Storage Values

The most basic storage type for a blockchain is a single StorageValue.

A StorageValue is used to place a single object into the blockchain storage.

A single object can be as simple as a single type like a u32, or more complex structures, or even vectors.

What is most important to understand is that a StorageValue places a single entry into the merkle trie. So when you read data, you read all of it. When you write data, you write all of it. This is in contrast to a StorageMap, which you will learn about next.

Construction

We constructed a simple StorageValue for you in the code, but let's break it down:

#[pallet::storage]
pub(super) type CountForKitties<T: Config> = StorageValue<Value = u32>;

As you can see, our storage is a type alias for a new instance of StorageValue.

Our storage value has a parameter Value where we can define the type we want to place in storage. In this case, it is a simple u32.

You will also notice CountForKitties is generic over <T: Config>. All of our storage must be generic over <T: Config> even if we are not using it directly. Macros use this generic parameter to fill in behind the scene details to make the StorageValue work. Think about all the code behind the scenes which actually sticks this storage into a merkle trie in the database of our blockchain.

Visibility of the type is up to you and your needs, but you need to remember that blockchains are public databases. So pub in this case is only about Rust, and allowing other modules to access this storage and its APIs directly.

You cannot make storage on a blockchain "private", and even if you make this storage without pub, there are low level ways to manipulate the storage in the database.