Index

When rolling out your own storage strategy — which virtually all smart contract developers do because they don’t use Store yet 🙃 — you must decide how to implement “indices”. These data structures can be found in most popular contracts. As an example, the balances mapping of the ERC-721 spec is an index: it is a derived state of the owners mapping: every time the owner of an ERC-721 token changes, the balances must be updated on both side.

Maintaining derived state up-to-date is usually done by putting indexing logic in functions that change their underlying state. As an example, most implementation of the ERC-721 specification add mutation logic to change the balances index in a transfer helper that also changes the ownership of the token.

// from the OZ ERC-721 contract
// https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol)
function _transfer(address from, address to, uint256 tokenId) internal virtual {
    [...]
    unchecked {
        _balances[from] -= 1;
        _balances[to] += 1;
	}
	_owners[tokenId] = to;
    [...]
}

This scales badly in terms of complexity: derived state cannot be added (or removed) after contract deployment, and one must be careful to only update state using internal functions that maintain their derived state up to date. If you were to change the _owners mapping directly, the balances would be incorrect.

Store exposes table hooks to make derived state less error-prone and more maintainable: logic can be registered — even at runtime — to run when records or fields of records are updated within a specific table. Store uses this strategy to expose simple onchain indices to developers, but it is also possible to design custom indexers with that pattern (Quad Trees, B-Trees, etc).