Let's imagine a field full of lobsters. Each lobster represents a server. Most of the lobsters like to hang around with each other, but some of them don't like certain other lobsters and will refuse to hang out with other lobsters.

Now each of these lobsters have at least one tiny little mite riding on them. These mites want to talk to the mites on the other lobsters, but they are too far away. So, they build a big sign on the back of their lobster that the other mites can see. Since they are on the back of their lobster, they can change what's on the sign or add to it or make a new sign, but the other mites can add to the signs by signaling with semaphore what they want written on the sign. However, the mites on the lobster that their lobster doesn't like are never close enough to send their semaphore signals, so although they can see the sign they can't change or add to it.