Liquidity pools

In brief

Pools are shared reserves that a market-making rule can trade against. They are the inventory layer many DeFi AMMs use; they can also serve lending, stable-swap and multi-asset designs that are not (x \cdot y = k). Providers receive pool shares (often an LP token or an NFT position). Fees accrue to those shares. Arbitrage rebalances reserves toward outside prices. Impermanent loss is the opportunity cost versus holding. Risks include the program, the tokens, and the curve. See Automated market makers for pricing math and Decentralized exchanges (Batch C) for venue architecture.

What a pool is

Uniswap’s documentation: the protocol holds reserves of two tokens and lets anyone deposit both sides or swap against them. That inventory is the pool.

More generally, a liquidity pool is:

• one or more asset balances controlled by a program;
• a rule for who may add, remove, or trade against those balances;
• an accounting system for who is owed what.

The pool is not the website. It is not the AMM formula (the formula is how some pools price). It is not your wallet.

On Ethereum, ERC-20 token contracts record balances assigned to the pool contract; allowances merely authorize transfers. On Solana, balances live in token accounts with an owner authority; pool programs commonly control that authority through a program-derived address. Same economic idea, different objects.

Why pool instead of wait

An order book needs a resting order at a price a taker will accept. A trading pool is willing to quote from its rule as long as it has usable inventory. That is why pool-based AMMs can provide continuous quotes without posting bids and asks.

The cost of that convenience is paid by liquidity providers (LPs) as inventory risk and by traders as price impact and fees.

Providers, shares and fees

An LP deposits assets the pool accepts. In Uniswap v2 they receive a fungible pool token — a receipt for a proportional slice of the reserves. In Uniswap v3/v4 a position is often an NFT (or similar) tied to a price range. Balancer-style weighted pools and Curve-style stable pools use their own share tokens.

Fees on swaps typically stay in the pool (raising the value of shares) or are credited to positions. Uniswap v2’s paper: traders paid 30 basis points, to LPs, unless a protocol fee was switched on. Other pools choose other fee tiers. Fees are not a promise that the LP ends up ahead of just holding.

Removing liquidity burns or closes the share and returns a slice of whatever the pool then holds — not a guaranteed basket of the original deposit.

Trading against a pool

A trader sends token A and receives token B from the reserves. The program updates balances according to its rule. The spot implied by those balances moves. That movement is price impact (see Slippage and Automated market makers).

Nobody “took your order” in the stock-exchange sense. You traded with the pot.

Rebalancing by arbitrage

If the pool’s implied price is away from other venues, arbitrageurs trade until it is close enough that further trades do not pay. That is how a constant-product pool tracks the outside market at all. LPs experience that as the mix of tokens changing: the pool sells the winner and accumulates the loser. EEA guidelines call the gap versus holding impermanent loss (and mention divergence loss / LVR as related measurements).

The loss is “impermanent” only if prices revert before you withdraw. Fees may offset it. They may not.

Not only (x \cdot y = k)

Pools exist under other rules:

• Concentrated liquidity (Uniswap v3/v4 docs): capital is active only inside a chosen tick range. Outside the range the position is idle and earns no fees; it can become a single asset.
• Stable-swap (Curve’s StableSwap paper): a curve flatter near balance so like-priced assets can trade with less impact than under a raw constant-product rule. The brief paper supports that qualitative comparison but omits the full invariant.
• Weighted / multi-asset (Balancer): an invariant based on token balances and chosen weights can support more than two assets and non-50/50 allocations.
• Lending pools (Aave-style): a pool of a single asset that is borrowed against collateral. That is inventory for credit, not an AMM. Do not mix the two just because both are called pools.

So: many familiar DeFi AMMs use pooled inventory, but pool and AMM are not universal synonyms. Not every pool is an AMM, and the broader AMM label can cover mechanisms without a conventional LP-funded reserve pool.

Why a pool is not a bank account

• No obligation to return the same assets 1:1.
• No deposit insurance.
• The program can have an admin, a pause, or a bug.
• A token in the pool can be frozen, rebasing, or infinitely minted.
• Share tokens can be staked elsewhere, adding more contracts.
• In a run, you exit at the then mix and the then price.

Treat a pool as a joint inventory account with a robot market maker, not as cash at a bank.

Risks

Smart-contract risk. The pot is the honeypot.

Token risk. Permissionless listing means the other asset may be junk. Fee-on-transfer and blacklist tokens can break accounting.

Inventory / IL / LVR. Prices move.

Oracle risk if the pool’s price is used as an oracle (don’t; see Oracles).

Admin / upgrade risk.

LP-token risk. The receipt can be lost, approved to a scammer, or used as collateral in something that liquidates.

How this sits next to the AMM article

This article is about inventory and shares. Automated market makers is about pricing rules. Slippage is about quoted versus executed price. Decentralized exchanges is about venues. Read them as a set, not as four copies of (x \cdot y = k).