First we need to install Ryu:

$ sudo pip install ryu

It is very simple as it can be installed with pip.

Confirm Ryu has been installed correctly by using the following command:

$ sudo mn --controller=ryu,ip=127.0.0.1 --topo linear,2
*** Creating network
*** Adding controller
warning: no Ryu modules specified; running simple_switch only
*** Adding hosts and stations:
h1 h2 
*** Adding switches and access point(s):
s1 s2 
*** Adding link(s):
(h1, s1) (h2, s2) (s2, s1) 
*** Configuring hosts
*** Starting controller(s)
c0 
*** Starting switches and/or access points
s1 s2 ...
*** Starting CLI:
mininet-wifi> 

The preceding command uses ryu as the controller in the network emulated by Mininet. It uses a simple switch in this process.

Now we will write a simple SDN application to get notified when a topology changes. We will use wscat for this recipe. You may install it using:

$ sudo apt install node-ws