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When building a custom pallet, storage of some kind will be needed. As with any other application, we need a place to store information after submitting state changes to the network. Whether we’re looking to store a simple value or complex data structures with mappings, a set of APIs are provided to implement it accordingly. Because the state is stored within this storage and replicated across the network, it is essential to realize that storage via blockchain is considered costly and should be used wisely by any pallet in the runtime.

Fun Fact: Polkadot and Kusama's Runtime is 1.5 MB

The runtime itself is stored using this storage instance of the Substrate node under a unique key within the underlying key-value database. The entirety of the runtime's Wasm code can be accessed on-chain.

FRAME storage items allow a developer to define and store new items in our blockchain’s state. Substrate's underlying sp_io is responsible, where they make calls accordingly to the storage layer of Substrate.

Substrate Storage Overview

Although FRAME makes declaring and using different storage primitives trivial, it's important to understand some context for what is occurring at a slightly lower level.

If you recall, the sp_io crate handles any needed I/O operations for the runtime. In a theoretical scenario (such as FRAME not being used or available), sp_io's underlying APIs could be called directly. This crate is not just simply logging to some arbitrary database - there are several layers to the storage of a Substrate node. This is out of the scope of this course, but it is prudent to know just how abstract some of the mechanisms in Substrate are.

FRAME Storage Abstraction

There are several ways to declare storage using FRAME. These are done by using several storage items, which are summarized below:

  • StorageMap - A type that stores value for a given key. Allowing to insert/remove/iterate on values.
  • StorageValue - A type that allows storing a single value.
  • StorageDoubleMap - A type that allows storing values for (key1, key2) couple. Similar to StorageMap, but allow to iterate and remove value associated with the first key.
  • StorageNMap - A type that allows to store values for an arbitrary number of keys in the form of (Key<Hasher1, key1>, Key<Hasher2, key2>, ..., Key<HasherN, keyN>).

For more information on their usage and APIs, look at the Substrate Rust documentation for further info.

Further in the course, you will see StorageMap and StorageValue being utilized.