The Automated Market Maker (AMM) systems are smart contracts using something more equivalent to a ballast system with pools - "liquidity pools" (LP, ironically

The Automated Market Maker (AMM) systems are smart contracts using something more equivalent to a ballast system with pools - "liquidity pools" (LP, ironically and perfectly these act like limited partner shares of a pooled investment) - allowing the current price to be more of a ratio, which is stored in the smart contract after every trade. The largest AMM's have user-spawned markets which contain two assets (this is in direct contrast and competition to the concept of begging exchanges to add your asset and pretending that market makers organically showed up, all while getting extorted by the exchanges and market makers, the market has chosen AMMs). When someone buys in an AMM market, they send AssetA adding to the pooled balance of AssetA, and receiving AssetB, reducing the pooled balance of AssetB. The ratio (simple division) of these two assets is stored at the end of that trade in the same smart contract (the spawned liquidity pool ballast), and can therefore be read by any other user and other smart contracts. Reading is a free operation. This user-spawned and self-replication smart contract factory, alongside the price storage, has been key to the DeFi boom over the past year. No oracle required. People (organizations) are using oracles to try to keep markets with similar assets in sync, and for smarter routing for the best price. The arbitragers take care of that too, which is simply Oracle + Fund movement. Flash loan users in the repo market are arbitraging as well, which is simply Oracle + Borrow + Fund movement + Repay in a single transaction included in a single block. It is a growth industry when you stop asking why and start asking how. A lot of oracles are ignoring some market pairs, and the oracles themselves could have their own inefficiency.