Execute the compiled producer binary in order to produce messages to the purchases topic. 
./out/producer
You should see output resembling this: 
Produced event to topic purchases: key = awalther   value = batteries
Produced event to topic purchases: key = htanaka    value = alarm clock
Produced event to topic purchases: key = awalther   value = t-shirts
Produced event to topic purchases: key = eabara     value = t-shirts
Produced event to topic purchases: key = jbernard   value = t-shirts
Produced event to topic purchases: key = eabara     value = alarm clock
Produced event to topic purchases: key = jbernard   value = t-shirts
Produced event to topic purchases: key = sgarcia    value = gift card
Produced event to topic purchases: key = jsmith     value = t-shirts
Produced event to topic purchases: key = jsmith     value = alarm clock