Pub/sub topology

P: box "Producer" rad 5px
arrow ">"
B: box "Broker" big bold fill 0xe0f2fe
arrow from B.e right 0.6
S1: box "Subscriber A"
move from B.e right 0.6 down 0.7
S2: box "Subscriber B"
move from B.e right 0.6 up 0.7
S3: box "Subscriber C"
line from B.e to S1.w "topic"
line from B.e to S2.w "topic"
line from B.e to S3.w "topic"