Substrate Storage Deep Dive Workshop/Tutorial

[shawntabrizi]

There are a lot of tricks and nuances to using storage in Substrate.

• Supported storage types (found here)

  • StorageValue
  • StorageMap
  • StorageDoubleMap
  • HashedStorage/UnhashedStorage (This may be just an implementation detail we should briefly talk about, but not go deep into)
  • LinkedMap/EnumerableStorageMap
  • other...?

• Implications of using different storage types for efficiency

• Using Vec, implications of letting users input the value for this

• Storage API: get, put, insert, take, append, etc...

• Storing Structs (advance things)

• Storage Upgrades or Migrations, how and when it is possible

• Charging fees for users storing items

• Genesis configuration, default value, "add extra genesis"

• decl_storage macro in general

• using the Polkadot UI for a number of different storage scenarios

• more?

@thiolliere may be able to help us here with the details of some of these things if he is so kind :)

[kianenigma]

I'd be happy to help with it but probably not as the main owner of the task.

An important idea worth noting: Both Indices and Council use a chunked map to deal with long Vecs. I've made an issue to standardize it but might be a while until anyone does that (if ever). Until then, I think introducing this to our users is super important.

Another one that I've learned the hard way: StorageDoubleMap vs literally storing a map (x, y) => value.

[shawntabrizi]

@Kianenigma can you share a code sample to get an idea of what you mean?

[kianenigma]

@Kianenigma can you share a code sample to get an idea of what you mean?

In cases where you have a very large vec, and you access indexes individually (no need to iterate all of them etc.), you can chunck the Vec into Sets (of 64 element per se) and fetch one set from mem. Indexing would simply be:

const SET_SIZE: u32 = 64;
decl_storage {  BigList map u32 => Vec<X> }

set_idx = global_idx / SET_SIZE;
vec_idxe = global_idx % SET_SIZE;
let set = Self::big_list(set_idx);
let target_value = set[vec_idx];

inserting and removing needs a bit bookkeeping though; one thing for sure is that you probably want a new storage item all the time that points to the next free set. Both Voters and ApprovalsOf have the same deal in council.

[ltfschoen]

And continue the conversation about off-chain storage, as we've already done here:

• https://docs.substrate.dev/docs/building-an-event-based-off-chain-storage
• https://docs.substrate.dev/discuss/5c9221c83276e500457b6314

[JoshOrndorff]

AFAIK, storing enums is not documented anywhere. TIL

// Can't derive Default like you can for structs.
#[derive(Encode, Decode, Clone, PartialEq)]
#[cfg_attr(feature = "std", derive(Debug))]
pub enum Status {
    Active,
    Sold,
    SellerReviewed,
    BuyerReviewed,
}

// Without a manual implementation, you can't use the enum as a storage value
impl Default for Status {
    fn default() -> Self {
        Self::Active
    }
}

[bkchr]

You can store it without a default. Either you specify the default yourself in the decl storage line or you store it as an optional.