Storing data

Smart contracts have their own private storage space. You can define different types of data members to use throughout your smart contract. These data members are committed to the chain and thus persist across transactions. For example, a first transaction can change a data member after which a second one reads the updated value.

All data members have to be defined on the top level of the contract and are identified by a unique name. No new data members can be introduced after the contract has been deployed. Clarity permits three different kinds of storage: constants, variables, and data maps.

• Constant values are unchangeable, defined on the top level of the contract. They are useful to define the contract owner, error codes, and other static values.
• Variables have an initial value and can be changed by means of future contract calls. One variable contains exactly one value of a predefined type.
• Maps are collections of data identified by other data. Think of variables where the variable names themselves are values. They are used to relate one value to another; for example, relating specific principals to unsigned integers to keep track of scores.

Although data members are _private_—meaning, only the current contract can use them—it does not mean that they are hidden. Anything on the blockchain is inherently public so data members should never be used to store sensitive information like passwords or private keys. The value of any data member can be extracted from the chain state effortlessly.