Halkyon Learn

Halkyon Learn is for creating, sharing and collaborating on study material. It schedules flashcards with FSRS to strengthen retention, and organises study material together with the external references it draws on. It is offline-first: everything works on the device, without a connection. And the whole stack is transparent: the app, the server and everything needed to host an instance are published, so anyone can see how an instance works and run their own.

• Offline first. Each device keeps its own database and syncs in the background (serverpod_offline_sync). Edits made on two devices while offline are never lost: card content is stored as append-only revisions, and a side changed differently on two devices is shown as a conflict until the person picks a version (ADR 0001, ADR 0003).
• No sign-up needed. The app starts with an anonymous account; signing in with GitHub later keeps everything. A GitHub sign-in stores only the numeric GitHub ID.
• No end-to-end encryption. Content is readable by the operator of the instance, as on Mastodon; whoever trusts no operator can run their own (ADR 0002).

Targets: Linux, Windows, macOS, Android and iOS. There is no web build.

Packages

Package	What it is	Licence
halkyon_learn_server	The Serverpod server: endpoints, models, migrations	SSPL-1.0
halkyon_learn_client	The client, generated from the server's models	AGPL-3.0-or-later
halkyon_learn_flutter	The app (app.halkyon.learn)	AGPL-3.0-or-later

Setting up

• Flutter v3.47.5, the version CI uses (FLUTTER_VERSION in .forgejo/workflows/ci.yaml). Set up each platform you build for with Flutter's own guides: Linux, Windows, Android, macOS, iOS.

• The Serverpod CLI v4.0.4: dart install serverpod_cli@4.0.4. Pass --no-analytics to every call; otherwise it reports the commands run to Serverpod.

• Linux, on top of Flutter's packages, the libraries of four plugins: WebKitGTK (pulled in by the sign-in package), libsecret (keeps the session), GStreamer (plays sounds) and libmpv (plays videos from Jellyfin; the app doesn't start without it). On Debian or Ubuntu, Flutter's and these together:

  sudo apt-get install clang cmake ninja-build pkg-config libgtk-3-dev libstdc++-12-dev \
    libwebkit2gtk-4.1-dev libsecret-1-dev libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev \
    libmpv-dev
  

• Windows needs Developer Mode (Settings › System › For developers): building makes symbolic links, as Flutter links each plugin's native code into the build and the PDF engine's build does too, and Windows lets ordinary users make them only in Developer Mode.

• No Docker for development: serverpod start and the server's tests run an embedded PostgreSQL. Docker Compose is only for running an instance (below).

• For the scripts in tool/: Python 3 (tool/app_icons.py also needs Pillow, python3-pil), and for releases what Releases lists.

The rich-text editor comes from our fork of AppFlowy Editor (project-halkyon/appflowy-editor), which flutter pub get fetches by itself.

Running it locally

The server's secrets live in halkyon_learn_server/config/passwords.yaml, which is never committed. In a fresh clone, create it with a development and a test section, each holding database, serverSideSessionKeyHashPepper and serviceSecret (for example from openssl rand -base64 32). GitHub sign-in is optional: add githubClientId and githubClientSecret of your own GitHub OAuth app to development, registered with the callback URLs app.halkyon.learn://github-callback and http://127.0.0.1/github-callback.

Then start the server, its embedded database and the app:

cd halkyon_learn_server
serverpod --no-analytics start

serverpod start applies pending migrations and reloads on changes. The app talks to the server named by --dart-define=SERVER_URL=…, or else to the one in halkyon_learn_flutter/assets/config.json (http://localhost:8080). Without --no-analytics, the Serverpod CLI reports usage to Serverpod.

Tests

tool/check.sh

runs the format check, the analyzer with the strict lints of leancode_lint, the server's tests (against an embedded PostgreSQL, no Docker needed) and the app's tests. To run it before every push: ln -s ../../tool/check.sh .git/hooks/pre-push. CI runs it on every push too (.forgejo/workflows/ci.yaml).

The sync tests need a running server (serverpod start); run them in halkyon_learn_flutter with flutter test -t requires-server.

Running an instance

deploy/ holds a Docker Compose setup: Caddy (HTTPS), the server and PostgreSQL. Copy deploy/.env.example to deploy/.env and fill it in; it explains every value. The licences require offering the source of what the instance runs to the people who use it. Assistants such as Claude read people's studysets through the instance's MCP connector at /mcp (ADR 0006): webServer in halkyon_learn_server/config/production.yaml names its public address, as apiServer does for the app.

learn.halkyon.eu itself deploys from main once CI is green: deploy/redeploy.sh, run by the CI runner described in deploy/runner/, deploys only commits signed by a key in deploy/allowed-signers. It backs up its database every hour, off-site and encrypted; deploy/backups.md says how to set that up and how to restore.

Releases

Releases are published on code.halkyon.app, with the APK and the snap, and the APK also in the project's F-Droid repository (https://halkyon.app/fdroid/repo?fingerprint=E8082788756F6B7E682AA22FCEAD24B53E2A8B0912AD66D65E8C2D4F07BB94B0, added in the F-Droid app as a repository). The Snap Store (halkyon-learn) follows. Their builds offer learn.halkyon.eu, the project's instance, at first start; people may choose another there, and keep accounts on several.

• Checking a download: SHA256SUMS lists the checksum of every file, signed with the maintainer's GPG key (SHA256SUMS.asc). The APK is signed with the app's key, whose certificate has the SHA-256 27:E2:43:FC:BD:C8:26:FF:98:AB:7D:BB:10:43:6C:98:C3:BE:A2:4B:2A:01:5E:94:E4:CF:C7:D1:FC:2A:00:6C (apksigner verify --print-certs shows it).
• Versions follow semantic versioning: version in halkyon_learn_flutter/pubspec.yaml, <major>.<minor>.<patch>+<build>. Since the last release, a commit typed feat raises the minor number, fix or perf the patch number, and a breaking one (feat!: or a BREAKING CHANGE: line) the major number from 1.0 on, the minor one before; other commits release nothing by themselves. Every release raises the build number: Android only installs a higher one over an older, and the server tells apps below the build it supports to update.
• Making one is for maintainers, on their own machine and never in CI: tool/release.sh, or tool/release.sh minor (or major, patch) to choose. It needs main clean, pushed and green in CI, the app's key, the F-Droid repository's key and a Forgejo token; its header says more. It shows the version and its commits and asks first, then writes the version into the pubspec in a signed release commit with a signed tag, builds the APK and the snap (snap/snapcraft.yaml, built in LXD; its icon comes from tool/app_icons.py), checks both, signs the checksums, pushes, and publishes on code.halkyon.app and in the F-Droid repository; the Snap Store once it is switched on in the script. Besides Flutter and the Android SDK, it uses snapcraft with LXD, a JDK (keytool), GnuPG, curl, jq, unzip, squashfs-tools, rsync and fdroidserver. Release builds made without the app's key are signed with the debug key and can't update a released app.
• The F-Droid repository is kept by tool/fdroid_repo.sh (fdroidserver v2.4.5, e.g. pipx install fdroidserver==2.4.5); its settings and the app's listing are in tool/release/fdroid/.

More

• AGENTS.md: how the code is organised and the rules it follows.
• docs/adr/: the architecture decisions and their reasons.
• CONTRIBUTING.md: licences of contributions and the sign-off.
• LICENSE.md: which licence covers what.