MinMon - an opinionated minimal monitoring and alarming tool (for Linux)

This tool is just a single binary and a config file. No database, no GUI, no graphs. Just monitoring and alarms. I wrote this because the existing alternatives I could find were too heavy, mainly focused on nice GUIs with graphs (not on alarming), too complex to setup, or targeted at cloud/multi-instance setups.

Checks

Implemented are these checks:

• Filesystem usage
• Memory usage
• Pressure average
• Process exit status

See Roadmap for further ideas.

Actions

• Email
• Log
• Process
• Webhook

Report

The absence of alarms can mean two things: everything is okay or the monitoring/alarming failed altogether. That's why MinMon can trigger regular report events to let you know that it's up and running.

Design decisions

• No complex scripting language.
• No fancy config directory structure - just a single TOML file.
• No users, groups or roles.
• No cryptic abbreviations. The few extra letters in the config file won't hurt anyone.
• There are no predefined threshold names like "Warning" or "Critical". You might might want more than just two, or only one. So that's up to you to define in the config.
• The same check plugin can be used multiple times. You might want different levels to trigger different actions for different filesystems at different intervals.
• Alarms are timed in "cycles" (i.e. multiples of the interval of the check) instead of seconds. It's not very user-friendly but helps to keep the internal processing and the code simple and efficient.
• Alarms stand for themselves - they are not related. This means that depending on your configuration, two (or more) events may be triggered at the same time for the same check. There are cases where this could be undesirable.
• Simple, clean, bloat-free code with good test coverage.
• Depending on your configuration, there may be similar or identical blocks in the config file. This is a consequence of the flexibility and simpleness of the config file format.
• All times and dates are UTC. No fiddling with local times and time zones.
• No internal state is stored between restarts.
• As of now it's only for Linux but it should be easy to adapt to other *NIXes or maybe even Windows.
• Some of the things mentioned above may change in the future (see Roadmap).

Config file

The config file uses the TOML format and has the following sections:

• log
• report
• actions
• checks

Architecture

System overview

graph TD
    A(Config file) --> B(Main loop)
    B -->|interval| C(Check 1)
    B -.-> D(Check 2..n)
    C -->|data| E(Alarm 1)
    C -.-> F(Alarm 2..m)
    E -->|cycles, repeat_cycles| G(Action)
    E -->|recover_cycles| H(Recover action)
    E -->|error_repeat_cycles| I(Error action)

    style C fill:green;
    style D fill:green;
    style E fill:red;
    style F fill:red;
    style G fill:blue;
    style H fill:blue;
    style I fill:blue;

Alarm state machine

Each alarm has 3 possible states. "Good", "Bad" and "Error".
It takes cycles consecutive bad data points to trigger the transition from "Good" to "Bad" and recover_cycles good ones to go back. These transitions trigger the action and recover_action actions. During the "Bad" state, action will be triggered again every repeat_cycles cycles (if repeat_cycles is not 0).

The "Error" state is a bit special as it only "shadows" the other states. An error means that there is no data available at all, e.g. the filesystem usage for /home could not be determined. Since this should rarely ever happen, the transition to the error state always triggers the error_action on the first cycle. If there is valid data on the next cycle, the state machine continues as if the error state did not exist.

stateDiagram-v2
    direction LR

    [*] --> Good
    Good --> Good
    Good --> Bad: action/cycles
    Good --> Error: error_action

    Bad --> Good: recover_action/recover_cycles
    Bad --> Bad: repeat_action/repeat_cycles
    Bad --> Error: error_action

    Error --> Good
    Error --> Bad
    Error --> Error: error_repeat_action/error_repeat_cycles

Example

Check the mountpoint at /home every minute. If the usage level exceeds 70% for 3 consecutive cycles (i.e. 3 minutes), the "Warning" alarm triggers the "Webhook 1" action. The action repeats every 100 cycles until the "Warning" alarm recovers. This happens after 5 consecutive cycles below 70% which also triggers the "Webhook 1" action. If there is an error while checking the filesystem usage, the "Log error" action is triggered. This is repeated every 200 cycles.

Config

[[checks]]
interval = 60
name = "Filesystem usage"
type = "FilesystemUsage"
mountpoints = ["/home"]

[[checks.alarms]]
name = "Warning"
level = 70
cycles = 3
repeat_cycles = 100
action = "Webhook 1"
recover_cycles = 5
recover_action = "Webhook 1"
error_repeat_cycles = 200
error_action = "Log error"

[[actions]]
name = "Webhook 1"
type = "Webhook"
url = "https://example.com/hook1"
body = """{"text": "{{alarm_name}}: {{check_name}} on mountpoint '{{check_id}}' reached {{level}}%."}"""
headers = {"Content-Type" = "application/json"}

[[actions]]
name = "Log error"
type = "Log"
level = "Error"
template = """{{check_name}} check didn't have valid data for alarm '{{alarm_name}}' and id '{{alarm_id}}'."""

The webhook text will be rendered into something like "Warning: Filesystem usage on mountpoint '/home' reached 70%."

Diagram

graph TD
    A(example.toml) --> B(Main loop)
    B -->|every 60 seconds| C(FilesystemUsage 1: '/srv')
    C -->|level '/srv': 60%| D(LevelAlarm 1: 70%)
    D -->|cycles: 3, repeat_cycles: 100| E(Action: Webhook 1)
    D -->|recover_cycles: 5| F(Recover action: Webhook 1)
    D -->|error_repeat_cycles: 200| G(Error action: Log error)

    style C fill:green;
    style D fill:red;
    style E fill:blue;
    style F fill:blue;
    style G fill:blue;

Some (more exotic) ideas

Just to give some ideas of what's possible:

• Run it locally on your workstation and let it send you notifications to your desktop environment using the Process action and notify-send when the filesystem fills up.
• Use the report in combination with the Webhook action and telepush and let it send you "I'm still alive, since {{minmon_uptime}} seconds!" once a week to your Telegram messenger for the peace of mind.

Placeholders

To improve the reusability of the actions, it's possible to define custom placeholders for the report, events, checks, alarms and actions. When an action is triggered, the placeholders (generic and custom) are merged into the final placeholder map. Inside the action (depending on the type of the action) the placeholders can be used in one or more config fields using the {{placeholder_name}} syntax. There are also some generic placeholders that are always available and some that are specific to the check that triggered the action. Placeholders that don't have a value available when the action is triggered will be replaced by an empty string.

Installation

Docker image

To pull the docker image use

docker pull ghcr.io/flo-at/minmon:latest

or the example docker-compose.yml file.
In both cases, read-only mount your config file to /etc/minmon.toml.

Build and install using cargo

Make sure cargo and OpenSSL are correctly installed on your local machine.
You can either install MinMon from crates.io using

cargo install --all-features minmon

Or if you already checked out the repository, you can build and install your local copy like this:

cargo install --all-features --path .

Copy the systemd.minmon.service file to /etc/systemd/system/minmon.service and place your config file at path /etc/minmon.toml. You can enable and start the service with systemctl daemon-reload && systemctl enable --now minmon.service.

If you don't want to include the systemd integration, leave out the --all-features option.

Install for the AUR (Arch Linux)

Use your package manager of choice to install the minmon package from the AUR.
Place your config file at path /etc/minmon.toml. You can enable and start the service with systemctl daemon-reload && systemctl enable --now minmon.service.\

systemd integration (optional)

• Logging to journal.
• Notify systemd about start-up completion (Type=notify).
• Periodically reset systemd watchdog (WatchdogSec=x).

Roadmap

Check ideas

• Filesystem inode usage
• Folder size
• S.M.A.R.T.
• Load
• Temperatures
• Ping
• HTTP response, keyword, ..
• HTTPS certificate expiration date
• systemd service status
• Docker/Podman container status

General ideas

• Store data/status in time-based database (e.g. rrdtool) and visualize on web interface or ncurses UI. This should be optional and separated from the existing code.

Contributions

Contributions are very welcome! Right now MinMon is pretty basic but it's also super easy to extend. Even if it's just a typo in the documentation, I'll be happy to merge your PR. If you're looking for a new check or action type, just open a new issue (if it doesn't exist yet) and tag it with the "enhancement" label.

If you have an idea for a new check or action please use use the discussions page instead of opening a new issue.