READ.. should i get payed before or after?

i got hired to dj a party for 8 hours charging 20 dollars an hour. so the total price is going to be 160.. i strongly doubt the party will last the whole 8 hours and im afraid that if the party dies after 3 hours hes only gonna pay me 60... should he just pay me the 160 dollars at the door and if the party dies in 3 hours i still get my money or would i be ripping him off since i didnt dj the remainding 5 hours