First-come first-sell pool, a.k.a. ICO. The basic idea is to put a fixed amount of base token (token to be sold) in the pool, set a fixed price between base token and quote token (token used to buy), and a start time. After the start time, users can buy the base token at the ratio using quote token. This was a very popular model in 2017 ICO boom. However, this model favors early buyers, which results in gas war (e.g., Bancor ICO). Further, the early big buyers may take most of the base tokens and then dump them later in secondary market.