MQTT

MQTT is a lightweight protocol on top of TCP/IP which is used to transfer data between nodes. It was built to be used by devices having low network bandwidth. The low overheads in using this protocol has made it a popular choice for use with IoT devices.

The protocol uses a subscribe/publish method wherein both the data source and the recipient subscribe to a topic on a broker. This model is similar to twitter where a tweet sent by a user can be read by multiple users who subscribe to the user.

A broker is a software run on a more capable hardware which acts as a data distributor. A topic is similar to a subject and is built in a tree structure with its components separated by a '/'. Wildcards are allowed when describing topics to subscribe to.  Mosquitto(http://mosquitto.org/) is the most popular opensource broker at this moment.

Clients connect to the broker and subscribe to the topic. A client wishing to transmit its data(the sender) will publish data for the topic on the broker. Subscribers to the topic on the broker receive the data and process it further. MQTT is data agnostic and the data can take any format the sender wishes.

MQTT apis are available for multiple programming languages at the eclipse paho website.

The PubSub client library is available for use with arduinos.

Some use cases for MQTT are
a) Sensors such as temperature sensors periodically sending information about its surroundings.
b) Sending commands to a device from various sources.

An example project which uses MQTT in an arduino setting is available on the Make website. Here  the automation software - openhab is configured with MQTT bindings to send commands specifying the colour required to the broker(mosquitto) running on a raspberry pi. The ESP8266 connected to a LED strip subscribes to this broker reads the colour data off the broker and sets the light colour accordingly.

More information is on the protocol is available in the mqtt man page at
http://mosquitto.org/man/mqtt-7.html

Using Mock

The article is from my notes on setting up a RHEL 7 build environment on a RHEL 6 machine. This can be used to setup a build environment for any of my guest on the host machine.

I have a RHEL 6 based server grade machine which hosts vms belonging to RHEL 6, RHEL 7 and Fedora rawhide. In some instances, I need to be able to compile kernels based on the git tree for a certain guest machine. Trying to do this on a vm usually means I need to export the git tree over NFS and given the resources the vm has, takes a long time. The idea is to compile kernels on the more capable host machine and simply run "make install modules_install" on the guest system.

I export the filesystem containing the git trees using NFS
# cat /etc/exports
/NotBackedUp/scratch 192.168.140.0/24(rw,no_root_squash)

To setup mock,

1) Install mock.

# yum install -y mock

2) Set the epel7 environment as the default environment.

cd /etc/mock; ln -s epel-7-kernel-x86_64.cfg default.cfg

3) Edit epel-7-kernel-x86_64.cfg and add the lines below.

Since the environment is intended to build kernels, install the required dependencies along with some other tools.

config_opts['install'] ='elfutils dwarves rpm-build flex bison inotify-tools bc openssl openssl-devel openssh-clients rsync'

Allow bind mounts. Mount the local directory /NotBackedUp/scratch to /scratch in the mock environment

config_opts['plugin_conf']['bind_mount_enable'] = True
config_opts['plugin_conf']['bind_mount_opts']['dirs'].append(('/NotBackedUp/scratch', '/scratch' ))

4) Add user and group mock

5) Switch over to user mock and initialise the mock environment using the command 

$ mock --init

6) Switch over to the mock shell

$ mock shell

At this point, we are in a chroot shell as a root user in the mock EPEL 7 environment. The directory containing the git tree is available at the directory specified in the mock configuration file. In my case, this is /scratch. 

7) On the guest machine, make sure you mount the exported NFS share and run 

make menuconfig/oldconfig/localmodconfig

to setup the .config file. 

8) Switch back over to the mock environment on the host machine and run 

make; make modules

to build the kernel and the modules.

9) Once the build is done, on the guest vm, go back to the git tree and run

make install modules_install

to install the newly built kernel.

10) Setup grub on the guest machine and reboot into the new kernel.