Data Storage / Collections · NEAR Documentation

Overview

All data stored on the NEAR blockchain is done in key / value pairs. There are several collection methods in the SDKs we've created that will help you store your data on chain.

near-sdk-as for AssemblyScript smart contracts

near-sdk-rs for Rust smart contracts

For information on storage costs, please see [ storage staking ].

AssemblyScript Collection Types

near-sdk-as

Type	Iterable	Clear All Values	Preserves Insertion Order	Range Selection
`PersistentVector`	✘	✘	✘	✘
`PersistentSet`	✘	✘	✘	✘
`PersistentMap`
`PersistentUnorderedMap`	✘	✘	✘	✘
`PersistentDeque`		✘		✘
`AVLTree`	✘	✘

Big-O Notation

The Big-O notation values in the chart below describe the time complexity of the various collection methods found in near-sdk-as. These method complexities correlate with gas consumption on NEAR, helping you decide which collection to utilize in your project. There are three types found in our collection methods:

O(1) - constant
O(n) - linear
O(log n) - logarithmic

Type	Access	Insert	Delete	Search	Traverse	Clear
`PersistentVector`	O(1)	O(1)*	O(1)**	O(n)	O(n)	O(n)
`PersistentSet`	O(1)	O(1)	O(1)	O(1)	O(n)	O(n)
`PersistentMap`	O(1)	O(1)	O(1)	O(1)	N/A	N/A
`PersistentUnorderedMap`	O(1)	O(1)	O(1)	O(1)	O(n)	O(n)
`PersistentDeque`	O(1)	O(1)*	O(1)**	O(1)	O(n)	O(n)
`AVLTree`	O(log n)	O(log n)	O(log n)	O(log n)	O(n)	O(n)

* - to insert at the end of the vector using push_back (or push_front for deque)

** - to delete from the end of the vector using pop (or pop_front for deque), or delete using swap_remove which swaps the element with the last element of the vector and then removes it.

Gas Consumption Examples

The examples below show differences in gas burnt storing and retrieving key/value pairs using the above methods. Please note that the gas cost of spinning up the runtime environment on chain has been deducted to show just data read/writes.

You can reproduce this and test out your own data set by visiting collection-examples-as.

AssemblyScript Set Data Gas Chart

AssemblyScript Get Data Gas Chart

`PersistentVector`

Implements a vector / persistent array built on top of the Storage class.

Uses the following map: index -> element.

To create:

import { PersistentVector } from "near-sdk-as";
let vector = new PersistentVector<string>("v"); // choose a unique prefix per collection

To use:

vector.push(value);
vector.pop(value);
vector.length;

[ SDK source ]

`PersistentSet`

Built on top of the Storage class, this implements a persistent set without iterators.

is not iterable

more efficient in the number of reads and writes

[ SDK source ]

`PersistentMap`

Implements a map built on top of the Storage.

To create:

import { PersistentMap } from "near-sdk-as";
let map = new PersistentMap<string, string>("pmap"); // choose a unique prefix per collection

To use:

map.set(key, value);
map.getSome(key);

Note: The Map doesn't store keys, so if you need to retrieve them, include keys in the values.

[ SDK source ]

`PersistentUnorderedMap`

Implements an unordered map built on top of the PersistentMap class and a PersistentVector, which stores the keys of the map. These keys are initially in the order they are inserted, however, a deleted key is swapped with the last key.

To create:

import { PersistentUnorderedMap } from "near-sdk-as";
let map = new PersistentUnorderedMap<string, string>("umap"); // note the prefix must be unique per collection

To use:

map.set(key, value);
map.getSome(key);

[ SDK source ]

`PersistentDeque`

Built on top of the Storage class, this implements a persistent bidirectional queue / double-ended queue / deque.

To create:

import { PersistentDeque } from "near-sdk-as";
let dq = new PersistentDeque<string>("deque"); // choose a unique prefix per collection

To use:

dq.pushFront(value);
dq.popBack();

[ SDK source ]

`AVLTree`

Implements a Tree Map based on AVL-tree Keys are ordered and iterable.

To create:

import { AVLTree } from "near-sdk-as";
map = new AVLTree<string, string>("avl"); // choose a unique prefix per account

To use:

map.set(key, value);
map.getSome(key)

[ SDK source ]

Rust Collection Types

near-sdk-rs module documentation

Type	Iterable	Clear All Values	Preserves Insertion Order	Range Selection
`Vector`	✘	✘	✘	✘
`LookupSet`
`UnorderedSet`	✘		✘	✘
`LookupMap`
`UnorderedMap`	✘	✘	✘	✘
`TreeMap`	✘	✘

Big-O Notation

The Big-O notation values in the chart below describe the time complexity of the various collection methods found in near-sdk-rs. These method complexities correlate with gas consumption on NEAR, helping you decide which collection to utilize in your project. There are three types found in our collection methods:

O(1) - constant
O(n) - linear
O(log n) - logarithmic

Type	Access	Insert	Delete	Search	Traverse	Clear
`Vector`	O(1)	O(1)*	O(1)**	O(n)	O(n)	O(n)
`LookupSet`	O(1)	O(1)	O(1)	O(1)	N/A	N/A
`UnorderedSet`	O(1)	O(1)	O(1)	O(1)	O(n)	O(n)
`LookupMap`	O(1)	O(1)	O(1)	O(1)	N/A	N/A
`UnorderedMap`	O(1)	O(1)	O(1)	O(1)	O(n)	O(n)
`TreeMap`	O(log n)	O(log n)	O(log n)	O(log n)	O(n)	O(n)

* - to insert at the end of the vector using push_back (or push_front for deque)

** - to delete from the end of the vector using pop (or pop_front for deque), or delete using swap_remove which swaps the element with the last element of the vector and then removes it.

Gas Consumption Examples

The examples below show differences in gas burnt storing and retrieving key/value pairs using the above methods. Please note that the gas cost of spinning up the runtime environment on chain has been deducted to show just data read/writes.

You can reproduce this and test out your own data set by visiting collection-examples-rs.

Rust Set Data Gas Chart

Rust Get Data Gas Chart

`Vector`

Implements a vector / persistent array.

can iterate using index

Uses the following map: index -> element.

[ SDK source ]

[ Implementation ]

`LookupSet`

Implements a persistent set without iterators.

can not iterate over keys

more efficient in reads / writes

[ SDK source ]

[ Implementation ]

`UnorderedSet`

Implements a persistent set with iterators for keys, values, and entries.

[ SDK source ]

[ Implementation Docs ]

`LookupMap`

Implements an persistent map.

can not iterate over keys

does not preserve order when removing and adding values

efficient in number of reads and writes

To add data:

pub fn add_lookup_map(&mut self, key: String, value: String) {
    self.lookup_map.insert(&key, &value);
}

To get data:

pub fn get_lookup_map(&self, key: String) -> String {
    match self.lookup_map.get(&key) {
        Some(value) => {
            let log_message = format!("Value from LookupMap is {:?}", value.clone());
            env::log(log_message.as_bytes());
            value
        },
        None => "not found".to_string()
    }
}

[ SDK source ]

[ Implementation ]

`UnorderedMap`

Implements an unordered map.

iterable

does not preserve order when removing and adding values

is able to clear all values

To add data:

pub fn add_unordered_map(&mut self, key: String, value: String) {
    self.unordered_map.insert(&key, &value);
}

To get data:

pub fn get_unordered_map(&self, key: String) -> String {
    match self.unordered_map.get(&key) {
        Some(value) => {
            let log_message = format!("Value from UnorderedMap is {:?}", value.clone());
            env::log(log_message.as_bytes());
            value
        },
        // None => "Didn't find that key.".to_string()
        None => "not found".to_string()
    }
}

[ SDK source ]

[ Implementation ]

`TreeMap`

Implements a Tree Map based on AVL-tree.

iterable

preserves order

able to clear all values

self balancing

To add data:

pub fn add_tree_map(&mut self, key: String, value: String) {
    self.tree_map.insert(&key, &value);
}

To get data:

pub fn get_tree_map(&self, key: String) -> String {
    match self.tree_map.get(&key) {
        Some(value) => {
            let log_message = format!("Value from TreeMap is {:?}", value.clone());
            env::log(log_message.as_bytes());
            // Since we found it, return it (note implicit return)
            value
        },
        // did not find the entry
        // note: curly brackets after arrow are optional in simple cases, like other languages
        None => "not found".to_string()
    }
}

[ SDK source ]

[ Implementation ]