Declaring a `StorageMap`

pallets/simple-map

We declare a single storage map with the following syntax:

#[pallet::storage]
#[pallet::getter(fn simple_map)]
pub(super) type SimpleMap<T: Config> =
    StorageMap<_, Blake2_128Concat, T::AccountId, u32, ValueQuery>;

Explanation of the code:

SimpleMap - the name of the storage map
#[pallet::getter(fn simple_map)] - a getter function simple_map is created using pallet getter macros.
Blake2_128Concat - its an hasher used in map. More on this below.

Map contains key and its value:

T::AccountId - its the data type of key of the map.
u64 - its the data type of value of the map.
ValueQuery - If you omit ValueQuery, when interacting with a simple map, you will get an Option<u32>, which means that if you try to get a value from your StorageMap, you will get either Some(value) or None. Using ValueQuery will always return a value, so you don't have to deal with unwrapping the get calls.

Choosing a Hasher

Hasher to use to hash keys to insert to storage.

Although the syntax above is complex, most of it should be straightforward if you've understood the recipe on storage values. The last unfamiliar piece of writing a storage map is choosing which hasher to use. In general you should choose one of the three following hashers. The choice of hasher will affect the performance and security of your chain. If you don't want to think much about this, just choose Blake2_128Concat and skip to the next section.

`Blake2_128Concat`

This is a cryptographically secure hash function, and is always safe to use. It is reasonably efficient, and will keep your storage tree balanced. You must choose this hasher if users of your chain have the ability to affect the storage keys. In this pallet, the keys are AccountIds. At first it may seem that the user doesn't affect the AccountId, but in reality a malicious user can generate thousands of accounts and use the one that will affect the chain's storage tree in the way the attacker likes. For this reason, we have chosen to use the Blake2_128Concat hasher.

`Twox64Concat`

This hasher is not cryptographically secure, but is more efficient than blake2. Thus it represents trading security for performance. You should not use this hasher if chain users can affect the storage keys. However, it is perfectly safe to use this hasher to gain performance in scenarios where the users do not control the keys. For example, if the keys in your map are sequentially increasing indices and users cannot cause the indices to rapidly increase, then this is a perfectly reasonable choice.

`Identity`

The Identity "hasher" is really not a hasher at all, but merely an identity function that returns the same value it receives. This hasher is only an option when the key type in your storage map is already a hash, and is not controllable by the user. If you're in doubt whether the user can influence the key just use blake2.

The Storage Map API

Documentation This pallet demonstrated some of the most common methods available in a storage map including insert, get, take, and contains_key.

// Insert
<SimpleMap<T>>::insert(&user, entry);

// Get
let entry = <SimpleMap<T>>::get(account);

// Take
let entry = <SimpleMap<T>>::take(&user);

// Contains Key
<SimpleMap<T>>::contains_key(&user)

// Mutate
<SimpleMap<T>>::mutate(&user, |entry_option| {
							 *entry_option = Some(entry);
            });

`insert` and `mutate`

When deciding between mutate and insert to update storage, consider the following:

Insert performs a simple write operation to the database, which is the more efficient option.

On the other hand, mutate involves a read operation followed by a write, making it a more expensive database operation.

Therefore, when you have the option to use insert (i.e., you don't need to read the existing value), it's recommended to use insert over mutate.

Insert is suitable for inserting or overwriting an existing value. If you simply want to store a specific value, insert is the way to go.

Mutate, however, is designed for scenarios where you need to modify the existing value or make decisions based on its current state. Use mutate when you need to perform conditional updates or modifications that depend on the current value."

Substrate-recipes