Transforming payloads

The embedded MQTT and HTTP endpoints allow the device agent to receive data from external sources using the esthesis Line Protocol. When the external sources are under your control, eLP is a simple protocol you can easily work with. However, when the external sources are not under your control, you may need to transform their payloads before they are sent to the device agent.

esthesis device agent allows you to transform payloads using external Lua scripts, for both MQTT and HTTP endpoints (see the LUA* parameters in Configuration parameters for more details).

Lua incoming payload variable

In your Lua script, you have access to the original payload in the payload variable; the variable is a string. You can modify the payload as you wish, and simply return it at the end.

Example Lua script

Let us consider an external data source that pushes data in the following format:

cpu
temperature=20
load=2

The first line is always the category name, whereas the remaining lines contain individual measurements for that category. Measurements are separated by an equal sign.

We want to transform this incoming payload to eLP format, such as:

cpu temperature=20,load=2

The following Lua script could be used to achieve this:

-- Splitting the payload into lines
local lines = {}
for line in payload:gmatch("[^\r\n]+") do
    table.insert(lines, line)
end

-- Extracting the category and measurements
local category = lines[1]
local measurements = {}
for i = 2, #lines do
    local measurement = lines[i]:gsub("%s+", "")  -- Remove whitespace
    table.insert(measurements, measurement)
end

-- Constructing the single-line format
local transformedPayload = category .. " " .. table.concat(measurements, ",")

return transformedPayload

Lua incoming endpoint variable

In your Lua script, you also have access to an additional variable called endpoint. This variable contains the MQTT topic or HTTP URI, depending on the source of the incoming payload. This can be used to help determine the logic to apply when modifying the payload based on the specific endpoint on which data was received.

Example Lua script making use of the endpoint variable

You have a system that sends data into the following MQTT topics:

sensor/hardware/cpu:

temperature=20
load=2

sensor/hardware/memory:

free=80
used=20
loadAverage=50

The format of the data is exactly the same, so you want to implement your Lua script logic only once, however the resulting eLP should be differentiated based on the topic in which the data was received.

Below is an example of how to utilise the endpoint variable in your Lua script to handle that:

-- Splitting the payload into lines

local lines = {}
for line in payload:gmatch("[^\r\n]+") do
    table.insert(lines, line)
end

-- Extracting the category and measurements
local measurements = {}
for i = 1, #lines do
    local measurement = lines[i]:gsub("%s+", "")  -- Remove whitespace
    table.insert(measurements, measurement)
end

-- Logic based on the endpoint
if endpoint == "sensor/hardware/cpu" then
    -- Specific logic for CPU sensor
    transformedPayload = "cpu " .. table.concat(measurements, ",")
elseif endpoint == "sensor/hardware/memory" then
    -- Specific logic for Chipset sensor
    transformedPayload = "memory " .. table.concat(measurements, ",")
else
    -- Default logic for other endpoints (if any)
    transformedPayload =  "hardware " .. table.concat(measurements, ",")
end

return transformedPayload

Last modified: 06 May 2025