RustNotMC/docs/server.md

Hosting a Horizon server: design

Horizon's dedicated server should be the easiest and lightest Minecraft 26.3 server to host. It should be one small native program with no Java and no Python, one command to install it, one config file, and a CLI, a web panel and a control API to manage it. At the same view and simulation distance and with the same players, it should beat vanilla, Paper and Fabric + Lithium on TPS/MSPT, RAM, CPU, chunk generation and start time. Gameplay stays vanilla: there are no Paper-style rule changes unless the owner turns one on.

This page covers what the server does today (measured), the design of horizon-server, and what ships for the first working version (2026-10-23). The owner's guide is docs/hosting.md, also at https://playhorizon.dev/server.

1. Today

How it runs

The dedicated server is the game itself, run headless: horizon-client --resources <prepared> --dedicated [--nogui] [--port N] [--world NAME] [--universe DIR] [--initSettings], started in the server folder (crates/horizon-client/src/game/dedicated.rs).

• Startup. It follows vanilla's Main.main and DedicatedServer.initServer. It reads server.properties and writes it straight back with vanilla's defaults filled in (horizon_client::server, byte-exact with vanilla's Properties.store), and refuses to start until eula.txt says eula=true. It then loads the operator, whitelist and ban lists in vanilla's order and format, and opens <universe>/<level-name> with the same Game as single player, minus the local player.
• Network. Players join through the play-state hub that Open to LAN uses (crates/horizon-client/src/lan). Status ping, legacy ping, compression, login gate (bans, whitelist, IP bans), operator permission levels, the player limit with bypassesPlayerLimit, MOTD and server-icon.png all follow server.properties.
• Main loop. It ticks at 20 TPS on one thread, with chunk generation and lighting on the worker pool. Chunks stay loaded around every player (lan_update_streaming), or around the spawn while nobody is online. It supports pause-when-empty-seconds, player-idle-timeout and the autosave.
• Console (stdin). stop, save-all, save-on/save-off, list [uuids], op/deop, whitelist on|off|list|reload|add|remove, ban, ban-ip, pardon, pardon-ip, banlist, kick and say behave as in DedicatedServer. Every other command goes to the game's own dispatcher as the server (permission level 4). The log follows vanilla's Log4j pattern, in logs/latest.log and on stdout.
• Player names. usercache.json in the universe folder works as vanilla's CachedUserNameToIdResolver. Players are added when they join and kept for a month, and the file keeps the 1000 most recently used, newest first. The console finds a player by name in it, in any case. A name it doesn't hold is asked of the profile service (getByName). If the service doesn't know the name, an offline server uses the offline player of the lower-case name; an online server answers "That player does not exist". tools/test_user_cache_parity.py checks this against vanilla, with a mock profile service.

Measured (2026-10-16, Mac mini M4 Pro, main at bd8dff20, release build)

The server ran in a scratch folder with online-mode=false, level-seed=12345 and view and simulation distance 10. The vanilla server was the pinned 26.3 server.jar on Java 25 with -Xmx4G.

	Horizon (horizon-client --dedicated)	Vanilla 26.3
Start, new world (wall clock, to Done)	11.4 s (Done (9.762s))	12.5 s (Done (2.066s), includes the bundler's first unpack)
Start, existing world (wall clock)	5.8 s (resources load in 1.6 s)	10.7 s (Done (0.446s): the JVM and DataFixerUpper take the rest)
RAM, idle with nobody online (RSS)	1.40–1.51 GB	0.93–0.97 GB
CPU, idle with nobody online	1.2 % of one core	0.6 %
stop to exit	0.6 s	0.6 s
What has to be installed	one 46.8 MB binary + 589 MB of prepared resources	Java 25 + the 62 MB jar
Crates linked	384 (wgpu, winit, cpal, gilrs, arboard, naga…)	n/a

On Linux the full game loads libasound.so.2 (cpal) and libudev.so.1 (gilrs) at startup. wgpu, winit and the Wayland/X11 libraries are loaded only when a window opens. The server build (horizon-engine, section 3) is the same program built without its audio and controllers features, so it needs neither library, and a minimal server image runs it as it is.

The two clear losses are RAM and idle CPU. The dedicated server loads everything the client needs to draw (the HUD atlas, item renderer, block-entity models, panorama, fog and music tables, entity models), and it builds the meshing tables at startup. None of these are used without a window. Loading them is startup work, not tick cost, so it is in scope here (see section 9).

Joining it

• Horizon client: works. A headless Horizon client (--server 127.0.0.1:PORT, the harness mode) logged in, received its 49 chunks at render distance 2, chatted, saw /list, and left. The server's log matched vanilla's lines (logged in with entity id, joined the game, [Not Secure] <Tester> …, lost connection: Disconnected, left the game).
• Vanilla 26.3 client: checked by tools/test_dedicated_server_parity.py. VanillaProtocolClient, a headless client built from vanilla's own codecs, pings and joins Horizon's server and vanilla's from the same world copy. It compares every login, configuration and play packet up to the first chunk batch, the chunk bytes, chat, /list, failing commands, the console lines and the files left behind. Result on main: see "Harnesses on main" below.
• The real vanilla 26.3 client (tools/vanilla_client.py, from the owner's own files, dev-only) joins too. On 2026-10-18 it joined the server build of the game (horizon-engine, section 3) on the Mac mini: python3 tools/vanilla_client.py --name Vanilla --join localhost:PORT. It logged in, drew the terrain around spawn and stayed connected for four minutes, with no warning or error on the server. The tool runs on Linux under vscreen and on macOS from the logged-in screen session, even a locked one, with vsync off.

What fails or is missing

Area	Status
online-mode=true	Works (ServerLoginPacketListenerImpl): a 1024-bit RSA key pair at start ("Generating keypair"), the encryption request, the shared secret and challenge, AES/CFB8, and the session service's hasJoined check (with the address when prevent-proxy-connections is on), with vanilla's log lines and messages. test_online_mode_parity.py runs the same logins against vanilla's server and Horizon's with a mock session service.
Secure chat	Works (SignedMessageChain, LastSeenMessagesValidator, MessageSignatureCache): the chat session update and its profile key, checked against the services' playerCertificateKeys; signed chat, say, me and msg, relayed with their signatures and last-seen lists; enforce-secure-profile (with online-mode) refuses unsigned chat; and vanilla's log lines ([Not Secure], the decoder's warnings, the kicks). Offline servers and players without a chat session chat unsigned, as on vanilla. test_secure_chat_parity.py runs the same signed and unsigned players against vanilla's server and Horizon's, with test keys and a mock services server.
Nether and End	Works: the server holds a level per dimension, and each player is in its own (portals, execute in … tp, respawn). test_server_dimension_players_parity.py checks it against vanilla. How it works, its cost and what's left: server-levels.md.
Refused logins	Logged as vanilla does: Disconnecting <name> (/address): <reason>, then <name> (/address) lost connection: <reason>, and the session errors (Username '<name>' tried to join with an invalid session, Couldn't verify username because servers are unavailable).
Gameplay for remote players	Checked against vanilla's server with a vanilla-codec client (#484, tools/test_dedicated_server_parity.py): a right click with an item on a block uses it once, a dead player's commands run, players on a team without friendly fire don't hurt each other, and a player's projectiles are theirs (their ender pearl takes them). The recipe book, status effects and block sync while digging (#500) work. Left: a player's projectiles don't hit other players (no bow PvP between guests), and a mob a guest kills counts in the host's statistics and advancements, not the guest's (mob_died runs as the host).
Defaults that surprise owners	white-list=true is vanilla 26.3's default, so a fresh server refuses everyone until they are added. That matches vanilla, so the first-run flow (section 3) asks about the whitelist up front.
Management	Only stdin. There is no service, no backups, no status beyond list, no restart after a crash, and a server can't be managed while it runs in the background.
Packaging	The server is the full game client. Resources come from horizon-prepare, which needs a JDK and the official launcher's client install, both awkward on a headless VPS.
Tick times	The dedicated loop never recorded its tick times in the game clock (only the windowed loop did), so /tick query on a dedicated server reported 0 ms. Fixed in PR 2b, which the benchmark needs (section 9).
Scale	Measured against vanilla on equal work (section 9, benchmarks/server-equal-work-2026-10-17/): 20 TPS up to 50 bots, 9 to 11 TPS at 100, where vanilla's watchdog stops it. Horizon is ahead with few players and when exploring; with 25 to 50 players near spawn vanilla's tick is shorter.

Harnesses on main

On the tip of the code PRs (2a to 3, rebased on b4ad6538), tools/parity.py dedicated_server online_mode server_files server_player_combat server_player_damage passed all five. That includes the vanilla protocol client joining Horizon's server and vanilla's from the same world. The full --changed origin/main set (309 harnesses, since the PRs touch the client crate) had passed 60 of 60 when it was stopped for disk space; CI runs it on every pull request.

2. Architecture: a supervisor and an engine

 owner ──► horizon-server (CLI, TUI)          browser ──► web panel           AI ──► MCP (stdio)
                 │                                │                               │
                 └────────── control API: HTTP + JSON on 127.0.0.1, token ────────┘
                                                  │
                                   horizon-server run   (the supervisor: no GPU, no game code)
                  server.toml ──► server.properties ──► │ ◄── stdout: the vanilla log
                  backups, updates, crash restart       │ ──► stdin: console commands
                                                        │ ◄─► engine endpoint: status, TPS/MSPT, players
                                                        ▼
                                   the engine: horizon-client --dedicated   (the game, headless)

horizon-server is a new crate and binary (crates/horizon-server). It is the only program an owner runs. It depends only on the light crates (horizon-core, horizon-protocol, horizon-world, horizon-bench) plus serde, toml, toml_edit and zip, which are already in the lockfile. It doesn't link wgpu, winit, audio or controller crates. It runs the engine as a child process.

The supervisor/engine split stays even after the engine becomes headless, for these reasons:

• Crash restart and diagnostics. The supervisor outlives an engine crash. It keeps the crash report, restarts the engine with backoff, and the panel stays up to say what happened.
• Safe updates. An update swaps the engine binary and restarts. If the new engine fails its start check, the supervisor rolls back to the previous one.
• Parity stays cheap. The engine keeps reading vanilla's own files (server.properties, the four JSON lists, eula.txt) exactly as test_dedicated_server_parity.py checks them today. The clean config is a layer on top and never changes what the engine sees.
• No conflicts. The supervisor touches none of the game code that the tick and integrated-server work (perf-server-1, perf-ticks-2) are changing.

The engine today is horizon-client --dedicated. It needs one small, additive change: a status endpoint. With --control-socket <dir>/.horizon/engine.json (the existing control protocol: TCP on 127.0.0.1 plus a token, on every OS), the dedicated loop answers server-status with TPS, MSPT (average, P50/P95/P99 from the tick clock /tick query uses), players (name, UUID, address, ping), loaded chunks and uptime. It answers console <line> with the command's output.

The headless engine comes after perf-ticks-2's work lands: Resources split into shared and per-game, the game built in Game::new, the integrated server on its own thread. On that base:

1. The dedicated path skips everything only a window uses: the HUD atlas, item renderer, block-entity and entity models, panorama, fog, music, and mesh tables (perf-ticks-2 already stops server-side meshing). This is the RAM and start-time item.
2. The game moves into a library crate, with the window parts behind a client feature, so horizon-engine links no wgpu or winit either. Sound and controllers are already optional (the audio and controllers features, on by default): horizon-engine today is the game built without them (section 3).

3. Packaging

Binaries. The release archives hold horizon-client, horizon-ctl, horizon-prepare and horizon-server. On Linux the release also has horizon-server-linux-x86_64.tar.gz: horizon-server, horizon-prepare and horizon-engine, the game built with --no-default-features (no sound, no controllers), which links neither libasound.so.2 nor libudev.so.1. release.yml builds it after the client archive (xtask package-server) and fails if tools/check_imports.py --server finds either library; CI lints the same build. horizon-server runs horizon-engine when it is beside it or on the PATH, else horizon-client. Build it yourself with cargo build --release -p horizon-client --no-default-features and copy target/release/horizon-client as horizon-engine. macOS and Windows have no such libraries to avoid, so they keep the one archive.

Game files: the owner's own copy, as with prepare. Nothing of Mojang's is redistributed and nothing is downloaded on the owner's behalf. horizon-server looks for prepared resources in this order:

1. [engine] resources in server.toml;
2. resources/ in the server folder;
3. .local/horizon-client/26.3 beside the engine or up to four folders above it: a release folder, the installer's game-files (below), or a checkout.

If none is found, init says where it looked. Today the owner runs horizon-prepare --prepare --server-jar PATH from the release folder (it needs a JDK and the client files, as for playing), or copies a folder prepared on their own computer to the server (scp -r). Planned wrappers:

• On this machine: horizon-server prepare --server-jar PATH runs horizon-prepare on the owner's own server jar plus their launcher install.
• Prepared elsewhere: horizon-server import-resources DIR copies a folder prepared on the owner's own computer, for example by the launcher, to a headless machine.

A server-only prepare (from server.jar alone, without the client jar or a JDK) is the follow-up that removes the last Java step. It needs a list of which prepared files the server really reads, and that list comes out of the headless-engine work.

One-command install (packaging/server/):

• Linux and macOS: curl -fsSL https://raw.githubusercontent.com/horizon-rust/horizon/main/packaging/server/install.sh | sh The script picks the release archive for the OS and architecture (on Linux x86_64 the server archive, on macOS arm64 the game's), downloads it from GitHub Releases, and checks it against the SHA-256 GitHub publishes for each asset. It never asks for sudo. Options: --version vX.Y.Z, --dir (default ~/.local/share/horizon), --bin (default ~/.local/bin), and --archive FILE to install an archive already on the machine (a build of your own; no download and no checksum). A Linux release from before the server archive installs the full game, which links ALSA's and udev's libraries (libasound.so.2, libudev.so.1); minimal server images often lack them (Debian and Ubuntu: sudo apt install libasound2 libudev1, libasound2t64 on Ubuntu 24.04 and later). Then the script lists any that are missing, init warns, and run stops at once with what to install, rather than restarting a game that can't load.
• Windows: irm https://raw.githubusercontent.com/horizon-rust/horizon/main/packaging/server/install.ps1 | iex The script does the same into %LOCALAPPDATA%\Horizon (-Version, -Dir), with junctions instead of links (no admin rights), and adds bin to the user's PATH.
• Layout. Each release goes to <dir>/releases/<tag> and <dir>/current points at the one in use, so an update or a rollback is one link. <bin>/horizon-server and <bin>/horizon-prepare are two-line launchers that run current. The prepared game files live in <dir>/game-files/horizon-client and each release's .local/horizon-client links there, so updates keep them. The script then prints the next steps (horizon-prepare, horizon-server init).

Docker (packaging/server/Dockerfile, built the same way as the relay's image): a multi-stage build that ends on debian:bookworm-slim as user horizon, with the server build as horizon-engine, so the image needs no ALSA or udev. It uses volume /data for the server folder, takes the owner's prepared game files as a read-only mount on /opt/horizon/.local/horizon-client/26.3, exposes port 25565/tcp, and keeps the control API (and later the panel) on 127.0.0.1 inside the container: docker exec <name> horizon-server status|cmd|backup. On the first start, HORIZON_EULA=true and HORIZON_WORLD, HORIZON_SEED, HORIZON_PORT, HORIZON_WHITELIST, HORIZON_PLAYERS and HORIZON_OP answer init's questions; without the EULA answer the container exits and says why. docker stop sends SIGTERM, which saves and stops cleanly (give it --stop-timeout 180). The image is published only with Ludovic's OK (it's hosted infrastructure).

Services.

• horizon-server service install writes and enables a systemd unit (--user, or a system unit with --system) on Linux, a launchd agent (~/Library/LaunchAgents/dev.horizon.server.<name>.plist) on macOS, or a Scheduled Task at logon on Windows (a real Windows service needs a wrapper and comes later).
• Each runs horizon-server run --dir <folder> through the installer's launcher, so an update applies at the next restart. The supervisor restarts the engine itself, and the service manager restarts the supervisor. service uninstall stops and removes it and leaves the server folder alone.
• service print shows the unit or agent it would write, for owners who'd rather install it by hand.

First run (horizon-server init, interactive on a terminal):

1. EULA. It shows the link to https://aka.ms/MinecraftEULA and asks "Do you agree? [y/N]". Only a yes writes eula=true.
2. World. It asks for the world name (world) and an optional seed. An existing world folder, or a vanilla server folder, is detected and kept: its server.properties is imported (section 4).
3. Port. 25565 by default, checked free right away.
4. Players. It asks whether to use a whitelist (yes by default, as vanilla) and who to add first, then offers to make the first player an operator.
5. Game files. It finds or explains the prepared resources (see above).
6. It writes server.toml, shows the panel address and the commands to start it in the background (horizon-server start) or as a service.

Every question has a flag (--eula, --world, --seed, --port, --whitelist/--no-whitelist, --player NAME, --op NAME, --yes) and an environment variable for Docker (HORIZON_EULA, …; flags win).

4. One config file: server.toml

The owner edits only server.toml. The supervisor writes server.properties from it before every engine start, so the engine stays exactly vanilla. The file is short: typed sections for what owners change, and a [properties] table that passes any other vanilla key through unchanged.

# Horizon server settings (docs/server.md). The server writes them into server.properties before
# every start: edit this file, not that one.

[server]
motd = "A Minecraft Server"
port = 25565
bind = ""                    # listen on every address, or only on this one (server-ip)
max_players = 20
online_mode = true           # check players' Minecraft accounts (false: anyone joins under any name)
whitelist = true             # only whitelisted players and operators can join
enforce_whitelist = false    # kick online players taken off the whitelist
view_distance = 10
simulation_distance = 10

[world]
name = "world"               # the world folder
seed = ""                    # empty: a random seed on the first start
game_mode = "survival"       # survival, creative, adventure or spectator
difficulty = "easy"          # peaceful, easy, normal or hard
hardcore = false

# Any other server.properties key, spelled as vanilla spells it, for example:
# spawn-protection = 16
[properties]

[engine]
path = ""                    # the game program; empty: the one installed beside horizon-server
resources = ""               # the prepared game files; empty: resources/ here, else the usual places
restart_on_crash = true

[backups]
every = "6h"                 # automatic backups while the server runs: 30m, 6h, 1d...; "0" turns them off
keep = 12                    # automatic backups kept (backups made by hand are never deleted)

[updates]
keep = 1                     # other releases kept by updates, besides the one in use and the one rollback goes to

This is the file init writes today (PR 4 adds [panel] bind = "127.0.0.1:8130" and allow_public = false, section 6). The defaults are vanilla 26.3's (a test checks each one). Unknown keys are errors, as are a typed key set again under [properties] and values vanilla would refuse. Planned: backups.before_update (section 7).

• Import. horizon-server init in a folder with a server.properties, or horizon-server import server.properties, reads it with vanilla's own parser (ServerProperties). Keys with typed fields go into their sections; any other key that differs from vanilla's default goes into [properties]. Keys left at their defaults are dropped, so the TOML stays short. The original file is kept as server.properties.imported.
• Changes made in game. The engine writes server.properties itself, for example after /whitelist on. After it stops, the supervisor compares the file with what it wrote and copies any changed keys back into server.toml, so an in-game change survives the next start. Edits go through toml_edit, so the owner's comments and order are kept.
• Changes without opening the file. horizon-server config set KEY=VALUE... (the launcher's Servers page uses it) writes through toml_edit, checks the result as server.toml is checked, and writes nothing if a value is wrong. On a running server it sends what vanilla changes live through its console: difficulty, defaultgamemode and whitelist on|off; the backup settings apply at once because the supervisor reads them each time. Everything else waits for the next start, and the command says which. A real hot reload (view distance, MOTD) needs engine support and comes later.
• server.properties, ops.json, whitelist.json, banned-*.json and world/ keep vanilla's names and formats. A vanilla server folder works as is, and so does going back to vanilla.

5. CLI and TUI

horizon-server init                      first run (section 3)
horizon-server run                       supervisor + engine in the foreground; the console on stdin
horizon-server start | stop | restart    in the background (detached `run`); each waits for the game
horizon-server status [--json]           up?, uptime, TPS, MSPT, players, RAM, CPU, crashes
horizon-server cmd <command…>            one console command, with its output
horizon-server logs [-f] [-n N]
horizon-server op|deop <player>
horizon-server whitelist on|off|list|add|remove [player]
horizon-server backup [NAME]             backups             restore <backup> [--yes]
horizon-server import <server.properties>
horizon-server config [--json]           config set KEY=VALUE... [--json]
horizon-server service install|uninstall|print [--system]
horizon-server panel                     the panel's address and a one-time login link
horizon-server update [--check] [--version vX.Y.Z] [--timeout S]     rollback [--timeout S]
horizon-server update --archive FILE --version NAME     an archive you have (your own build)

Planned:
horizon-server console                   attach: the live log, and send commands (Ctrl-D detaches)
horizon-server ban|pardon <player> [reason]      ban-ip|pardon-ip <address>   (`cmd ban …` works now)
horizon-server import-resources <dir>      prepare --server-jar PATH
horizon-server mcp                       MCP server on stdio (section 8)

• Every command takes --dir (the server folder; default: the current folder).
• While the server runs, management commands go through the control API, so the engine applies them as vanilla's console would and its lists stay right. While it's stopped, they edit the vanilla files directly with the same code the engine uses (horizon_server_files::user_lists).
• The TUI is horizon-server console: the log, a one-line header (TPS, MSPT, players, RAM) that updates every second, and a command line. It uses plain ANSI with no TUI framework, and falls back to plain lines when stdout isn't a terminal.

6. Web panel

Scope (MVP, PR 4): status (phase, uptime, version, crashes), live TPS, MSPT, RAM and CPU graphs (the last 10 minutes, sampled once a second), players (list with ping, op, kick), the console (live log plus a command line), and backups (list, back up now, restore with confirmation), with restart and stop. The panel lives in the supervisor, so it is up exactly when the server is: starting a stopped server stays a command (or the service manager's job). Whitelist and bans go through the console for now.

Updates: when a newer release is out, the panel says so with an Update button, which runs the same path as horizon-server update (section 7) and then shows how it went.

Later: bans and IP bans, worlds, server.properties import, logs and crash reports, and modpacks (with the modpack work).

Tech: one HTML page with plain JS and CSS (crates/horizon-server/panel/), embedded in the binary with include_str!. No build step, no framework, no CDN. It uses Horizon's design system tokens (horizon-theme's colours, light and dark) and draws charts on a canvas. It polls the control API's routes once a second (status, metrics?since=, logs?since=), served by the same small HTTP code on std::net, with no async runtime or new dependency. Server-sent events can replace the polling later if a tab ever costs too much.

Auth: local or tailnet only, never an open admin port.

• Bind. The panel listens on 127.0.0.1:8130 by default. panel.bind can name a tailnet address (100.64.0.0/10 or Tailscale's fd7a:115c:a1e0::/48) or another loopback address. Any other address, such as 0.0.0.0 or a public IP, is refused unless panel.allow_public = true is set, and even then the panel logs a warning at every start.
• Who may act. The API token in .horizon/api.json (file mode 0600) is what the CLI and MCP use. Browsers get a one-time login link from horizon-server panel: a code good for 10 minutes, swapped for an HttpOnly; SameSite=Strict session cookie. Anyone who can read the server folder can manage the server, and no one else can. Sessions end when the supervisor stops.
• Protection. The Host header must name the panel itself: its address, localhost on loopback, or a tailnet name (a MagicDNS short name or *.ts.net). That stops DNS rebinding. Changes are POST only and need the X-Horizon: 1 header, which stops CSRF. After 10 failed logins in a minute, logins are refused for a while. The page is served with a strict Content-Security-Policy (nothing from elsewhere) and can't be framed.
• Remote access goes over Tailscale/SSH (ssh -L 8130:127.0.0.1:8130 host), with no TLS or passwords to manage. HTTPS comes for free with tailscale serve, and the doc shows how to set it up.

7. Backups, updates and crash restart

• Backups. The supervisor runs save-off, then save-all flush, then zips the world plus the vanilla files and server.toml into backups/<UTC time>-<name>.zip, then runs save-on. Players stay online. Zip works for everyone, Windows included. It runs on the every schedule and on demand (later also before an update). It keeps the last keep automatic backups and never deletes one made by hand. A backup is written to a temporary name and renamed when complete, so a crash never leaves a half-written backup that looks finished. A stopped server is backed up straight from its files.

• Restore. restore NAME takes a file name, a unique prefix, or the name given to backup (the newest with it). The server stops. The current world is moved to backups/replaced-<time>/ (never deleted), the backup is unpacked into a temporary folder and checked (level.dat is there), then moved in, and the server starts again if it was running. One command, or one click with confirmation in the panel.

• Crash restart. If the engine exits without being asked to stop, the supervisor keeps the last 200 log lines and the engine's crash report (crash-reports/), restarts it after 5 s, and doubles the wait up to 5 minutes. It gives up after 5 crashes in 10 minutes, so it doesn't loop on a broken world, and shows the reason in status and the panel.

• Updates (crates/horizon-server/src/update.rs), for a horizon-server installed by install.sh or install.ps1. Releases ship often (release-flow.md), so updates stay opt-in: a running server looks for a newer release a minute after it starts and then every 6 hours, and only says so, in its log, in status and in the panel (HORIZON_NO_UPDATE_CHECK=1 turns the check off). update --check asks GitHub once. update:

  1. downloads the newest stable release's archive for this computer (on Linux the server build; --version picks another, a beta included) and checks it against the release's SHA256SUMS, whose Ed25519 signature must check with a key built into horizon-server (docs/release-flow.md, "Signed checksums"): an unsigned or tampered release is refused;
  2. unpacks it into <dir>/releases/<tag>, beside the others, with its game files linked to <dir>/game-files as the installers do;
  3. stops the game, backs up the world (backups/<time>-before-<tag>.zip), records the release in use in <dir>/previous and points <dir>/current at the new one;
  4. checks that the new game loads on this system (--help), then starts it;
  5. waits until it has logged Done and answered a status request, for 300 s (--timeout).

  If the new game doesn't load, stops, or isn't up in time, the supervisor goes back by itself: current points at the release before, previous is left as it was, the world is put back from the backup (the one the new game left is kept in backups/replaced-<time>/), and the game starts again on the old release. rollback switches to previous by hand, through the same path (the world is backed up first and kept as it is). The supervisor keeps running as it was: the new horizon-server itself takes over the next time the server starts (stop and start, or the service's restart). A stopped server is switched and its new game checked to load; it starts on the new release next time. One install serves every server folder on the machine, so each one runs the new release from its next start. Once the new game is up (or a stopped server is switched), old releases are deleted from <dir>/releases/: the one in use, the one previous names (where rollback goes), the one the running horizon-server itself came from, and the updates.keep newest of the others (1 by default) stay. A release that can't be deleted (a program still running from it on Windows) is left for the next update.

8. Control API and MCP

• Control API. The control API is the panel's HTTP API, documented as stable (/api/v1/...). The supervisor serves it on 127.0.0.1 at a port of the system's choosing, written with the token to .horizon/api.json. It's plain HTTP/1.1 on std::net (crates/horizon-server/src/http.rs), no runtime.
  • built (PR 2b, 2c): GET status (phase, uptime, crashes, RSS, CPU, and the engine's TPS, MSPT average/P50/P95/P99, players, chunks), GET logs?since=, POST command {line} (returns the output), POST stop, POST restart, GET backups, POST backups {name}, POST restore {name}, POST update {tag, timeout} (without a tag: the newest release, downloaded first; status has an update object that says how it went);
  • planned: GET metrics?since= (the panel's graphs), GET events (SSE: log lines and metrics), GET/PATCH config (server.toml fields, validated).
• Auth. Every call needs Authorization: Bearer <token from .horizon/api.json> (file mode 0600), and the Host header must be the loopback address it listens on.
• MCP. horizon-server mcp is an MCP server on stdio, for Claude Code or any MCP client run as the same OS user. It reads the token itself, and its tools map one to one onto the API: status, players, run_command, logs, backup, list_backups, restore (needs confirm: true), start, stop, restart, get_config and set_config. It's the same idea as the game's horizon-ctl (docs/agent-control.md): an AI can run and diagnose a server, and every action it takes shows in the log.
• Vanilla's own management protocol. Vanilla 26.3 has a server management protocol: JSON-RPC over a WebSocket, set up by the management-server-* keys in server.properties. Horizon's engine already parses those keys but doesn't serve the protocol. Serving it later, on the engine or the supervisor, would let tools written for vanilla manage a Horizon server too. That is not part of the MVP.

9. Benchmark plan (bots against vanilla and Paper)

The goal is the published claim: "runs 50–100 players on hardware where vanilla struggles". It must be reproducible by anyone, and show the same gameplay on both sides.

• Servers:

  • Horizon (horizon-server);
  • vanilla 26.3 (the owner's server.jar);
  • Paper (a jar the owner downloads; Paper's defaults are kept, and noted as non-vanilla);
  • optionally Fabric + Lithium (via the pinned tools/benchmark-deps/mods.lock.json).

  Java servers run on Java 25 with -Xms = -Xmx (the same heap for all, 4 GB by default), and the heap is reported.

• Same conditions:

  • the same seed (12345) and a fresh world, plus a second pass on a pre-generated copy of the same world;
  • view-distance 10 and simulation-distance 10;
  • online-mode=false and the same server.properties;
  • one server at a time on the same machine, with the bots on another machine on the LAN, or pinned to other cores when they share one (the report records which).

• Bots: horizon-bots (PR 5), a small Rust load client on horizon-protocol. It handles handshake, offline login and configuration, accepts teleports, answers keep-alives and pings, acknowledges chunk batches as vanilla's client does, and sends movement at 20 Hz. The bots play in creative and fly 24 blocks above their spawn point: they never touch terrain, so they need no copy of the world, and no server can refuse a move for walking into a block. Each behaviour is deterministic per bot from a seed:

  • idle: hovers above spawn;
  • walk: flies to random points within 48 blocks of spawn at sprint speed (5.6 blocks/s);
  • explore: flies straight outwards at creative flying speed (22 blocks/s), which forces chunk generation;
  • mixed: 60 % walk, 20 % explore, 20 % idle.

  A build behaviour (breaking and placing blocks) needs the bots to read terrain and comes later. Runs use 1, 10, 25, 50 and 100 bots, joining 1 per second. Bots are needed because 100 real vanilla clients can't run on one machine.

• Measured (once a second, for 5 minutes after the last bot joins, after a 1-minute warm-up):

  • TPS and MSPT (average, P50/P95/P99), from each server's own /tick query (vanilla's ServerTickRateManager; Horizon implements the same command and clock), so every server measures itself the same way;
  • RSS and CPU (process tree, horizon_bench::rss_bytes/cpu_seconds);
  • chunk generation: chunks per second while the explore bots run, and time until each new bot has its full view loaded;
  • start time: cold and warm, to Done;
  • bot-side: chunk arrival, keep-alive round trip, and disconnects (a run with disconnects is reported as failed).

• Output. tools/server_bench.py drives it: a fresh server per run from the same server.properties, Done, the bots, a warm-up, then the measurement. It writes a horizon-server-benchmark JSON report (the machine, commit, Java version, settings and every run) and prints a comparison table. Results are published like the client benchmark, with the machine, the versions and the exact commands. TPS is derived from /tick query's average (20 while it fits in 50 ms), since the command reports times, not the rate.

• First run (PR 5, on a busy shared Mac mini, so the numbers are noisy). With 1 bot Horizon ticks in 3.5 ms against vanilla's 11.2 ms and uses 1.6 GB against 3.6 GB. At 10 and 25 mixed bots Horizon falls to about 10 TPS where vanilla keeps 20, and at 50 both land at about 15 TPS. The work isn't equal yet:

  • Vanilla pulls fast players back when it lags (moved too quickly), which Horizon never does, so vanilla's explore bots generated less terrain.
  • Vanilla's chunk sending yields under load.
  • Horizon sent each nearby walk bot 1.5–4 times as many chunks.

  Horizon sent 2–4 times the chunks per second in every multi-bot run.

• Equal-work run (2026-10-17, benchmarks/server-equal-work-2026-10-17/, on the ThinkPad with the bots on the Mac mini). Horizon now checks moves and paces chunks as vanilla does (#586), the bots wait for the chunk they stand in, an --explore-speed pass keeps both servers at the same rate, and vanilla runs with gamerule locator_bar false (Horizon has no locator bar yet). Results:

  • Memory: Horizon's RSS is 1.9 to 2.6 times smaller in every run (vanilla's heap is fixed at 4 GB).
  • Walking near spawn: Horizon's tick is shorter with 1 and 10 bots (5.5 against 18.3 ms, 11.5 against 16.8), and longer with 25 and 50 (20.7 against 16.0, 31.4 against 20.5). Both keep 20 TPS up to 50.
  • Exploring at the same rate: Horizon's tick is about half of vanilla's (13.5 against 29.2 ms at 10 bots, 20.2 against 37.0 at 25).
  • At 100 bots: vanilla's watchdog stops it in every pass; Horizon runs at 9 to 11 TPS.

  Two server costs were found and fixed on the way: every chunk section was encoded once per player (#623), and every packet woke its connection's writer thread (#649). Together they took the 50-bot walking tick from 281 ms to 31 ms. At 25 walking bots most of what remains is the engine's streaming and block entity ticks, both of which grow with players (the README has the profile).

• Mixed bots, as installed (2026-10-18, benchmarks/server-mixed-2026-10-18/, the first published run for the working version). It used the v2026.10.18.15 release as install.sh installs it, against vanilla 26.3, with 1, 10, 25 and 50 mixed bots and vanilla's default game rules on both (Horizon has the locator bar now). The servers ran on the ThinkPad and the bots on the Mac mini. Results:

  • Both kept 20 TPS in every run.
  • Horizon's RSS was 3.7 to 5.2 times smaller (1.0 GB against 4.0 GB at 50).
  • Horizon's average tick was shorter with 1, 10 and 25 bots (2.4 against 17.9 ms, 12.4 against 34.5, 24.0 against 31.6) and longer with 50 (35.2 against 29.7). Its P99 was lower at every count.
  • Horizon also did more work: it kept up with the exploring bots, so it generated 1.9 to 2.9 times the new terrain a second.

• Mixed bots, equal work (2026-10-18, benchmarks/server-equal-mixed-2026-10-18/). This is the same run with the load made equal. The explore bots fly at 0.8 blocks a second, so every bot follows the same path on both servers and none waits for terrain. A 300-second warm-up lets both finish the bots' first views before the measurement. server_bench.py now prints a work table that compares each count's chunks a second across servers and flags a run that isn't equal. Horizon ran as horizon-engine, the server build, installed from its archive. Results, on the ThinkPad again (the desktop didn't wake):

  • The work was equal at 1 and 10 bots. At 25 the chunks sent a second were equal (158.6 against 157.6), and vanilla still sent 2.5 more first-time chunks a second while it caught up.
  • Tick: 1.8 against 11.3 ms at 1 bot, 4.7 against 34.3 at 10, 11.7 against 50.5 at 25 (vanilla 19.8 TPS). At 50 Horizon ran at 23.8 ms. Vanilla's watchdog stopped its server 30 s after the last bot joined (a 60-second tick). This is one pass, to repeat on a bigger machine.
  • Horizon's RSS was 4.1 to 4.9 times smaller (1.03 GB at 50).
  • Served in full, vanilla's walking bots got 5400 to 6100 chunks each, against 640 to 1100 in the first run. That is why vanilla held 50 bots there and not here.

• Four servers, equal work (2026-10-19, benchmarks/server-four-way-2026-10-19/). The same load against vanilla, Paper (build 205, its defaults except the connection throttle, which refused bots that share an address) and vanilla on Fabric with Lithium. Horizon was release v2026.10.19, which spawns mobs and ticks blocks around every player as vanilla does (#950, #931), so it now does vanilla's work away from spawn too. Results, on the ThinkPad (the desktop didn't wake):

  • The work was equal on all four at every count (chunks a second within 4 %).
  • Horizon's RSS was 3.8 to 4.8 times smaller (0.72 to 1.11 GB). It started in 8.2 to 8.5 s, against 14.7 to 22.4 s for the Java servers. From 10 bots up it used the least CPU.
  • Average tick: Paper was ahead of Horizon at every count (10.3 against 13.7 ms at 1 bot, 28.3 against 30.9 at 25, 46.9 against 47.8 at 50). Fabric + Lithium was ahead at 1 and 10 bots (12.5 and 20.2 against 13.7 and 22.9), behind at 25, and fell to 17.2 TPS at 50. Vanilla was slowest, and its watchdog stopped it at 50.
  • Horizon's P99 was the lowest from 10 bots up (73 ms at 50, against Paper's 110 and Fabric's 174), and its worst single P99 the lowest at every count.

• Ownership. perf-server-1 owns making the tick faster. This work only measures it, files what it finds, and fixes startup, RAM and packaging.

10. Working version for 2026-10-23

Goal: horizon-server installs with one command, starts from a clean config, serves Horizon and vanilla 26.3 clients, backs up and restores, has a CLI and a basic web panel, and has a first published benchmark against vanilla.

In:

1. horizon-server crate and binary, horizon-server-files (the vanilla files, moved out of the client unchanged), server.toml with import, the CLI (init, run, start, stop, status, cmd, op, whitelist, backup, backups, restore, logs), crash restart, and the engine status endpoint.
2. install.sh and install.ps1, the Dockerfile, systemd and launchd units, service install, and the server archive in the release.
3. The web panel MVP: status, live TPS/MSPT/RAM/CPU, players, console, backups and login links.
4. horizon-bots and tools/server_bench.py, with a first run against vanilla (1/10/25/50 bots, mixed) published in benchmarks/: done, benchmarks/server-mixed-2026-10-18/, and with equal work benchmarks/server-equal-mixed-2026-10-18/ (section 9).
5. Small engine fixes found on the way: logging refused logins (vanilla's lines).

Where it stands (2026-10-16): items 1 to 4 are built and in review (section 11). What's left for the 23rd: run the Dockerfile and install.ps1 once on a machine that has Docker and on Windows; publish a release that holds horizon-server, so the installers have something to install; a clean benchmark run on a quiet machine, then published in benchmarks/; and item 5.

Out (next):

• the Nether and End for remote players;
• the headless engine and its RAM win;
• a server-only prepare without a JDK;
• a Windows service, hot reload, modpacks in the panel;

Online mode works, so a public server can check accounts. An offline server should still run only on a LAN, a tailnet, or behind a whitelist.

11. Pull requests

In order, each one small and shippable alone:

1. This design (docs only).
2. horizon-server, stacked in three parts:
   • a. The crate, the vanilla files moved out of the client unchanged (horizon-server-files), server.toml with import, init, run (crash restart, clean stop on Ctrl-C and SIGTERM), and op and whitelist for a stopped server: #551.
   • b. The control API on 127.0.0.1 with its token, start, stop, restart, status, cmd and logs, plus the engine status endpoint. That endpoint is about 30 additive lines in game/dedicated.rs plus a new game/dedicated/status.rs. perf-ticks-2's branches touch dedicated.rs elsewhere (listen: true), so the edits don't overlap. It also makes the dedicated loop record its tick times, so /tick query reports them: #552.
   • c. Backups, restore and automatic backups: #553.
3. Install scripts, Docker and services (service install), and horizon-server in the releases: #554.
4. Web panel MVP: #555.
5. Bots and the first benchmark against vanilla: #556.

Each PR is stacked on the one before. Merging them in order keeps every step shippable alone.