Version: Current

storage

Account Storage

All accounts have storage. Both resources and structures can be stored in account storage.

Paths

Objects are stored under paths. Paths consist of a domain and an identifier.

Paths start with the character /, followed by the domain, the path separator /, and finally the identifier. For example, the path /storage/test has the domain storage and the identifier test.

There are only three valid domains: storage, private, and public.

Objects in storage are always stored in the storage domain.

Paths in the storage domain have type StoragePath, in the private domain PrivatePath, and in the public domain PublicPath.

PrivatePath and PublicPath are subtypes of CapabilityPath.

Both StoragePath and CapabilityPath are subtypes of Path.

Path
CapabilityPath		StoragePath
PrivatePath	PublicPath

Path Functions

fun toString(): String

Returns the string representation of the path.

let storagePath = /storage/path
storagePath.toString()  // is "/storage/path"

There are also utilities to produce paths from strings:

fun PublicPath(identifier: string): PublicPath?
fun PrivatePath(identifier: string): PrivatePath?
fun StoragePath(identifier: string): StoragePath?

Each of these functions take an identifier and produce a path of the appropriate domain:

let pathID = "foo"
let path = PublicPath(identifier: pathID) // is /public/foo

Account Storage API

Account storage is accessed through the following functions of AuthAccount. This means that any code that has access to the authorized account has access to all its stored objects.

fun save<T>(_ value: T, to: StoragePath)

Saves an object to account storage. Resources are moved into storage, and structures are copied.

T is the type parameter for the object type. It can be inferred from the argument's type.

If there is already an object stored under the given path, the program aborts.

The path must be a storage path, i.e., only the domain storage is allowed.

fun type(at: StoragePath): Type?

Reads the type of an object from the account's storage which is stored under the given path, or nil if no object is stored under the given path.

If there is an object stored, the type of the object is returned without modifying the stored object.

The path must be a storage path, i.e., only the domain storage is allowed

fun load<T>(from: StoragePath): T?

Loads an object from account storage. If no object is stored under the given path, the function returns nil. If there is an object stored, the stored resource or structure is moved out of storage and returned as an optional. When the function returns, the storage no longer contains an object under the given path.

T is the type parameter for the object type. A type argument for the parameter must be provided explicitly.

The type T must be a supertype of the type of the loaded object. If it is not, execution will abort with an error. The given type does not necessarily need to be exactly the same as the type of the loaded object.

The path must be a storage path, i.e., only the domain storage is allowed.

fun copy<T: AnyStruct>(from: StoragePath): T?

Returns a copy of a structure stored in account storage, without removing it from storage.

If no structure is stored under the given path, the function returns nil. If there is a structure stored, it is copied. The structure stays stored in storage after the function returns.

T is the type parameter for the structure type. A type argument for the parameter must be provided explicitly.

The type T must be a supertype of the type of the copied structure. If it is not, execution will abort with an error. The given type does not necessarily need to be exactly the same as the type of the copied structure.

The path must be a storage path, i.e., only the domain storage is allowed.

// Declare a resource named `Counter`.
//
resource Counter {
    pub var count: Int
    pub init(count: Int) {
        self.count = count
    }
}
// In this example an authorized account is available through the constant `authAccount`.
// Create a new instance of the resource type `Counter`
// and save it in the storage of the account.
//
// The path `/storage/counter` is used to refer to the stored value.
// Its identifier `counter` was chosen freely and could be something else.
//
authAccount.save(<-create Counter(count: 42), to: /storage/counter)
// Run-time error: Storage already contains an object under path `/storage/counter`
//
authAccount.save(<-create Counter(count: 123), to: /storage/counter)
// Load the `Counter` resource from storage path `/storage/counter`.
//
// The new constant `counter` has the type `Counter?`, i.e., it is an optional,
// and its value is the counter resource, that was saved at the beginning
// of the example.
//
let counter <- authAccount.load<@Counter>(from: /storage/counter)
// The storage is now empty, there is no longer an object stored
// under the path `/storage/counter`.
// Load the `Counter` resource again from storage path `/storage/counter`.
//
// The new constant `counter2` has the type `Counter?` and is `nil`,
// as nothing is stored under the path `/storage/counter` anymore,
// because the previous load moved the counter out of storage.
//
let counter2 <- authAccount.load<@Counter>(from: /storage/counter)
// Create another new instance of the resource type `Counter`
// and save it in the storage of the account.
//
// The path `/storage/otherCounter` is used to refer to the stored value.
//
authAccount.save(<-create Counter(count: 123), to: /storage/otherCounter)
// Load the `Vault` resource from storage path `/storage/otherCounter`.
//
// The new constant `vault` has the type `Vault?` and its value is `nil`,
// as there is a resource with type `Counter` stored under the path,
// which is not a subtype of the requested type `Vault`.
//
let vault <- authAccount.load<@Vault>(from: /storage/otherCounter)
// The storage still stores a `Counter` resource under the path `/storage/otherCounter`.
// Save the string "Hello, World" in storage
// under the path `/storage/helloWorldMessage`.
authAccount.save("Hello, world!", to: /storage/helloWorldMessage)
// Copy the stored message from storage.
//
// After the copy, the storage still stores the string under the path.
// Unlike `load`, `copy` does not remove the object from storage.
//
let message = authAccount.copy<String>(from: /storage/helloWorldMessage)
// Create a new instance of the resource type `Vault`
// and save it in the storage of the account.
//
authAccount.save(<-createEmptyVault(), to: /storage/vault)
// Invalid: Cannot copy a resource, as this would allow arbitrary duplication.
//
let vault <- authAccount.copy<@Vault>(from: /storage/vault)

As it is convenient to work with objects in storage without having to move them out of storage, as it is necessary for resources, it is also possible to create references to objects in storage: This is possible using the borrow function of an AuthAccount:

fun borrow<T: &Any>(from: StoragePath): T?

Returns a reference to an object in storage without removing it from storage. If no object is stored under the given path, the function returns nil. If there is an object stored, a reference is returned as an optional.

T is the type parameter for the object type. A type argument for the parameter must be provided explicitly. The type argument must be a reference to any type (&Any; Any is the supertype of all types). It must be possible to create the given reference type T for the stored / borrowed object. If it is not, execution will abort with an error. The given type does not necessarily need to be exactly the same as the type of the borrowed object.

The path must be a storage path, i.e., only the domain storage is allowed.

// Declare a resource interface named `HasCount`, that has a field `count`
//
resource interface HasCount {
    count: Int
}
// Declare a resource named `Counter` that conforms to `HasCount`
//
resource Counter: HasCount {
    pub var count: Int
    pub init(count: Int) {
        self.count = count
    }
}
// In this example an authorized account is available through the constant `authAccount`.
// Create a new instance of the resource type `Counter`
// and save it in the storage of the account.
//
// The path `/storage/counter` is used to refer to the stored value.
// Its identifier `counter` was chosen freely and could be something else.
//
authAccount.save(<-create Counter(count: 42), to: /storage/counter)
// Create a reference to the object stored under path `/storage/counter`,
// typed as `&Counter`.
//
// `counterRef` has type `&Counter?` and is a valid reference, i.e. non-`nil`,
// because the borrow succeeded:
//
// There is an object stored under path `/storage/counter`
// and it has type `Counter`, so it can be borrowed as `&Counter`
//
let counterRef = authAccount.borrow<&Counter>(from: /storage/counter)
counterRef?.count // is `42`
// Create a reference to the object stored under path `/storage/counter`,
// typed as `&{HasCount}`.
//
// `hasCountRef` is non-`nil`, as there is an object stored under path `/storage/counter`,
// and the stored value of type `Counter` conforms to the requested type `{HasCount}`:
// the type `Counter` implements the restricted type's restriction `HasCount`
let hasCountRef = authAccount.borrow<&{HasCount}>(from: /storage/counter)
// Create a reference to the object stored under path `/storage/counter`,
// typed as `&{SomethingElse}`.
//
// `otherRef` is `nil`, as there is an object stored under path `/storage/counter`,
// but the stored value of type `Counter` does not conform to the requested type `{Other}`:
// the type `Counter` does not implement the restricted type's restriction `Other`
let otherRef = authAccount.borrow<&{Other}>(from: /storage/counter)
// Create a reference to the object stored under path `/storage/nonExistent`,
// typed as `&{HasCount}`.
//
// `nonExistentRef` is `nil`, as there is nothing stored under path `/storage/nonExistent`
//
let nonExistentRef = authAccount.borrow<&{HasCount}>(from: /storage/nonExistent)

Storage Iteration

It is possible to iterate over an account's storage using the following iteration functions:

fun forEachPublic(_ function: ((PublicPath, Type): Bool))
fun forEachPrivate(_ function: ((PrivatePath, Type): Bool))
fun forEachStored(_ function: ((StoragePath, Type): Bool))

Each of these iterates over every element in the specified domain (public, private, and storage), applying the function argument to each. The first argument of the function is the path of the element, and the second is its runtime type. In the case of the private and public path iteration functions, this is the runtime type of the capability linked at that path. The Bool return value determines whether iteration continues; true will proceed to the next stored element, while false will terminate iteration. The specific order in which the objects are iterated over is undefined, as is the behavior when a path is added or removed from storage.

warning

The order of iteration is undefined. Do not rely on any particular behaviour.

Saving to or removing from storage during iteration can cause the order in which values are stored to change arbitrarily.

Continuing to iterate after such an operation will cause Cadence to panic and abort execution. In order to avoid such errors, we recommend not modifying storage during iteration. If you do, return false from the iteration callback to cause iteration to end after the mutation like so:

account.save(1, to: /storage/foo1)
account.save(2, to: /storage/foo2)
account.save(3, to: /storage/foo3)
account.save("qux", to: /storage/foo4)
account.forEachStored(fun (path: StoragePath, type: Type): Bool {
    if type == Type<String>() {
        account.save("bar", to: /storage/foo5)
        // returning false here ends iteration after storage is modified, preventing a panic
        return false
    }
    return true
})

info

The iteration will skip any broken elements in the storage. An element could be broken due to invalid types associated with the stored value. e.g: A value belongs to type T of a contract with syntax/semantic errors.

Storage limit

An account's storage is limited by its storage capacity.

An account's storage used is the sum of the size of all the data that is stored in an account (in MB). An account's storage capacity is a value that is calculated from the amount of FLOW that is stored in the account's main FLOW token vault.

At the end of every transaction, the storage used is compared to the storage capacity. For all accounts involved in the transaction, if the account's storage used is greater than its storage capacity, the transaction will fail.

An account's storage used and storage capacity can be checked using the storageUsed and storageCapacity fields. The fields represent current values of storage which means this would be true:

let storageUsedBefore = authAccount.storageUsed
authAccount.save(<-create Counter(count: 123), to: /storage/counter)
let storageUsedAfter = authAccount.storageUsed
let storageUsedChanged = storageUsedBefore != storageUsedAfter // is true

Account Storage​

Paths​

Path Functions​

Account Storage API​

Storage Iteration​

Storage limit​

Account Storage

Paths

Path Functions

Account Storage API

Storage Iteration

Storage limit