Scripting with Lua

Monsta allows you to create custom logic for data collection, metric processing and complex alert rules using the programming language Lua.

If the standard templates or native expressions are not sufficient for your needs (such as parsing complex strings, advanced mathematical calculations or loops), the Lua scripting engine offers full flexibility to extend the platform.

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Why Lua?

Lua is a Brazilian scripting language, widely used in the tech industry (especially in games and embedded systems) for three main reasons:

• Extreme Speed: It is one of the fastest scripting languages in the world, ensuring your scripts run in milliseconds without overloading the Monsta server.
• Lightweight: The interpreter occupies very little memory space, allowing large-scale concurrent executions.
• Simple Syntax: It is very easy to read and learn, even for those with little programming experience.

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Quick Guide to Lua Syntax

To help you start creating your first scripts in Monsta, here are the fundamental concepts of the language:

Variables and Data Types

In Lua, you do not need to define the variable type beforehand (dynamic typing). Always use the local keyword to ensure the variable exists only within that script (avoiding memory conflicts).

local nome_dispositivo = "Servidor Core" -- String
local latencia = 45                     -- Number
local ativo = true                      -- Boolean

Conditional Structures (if / else)

The syntax uses the keywords then, elseif and requires the block to be closed with an end.

if latencia > 100 then
    print("Alerta: Latência Alta!")
elseif latencia > 50 then
    print("Aviso: Latência Moderada.")
else
    print("Status: OK.")
end

Tables (Dictionaries and Arrays)

Tables are the only complex data structure in Lua. They serve both as indexed lists and as key/value objects.

-- Example of Array/List
local sensores = {"Ping", "CPU", "Memória"}

-- Example of Dictionary (Key/Value)
local dados_coleta = {
    ip = "192.168.1.50",
    status = "Online",
    portas_abertas = 4
}

Attention to Indices!

Unlike most programming languages (such as JavaScript or Python) where lists start at index 0, in Lua tables start at index 1. Therefore, sensores[1] will return “Ping”.

How does Monsta interact with the script?

Usually, Monsta injects predefined global variables or tables into your script (containing the collected device data) and expects your script to return a specific value (such as a number, text or boolean) using the return command.

-- Example of an SNMP query
local load_linux = snmp.get("1.3.6.1.4.1.2021.10.1.3.2.0") -- Monsta function
return load_linux

The return can also define the metric status. To return a status, the return must return a dictionary containing value (the value to be returned) and status (a string with predefined values).

The accepted values for status are:

status	Color	Metric Status
”MetricOk”	Green	Normal
”MetricWarning”	Orange	Warning
”MetricCritical”	Red	Critical

-- Example of an SNMP query with status handling
local volt = snmp.get("1.3.6.1.2.1.33.1.3.3.1.3.1.0")

if volt > 230 then
   -- If the voltage is higher, it will enter Warning status
   return { value=volt, status="MetricWarning" }
elseif volt < 100 then
   -- If the voltage is lower, it will enter Critical status
   return { value=volt, status="MetricCritical" }
else
   -- Otherwise, the status will be Normal
   return { value=volt, status="MetricOk" }
end

Attention!

When the status is defined inside the script, it is not possible to configure the alert status through the monitor settings, because the script will define that information.

Resources and Complete Manuals

To dive deeper into the language, consult the official Lua language documentation.

Monsta has its own functions that are used in scripts. To learn about the specific internal modules that Monsta provides (network functions, string handling and connection), check the upcoming articles in this area (Lua Script Modules).