Multithreaded HTTP Server for the BeagleBone Black

A multithreaded HTTP server, running on the BeagleBone Black, that serves the current temperature and humidity to its clients in JSON format.

The server is built using the Poco C++ Libraries.

Data from the SHT21 temperature and humidity sensor is read through sysfs using the pre-built LKM.

An ExpireCache, whose access is protected by a mutex, prevents simultaneous http requests from concurrently accessing the sensor.

Last but not least, the server can be connected to a fat-client Qt desktop application, whose job is to retrieve the information and display it nicely to the user.

JSON Structure

Data is served at /.
Any other endpoint will serve a 404 response.

Here's how the JSON looks like if everything went fine:

{
    "version": <number>,
    "data": {
        "temperature": <number>,
        "humidity": <number>
    }
}

For an error:

{
    "version": <number>,
    "error": <string>
}

Env Variables

Both the server's port number and the timeout for the ExpireCache can be adjusted in http-server.properties

Circuit

Setup, Building, and Flashing

This document goes in detail about how I set up my machine for cross-compilation.

The project uses a global CMake file with different targets.
Dependencies are managed through Conan.
The file profile_crossbuild tells the latter which platform we're generating binary files for.

To build:

#
# On the VM
#

mkdir build
cd build

conan install .. --build=missing --profile=../profile_crossbuild

cmake .. -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Build
cmake --build .

Finally sftp the executable and the environment variables to the board:

#
# On the VM
#

sftp-bbb
> put build/bin/http-server
> put http-server.properties

Running the server

The program expects the SHT21 LKM to be loaded before starting:

#
# On the BeagleBone
#

# get root permissions
sudo su

# verify that the sht21 kernel module exists
find "/lib/modules/$(uname -r)/kernel/drivers/hwmon" -name sht21.ko.xz

# load the module
modprobe sht21

# let the kernel know to which bus the sensor is connected
echo sht21 0x40 > /sys/bus/i2c/devices/i2c-2/new_device

# no need for root permissions anymore
exit

# start the server
./http-server

Details about the BBB and the OS

Processor: AM3358 ARM Cortex-A8

OS: Debian 10.3 Buster IoT (without graphical desktop)

Kernel release: 4.19.94-ti-r42

GCC version: 8.3.0

Link to the official docs here

Yet another project with this damn SHT21 sensor?

Yes, mainly because:

1. it's cheap and easy to find;
2. I got to know it quite well;
3. it's straightforward to connect and use;
4. the pre-built LKM abstracts over many details.

Focus of this project was the combination of a remote sensor running on Linux with a fat-client GUI app.
Using the SHT21 has simply been the quickest way to get it done.

ASIDE: The walrus...

The nice cartoon sketch is offered by Pixabay users under their generous free license.