch Announcement: https://skiplabs.io/blog/press_release

Hack: https://hacklang.org/

HHVM: https://hhvm.com/

OCaml: https://ocaml.org/

OCaml Functors: https://ocaml.org/docs/functors

Haskell: https://www.haskell.org/

Clojure: https://clojure.org/

Scala Implicit Parameters: https://docs.scala-lang.org/tour/implicit-parameters.html

Salsa (Incremental Computation for Rust): https://github.com/salsa-rs/salsa

ReactiveX: https://reactivex.io/

SQLite: https://www.sqlite.org/

TPC-H Benchmark: https://www.tpc.org/tpch/

PostgreSQL: https://www.postgresql.org/

PostgreSQL NOTIFY: https://www.postgresql.org/docs/current/sql-notify.html

Server-Sent Events: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events

JavaScript Proxy Objects: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

Doxygen: https://www.doxygen.nl/

SKDB and Reactive Databases (Materialized View): https://materializedview.io/p/skdb-reactive-databases-julien-verlaguet

Julien on LinkedIn: https://www.linkedin.com/in/julien-verlaguet-b5710a20/

Julien on X: https://x.com/JulienVerlaguet

Kris on Bluesky: https://bsky.app/profile/krisajenkins.bsky.social

Kris on Mastodon: http://mastodon.social/@krisajenkins

Kris on LinkedIn: https://www.linkedin.com/in/krisjenkins/

Building something people actually want is supposed to be the happy ending. But it arrives with a bill attached: feature requests you didn't ask for, pull requests you'd rather not maintain forever, users demanding the one thing you swore you'd never build, and — if you're unlucky — a company where the sales team quietly starts deciding what engineering works on. DuckDB has spent the last two and a half years working through that list. So how do you stay a database engineering team when success keeps trying to turn you into something else?

Hannes Mühleisen, co-creator of DuckDB, is back to talk through the answers they've landed on. Their fix for unwanted pull requests was an extension mechanism, which then forced them to make every part of the engine pluggable — including the parser, which meant ripping out 20,000 lines of Postgres' yacc grammar and rewriting SQL parsing on top of PEG. Their fix for the users demanding client-server was Quack, a protocol designed by people who'd already published a paper on why every existing database wire protocol is wrong. And their answer to Apache Iceberg, after three years of implementing it, was DuckLake: throw out the Avro-and-JSON metadata files and keep the metadata in a database, on the grounds that the Iceberg REST catalog has a Postgres in it anyway.

Which brings us to the news Hannes breaks in this episode: DuckDB Labs is being acquired by AWS, while the DuckDB Foundation, the project and its licence stay where they are. There's the question of why a profitable, self-funded, 30-person company in Amsterdam would take that deal, what commitments you write into the contracts when you're worried today's promises might outlive today's management, and what it's actually like to have a boss again after five years without one. If you're curious how an open source project keeps its technical soul once the enterprise arrives — or you just want to know why parsing SQL is harder than parsing almost anything else — Hannes has some good answers.

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Our previous episode with Hannes: https://youtu.be/pZV9FvdKmLc

DuckDB: https://duckdb.org/

DuckDB Foundation: https://duckdb.foundation/

DuckLabs (formerly DuckDB Labs): https://ducklabs.com/

DuckLake: https://ducklake.select/

Quack (DuckDB's client-server protocol): https://duckdb.org/quack/

DuckDB v2.0: Your Database Deserves a Better Parser: https://duckdb.org/2026/08/20/duckdb-20-peg-parser

Runtime-Extensible Parsers (CIDR 2025 paper): https://duckdb.org/pdf/CIDR2025-muehleisen-raasveldt-extensible-parsers.pdf

Don't Hold My Data Hostage (VLDB 2017 paper): https://www.vldb.org/pvldb/vol10/p1022-muehleisen.pdf

cpp-peglib: https://github.com/yhirose/cpp-peglib

GNU Bison: https://www.gnu.org/software/bison/

PEP 617 – New PEG parser for CPython: https://peps.python.org/pep-0617/

PRQL: https://prql-lang.org/

Apache Iceberg: https://iceberg.apache.org/

CWI (Centrum Wiskunde & Informatica): https://www.cwi.nl/en/

DuckCon #7, Amsterdam: https://duckdb.org/events/2026/06/24/duckcon7/

Kris on Bluesky: https://bsky.app/profile/krisajenkins.bsky.social

Kris on Mastodon: http://mastodon.social/@krisajenkins

Kris on LinkedIn: https://www.linkedin.com/in/krisjenkins/

My guest this week is James Brown, an engineering lead at Schroders, and that's a useful vantage point — asset managers carry all the regulation and organisational weight of a bank, mixed with the first-to-market pressure of a startup. James starts with his own "Claude mania": months of agents running around the clock, and the wave of anxiety that hit him one morning walking to the shop without one running at home. From there, Clair — the plugin he's building to give coding agents proximal awareness of each other, using git orphan branches as a zero-infrastructure message bus; why AI behaves like oxygen in a room full of tiny fires; what "going well" actually measures inside a regulated firm; and why he thinks team sizes won't change, even when the number of teams does.

There's a darker thread running under all of it. The collapse in junior hiring, the advice we no longer know how to give a 20-year-old, dark factories and evolutionary harnesses that might make the AI's ideas better than ours, and the burnout James expects to be our dominant topic for the next couple of years. If AI is working beautifully on your side projects but landing with a thud at work, James has some honest answers — including several about what he doesn't know yet.

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Clair (James's multi-agent proximity plugin): https://github.com/JBJamesBrownJB/clair

Clair product docs: https://github.com/JBJamesBrownJB/clair/blob/main/docs/product.md

James's blog: https://medium.com/@jameskinnahbrown

"Milk, Eggs and Claude Mania": https://medium.com/@jameskinnahbrown/milk-eggs-and-claude-mania-49f445c5a77e

Schroders: https://www.schroders.com/

Claude Code: https://www.claude.com/product/claude-code

Agent Skills & progressive disclosure: https://platform.claude.com/docs/en/agents-and-tools/agent-skills/overview

Git orphan branches (git checkout --orphan): https://git-scm.com/docs/git-checkout

Moltbook: https://www.moltbook.com/

Team Topologies: https://teamtopologies.com/

"Expert Panel: How Far Can We Accelerate with AI?": https://youtu.be/Bg7L4vmmSKg

XT26 (the conference the panel was part of): https://www.juxt.pro/xt26/

Kris on Bluesky: https://bsky.app/profile/krisajenkins.bsky.social

Kris on Mastodon: http://mastodon.social/@krisajenkins

Kris on LinkedIn: https://www.linkedin.com/in/krisjenkins/