Overview

There are multiple different implementations of key-value maps inside the framework, suited for different usecases. We will go over their differences and similarities, and how to choose which one to use.

Aptos Blockchain performance and gas cost considerations

Aptos Blockchain state is managed through on-chain resources. Furthermore, transaction performance and gas cost is heavily influenced by how these resources are read and written. Breaking down the gas costs further, we have:

1. Storage fee, which are determined by the number and size of resources (i.e., writing to a new resource incurs the highest storage fee, whereas deleting an existing resource provides the largest refund.)
2. IO gas costs —generally much lower— which depend on the number and size of resources read and modified.
3. execution gas costs are based on the computation needed, and are generally in the similar scale as io gas costs.

Transactions that modify the same resource cannot be executed in parallel, as they conflict with one another.

One useful analogy is thinking about each resource being a file on a disk, then performance of smart contract would correlate well to a program that operates on files in the same way.

Different Map implementations

Note:

• SimpleMap has been deprecated, and replaced with OrderedMap.
• SmartTable has been deprecated, and replaced with BigOrderedMap.

Performance comparison

We measured performance at small scale, measuring microseconds taken for a single pair of insert + remove operation, into a map of varied size.

You can see that overhead of BigOrderedMap compared to OrderedMap, when both are in the single resource, is around 1.5-2x. So you can generally used BigOrdredMap when it is unknown if data will be too large to be stored in a single resource.x

Common map operations:

Most maps above support the same set of functions (for actual signatures and restrictions, check out the corresponding implementations):

Creating Maps

• new<K, V>(): Self: creates an empty map

Destroying Maps

All except Table support:

• destroy_empty<K, V>(self: Self<K, V>): Destroys an empty map. (not supported by Table)
• destroy<K, V>(self: Self<K, V>, dk: |K|, dv: |V|): Destroys a map with given functions that destroy correponding elements. (not supported by Table and TableWithLength)

Managing Entries

• add<K, V>(self: &mut Self<K, V>, key: K, value: V): Adds a key-value pair to the map.
• remove<K, V>(self: &mut Self<K, V>, key: K): V: Removes and returns the value associated with a key.
• upsert<K, V>(self: &mut Self<K, V>, key: K, value: V): Option<V>: Inserts or updates a key-value pair.
• add_all<K, V>(self: &mut Self<K, V>, keys: vector<K>, values: vector<V>): Adds multiple key-value pairs to the map. (not supported by Table and TableWithLength)

Retrieving Entries

• contains<K, V>(self: &Self<K, V>, key: &K): bool: Checks whether key exists in the map.
• borrow<K, V>(self: &Self<K, V>, key: &K): &V: Returns an immutable reference to the value associated with a key.
• borrow_mut<K: drop, V>(self: &mut Self<K, V>, key: K): &mut V: Returns a mutable reference to the value associated with a key. (BigOrderedMap only allows borrow_mut when value type has a static constant size, due to modification being able to break it’s invariants otherwise. Use remove() and add() combination instead)

Order-dependant functions

These set of functions are only implemented by OrderedMap and BigOrderedMap.

• borrow_front<K, V>(self: &Self<K, V>): (&K, &V)
• borrow_back<K, V>(self: &Self<K, V>): (&K, &V)
• pop_front<K, V>(self: &mut Self<K, V>): (K, V)
• pop_back<K, V>(self: &mut Self<K, V>): (K, V)
• prev_key<K: copy, V>(self: &Self<K, V>, key: &K): Option<K>
• next_key<K: copy, V>(self: &Self<K, V>, key: &K): Option<K>

Utility Functions

• length<K, V>(self: &Self<K, V>): u64: Returns the number of entries in the map. (not supported by Table)

Traversal Functions

These set of functions are not implemented by Table and TableWithLength.

• keys<K: copy, V>(self: &Self<K, V>): vector<K>

• values<K, V: copy>(self: &Self<K, V>): vector<V>

• to_vec_pair<K, V>(self: Self<K, V>): (vector<K>, vector<V>)

• for_each_ref<K, V>(self: &Self<K, V>, f: |&K, &V|)

• to_ordered_map<K, V>(self: &BigOrderedMap<K, V>): OrderedMap<K, V>: Converts BigOrderedMap into OrderedMap

Example Usage

Creating and Using a OrderedMap

module 0x42::map_usage {
    use aptos_framework::ordered_map;
 
    public entry fun main() {
        let map = ordeded_map::new<u64, u64>();
        map.add(1, 100);
        map.add(2, 200);
 
        let length = map.length();
        assert!(length == 2, 0);
 
        let value1 = map.borrow(&1);
        assert!(*value1 == 100, 0);
 
        let value2 = map.borrow(&2);
        assert!(*value2 == 200, 0);
 
        let removed_value = map.remove(&1);
        assert!(removed_value == 100, 0);
 
        map.destroy_empty();
    }
}

Additional details for BigOrderedMap

Its current implementation is B+ tree, which is chosen as it is best suited for the onchain storage layout - where the majority of cost comes from loading and writing to storage items, and there is no partial read/write of them.

Implementation has few characteristics that make it very versatile and useful across wide range of usecases:

• When it has few elements, it stores all of them within the resource that contains it, providing comparable performance to OrderedMap itself, while then dynamically growing to multiple resources as more and more elements are added
• It reduces amount of conflicts: modifications to a different part of the key-space are generally parallel, and it provides knobs for tuning between parallelism and size
• All operations have guaranteed upper-bounds on performance (how long they take, as well as how much execution and io gas they consume), allowing for safe usage across a variety of use cases.
  • One caveat, is refundable storage fee. By default, operation that requires map to grow to more resources needs to pay for storage fee for it. Implementation here has an option to pre-pay for storage slots, and to reuse them as elements are added/removed, allowing applications to achieve fully predictable overall gas charges, if needed.
• If key/value is within the size limits map was configured with, inserts will never fail unpredictably, as map internally understands and manages maximal resource size limits.

Creating BigOrderedMap

Because it’s layout affects what can be inserted and performance, there are a few ways to create and configure it:

• new<K, V>(): Self<K, V>: Returns a new BigOrderedMap with the default configuration. Only allowed to be called with constant size types. For variable sized types, another constructor is needed, to explicitly select automatic or specific degree selection.

• new_with_type_size_hints<K, V>(avg_key_bytes: u64, max_key_bytes: u64, avg_value_bytes: u64, max_value_bytes: u64): Self<K, V>: Returns a map that is configured to perform best when keys and values are of given avg sizes, and guarantees to fit elements up to given max sizes.

• new_with_config<K, V>(inner_max_degree: u16, leaf_max_degree: u16, reuse_slots: bool): Self<K, V>: Returns a new BigOrderedMap with the provided max degree consts (the maximum # of children a node can have, both inner and leaf). If 0 is passed for either, then it is dynamically computed based on size of first key and value, and keys and values up to 100x times larger will be accepted. If non-0 is passed, sizes of all elements must respect (or their additions will be rejected):

  • key_size * inner_max_degree <= MAX_NODE_BYTES
  • entry_size * leaf_max_degree <= MAX_NODE_BYTES

  reuse_slots means that removing elements from the map doesn’t free the storage slots and returns the refund. Together with allocate_spare_slots, it allows to preallocate slots and have inserts have predictable gas costs. (otherwise, inserts that require map to add new nodes, cost significantly more, compared to the rest)

Source Code

• ordered_map.move
• table.move
• table_with_length.move
• big_ordered_map.move