Cache Multiple Calls

pallets/storage-cache

Calls to runtime storage have an associated cost and developers should strive to minimize the number of calls.

decl_storage! {
	trait Store for Module<T: Config> as StorageCache {
		// copy type
		SomeCopyValue get(fn some_copy_value): u32;

		// clone type
		KingMember get(fn king_member): T::AccountId;
		GroupMembers get(fn group_members): Vec<T::AccountId>;
	}
}

Copy Types

For Copy types, it is easy to reuse previous storage calls by simply reusing the value, which is automatically cloned upon reuse. In the code below, the second call is unnecessary:

fn increase_value_no_cache(origin, some_val: u32) -> DispatchResult {
	let _ = ensure_signed(origin)?;
	let original_call = <SomeCopyValue>::get();
	let some_calculation = original_call.checked_add(some_val).ok_or("addition overflowed1")?;
	// this next storage call is unnecessary and is wasteful
	let unnecessary_call = <SomeCopyValue>::get();
	// should've just used `original_call` here because u32 is copy
	let another_calculation = some_calculation.checked_add(unnecessary_call).ok_or("addition overflowed2")?;
	<SomeCopyValue>::put(another_calculation);
	let now = <frame_system::Module<T>>::block_number();
	Self::deposit_event(RawEvent::InefficientValueChange(another_calculation, now));
	Ok(())
}

Instead, the initial call value should be reused. In this example, the SomeCopyValue value is Copy so we should prefer the following code without the unnecessary second call to storage:

fn increase_value_w_copy(origin, some_val: u32) -> DispatchResult {
	let _ = ensure_signed(origin)?;
	let original_call = <SomeCopyValue>::get();
	let some_calculation = original_call.checked_add(some_val).ok_or("addition overflowed1")?;
	// uses the original_call because u32 is copy
	let another_calculation = some_calculation.checked_add(original_call).ok_or("addition overflowed2")?;
	<SomeCopyValue>::put(another_calculation);
	let now = <frame_system::Module<T>>::block_number();
	Self::deposit_event(RawEvent::BetterValueChange(another_calculation, now));
	Ok(())
}

Clone Types

If the type was not Copy, but was Clone, then it is still better to clone the value in the method than to make another call to runtime storage.

The runtime methods enable the calling account to swap the T::AccountId value in storage if

1. the existing storage value is not in GroupMembers AND
2. the calling account is in GroupMembers

The first implementation makes a second unnecessary call to runtime storage instead of cloning the call for existing_key:

fn swap_king_no_cache(origin) -> DispatchResult {
	let new_king = ensure_signed(origin)?;
	let existing_king = <KingMember<T>>::get();

	// only places a new account if
	// (1) the existing account is not a member &&
	// (2) the new account is a member
	ensure!(!Self::is_member(&existing_king), "current king is a member so maintains priority");
	ensure!(Self::is_member(&new_king), "new king is not a member so doesn't get priority");

	// BAD (unnecessary) storage call
	let old_king = <KingMember<T>>::get();
	// place new king
	<KingMember<T>>::put(new_king.clone());

	Self::deposit_event(RawEvent::InefficientKingSwap(old_king, new_king));
	Ok(())
}

If the existing_key is used without a clone in the event emission instead of old_king, then the compiler returns the following error:

error[E0382]: use of moved value: `existing_king`
  --> src/lib.rs:93:63
   |
80 |             let existing_king = <KingMember<T>>::get();
   |                 ------------- move occurs because `existing_king` has type `<T as frame_system::Config>::AccountId`, which does not implement the `Copy` trait
...
85 |             ensure!(!Self::is_member(existing_king), "is a member so maintains priority");
   |                                      ------------- value moved here
...
93 |             Self::deposit_event(RawEvent::InefficientKingSwap(existing_king, new_king));
   |                                                               ^^^^^^^^^^^^^ value used here after move

error: aborting due to previous error

For more information about this error, try `rustc --explain E0382`.
error: Could not compile `storage-cache`.

To learn more, run the command again with --verbose.

Fixing this only requires cloning the original value before it is moved:

fn swap_king_with_cache(origin) -> DispatchResult {
	let new_king = ensure_signed(origin)?;
	let existing_king = <KingMember<T>>::get();
	// prefer to clone previous call rather than repeat call unnecessarily
	let old_king = existing_king.clone();

	// only places a new account if
	// (1) the existing account is not a member &&
	// (2) the new account is a member
	ensure!(!Self::is_member(&existing_king), "current king is a member so maintains priority");
	ensure!(Self::is_member(&new_king), "new king is not a member so doesn't get priority");

	// <no (unnecessary) storage call here>
	// place new king
	<KingMember<T>>::put(new_king.clone());

	Self::deposit_event(RawEvent::BetterKingSwap(old_king, new_king));
	Ok(())
}

Not all types implement Copy or Clone, so it is important to discern other patterns that minimize and alleviate the cost of calls to storage.