Add example systemd unit and documentation on using it

Author: DanielSidhion

Diff for: docs/deploy/hardware.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 4
+sidebar_position: 1
 ---
 
 # Hardware

Diff for: docs/deploy/linux.md

@@ -0,0 +1,11 @@
+---
+sidebar_position: 2
+---
+
+# Deploying on Linux
+
+## systemd
+
+TigerBeetle includes an example [systemd unit](https://github.com/tigerbeetle/tigerbeetle/tree/main/tools/systemd/) for use with Linux systems that use systemd.
+The unit is configured to start a [single-node cluster](../quick-start/single-binary.md), so you may need to adjust it for other cluster configurations.
+Instructions to do so are in our GitHub repository.

Diff for: scripts/.cspell.json

@@ -83,6 +83,8 @@
 	"myscript",
 	"nacks",
 	"Nearline",
+	"networkd",
+	"nounset",
 	"otherstuff",
 	"paxos",
 	"POCPOU",

Diff for: tools/systemd/README.md

@@ -0,0 +1,66 @@
+# systemd Service Unit
+
+This is an example of a systemd service that runs TigerBeetle.
+You will likely have to adjust it if you're deploying TigerBeetle.
+
+## Adjusting
+
+You can adjust multiple aspects of this systemd service.
+Each specific adjustment is listed below with instructions.
+
+It is not recommended to adjust some values directly in the service file.
+When this is the case, the instructions will ask you to instead use systemd's drop-in file support.
+Here's how to do that:
+
+1. Install the service unit in systemd (usually by adding it to `/etc/systemd/system`).
+2. Create a drop-in file to override the environment variables.
+   Run `systemctl edit tigerbeetle.service`.
+   This will bring you to an editor with instructions.
+3. Add your overrides.
+   Example:
+   ```
+   [Service]
+   Environment=TIGERBEETLE_CACHE_GRID_SIZE=4GB
+   Environment=TIGERBEETLE_ADDRESSES=0.0.0.0:3001
+   ```
+
+### Pre-start script
+
+This service executes the `tigerbeetle-pre-start` script before starting TigerBeetle.
+This script is responsible for ensuring that a replica data file exists.
+It will create a data file if it doesn't exist.
+
+The script assumes that `/bin/sh` exists and points to a POSIX-compliant shell, and the `test` utility is either built-in or in the script's search path.
+If this is not the case, adjust the script's shebang.
+
+The service assumes that this script is installed in `/usr/local/bin`.
+If this is not the case, adjust the `ExecStartPre` line in `tigerbeetle.service`.
+
+### TigerBeetle executable
+
+The `tigerbeetle` executable is assumed to be installed in `/usr/local/bin`.
+If this is not the case, adjust both `tigerbeetle.service` and `tigerbeetle-pre-start` to use the correct location.
+
+### Environment variables
+
+This service uses environment variables to provide default values for a simple single-node cluster.
+To configure a different cluster structure, or a cluster with different values, adjust the values in the environment variables.
+It is **not recommended** to change these default values directly in the service file, because it may be important to revert to the default behavior later.
+Instead, use systemd's drop-in file support.
+
+### State directory and replica data file path
+
+This service configures a state directory, which means that systemd will make sure the directory is created before the service starts, and the directory will have the correct permissions.
+This is especially important because the service uses systemd's dynamic user capabilities.
+systemd forces the state directory to be in `/var/lib`, which means that this service will have its replica data file at `/var/lib/tigerbeetle/`.
+It is **not recommended** to adjust the state directory directly in the service file, because it may be important to revert to the default behavior later.
+Instead, use systemd's drop-in file support.
+If you do so, remember to also adjust the `TIGERBEETLE_DATA_FILE` environment variable, because it also hardcodes the `tigerbeetle` state directory value.
+
+Due to systemd's dynamic user capabilities, the replica data file path will not be owned by any existing user of the system.
+
+### Hardening configurations
+
+Some hardening configurations are enabled for added security when running the service.
+It is **not recommended** to change these, since they have additional implications on all other configurations and values defined in this service file.
+If you wish to change those, you are expected to understand those implications and make any other adjustments accordingly.

Diff for: tools/systemd/tigerbeetle-pre-start.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+set -eu
+
+if ! test -e "${TIGERBEETLE_DATA_FILE}"; then
+  /usr/local/bin/tigerbeetle format --cluster="${TIGERBEETLE_CLUSTER_ID}" --replica="${TIGERBEETLE_REPLICA_INDEX}" --replica-count="${TIGERBEETLE_REPLICA_COUNT}" "${TIGERBEETLE_DATA_FILE}"
+fi

Diff for: tools/systemd/tigerbeetle.service

@@ -0,0 +1,26 @@
+[Unit]
+Description=TigerBeetle Replica
+Documentation=https://docs.tigerbeetle.com/
+After=network-online.target
+Wants=network-online.target systemd-networkd-wait-online.service
+
+[Service]
+DynamicUser=true
+DevicePolicy=closed
+ProtectHome=true
+StateDirectory=tigerbeetle
+StateDirectoryMode=700
+
+Environment=TIGERBEETLE_CACHE_GRID_SIZE=1GB
+Environment=TIGERBEETLE_ADDRESSES=3001
+Environment=TIGERBEETLE_REPLICA_COUNT=1
+Environment=TIGERBEETLE_REPLICA_INDEX=0
+Environment=TIGERBEETLE_CLUSTER_ID=0
+Environment=TIGERBEETLE_DATA_FILE=%S/tigerbeetle/0_0.tigerbeetle
+
+Type=exec
+ExecStartPre=/usr/local/bin/tigerbeetle-pre-start.sh
+ExecStart=/usr/local/bin/tigerbeetle start --cache-grid=${TIGERBEETLE_CACHE_GRID_SIZE} --addresses=${TIGERBEETLE_ADDRESSES} ${TIGERBEETLE_DATA_FILE}
+
+[Install]
+WantedBy=multi-user.target