# App Storage

Because Beanstalk is a single contract, all of the Facets can share a common state through the [`AppStorage`](https://dev.to/mudgen/appstorage-pattern-for-state-variables-in-solidity-3lki) pattern.

Each Facet should define the `AppStorage` storage variable in the base contract and define **no other** `internal` or `public` state variables.

```solidity
AppStorage internal s;
```

The `AppStorage` struct can be found in the [`AppStorage.sol`](https://github.com/BeanstalkFarms/Beanstalk/tree/master/protocol/contracts/beanstalk/storage) file.

When adding new state variables, always add them to the end of the `AppStorage` struct to ensure consistent mapping of state variables to storage slots. When deprecating an unnecessary state variables, either replace it with a different variable denoting that the state variable is deprecated or leave it as is.

In the EVM, data is accessed 32 bytes at a time (known as a slot). Because of this, in Beanstalk, many state variables are packed within the `AppStorage` struct to reduce gas costs. For example, say that in function `foo`, the state of 2 `uint256` values is changed. Changing a non-zero value to another non-zero value costs 10000 gas (`2 * 5000`). If the 2 values could be stored in `uint128`s instead (meaning the data is in one slot), 2100 gas is shaved off, as the state slot is now warm. (If a slot is touched for the first time in a transaction, it is turned from cold to warm for the rest of that transaction. Warm touches are cheaper than cold touches.)


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