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Key-Value Storage for contracts

An alternative way for storing state for contracts.


Consider this as an experimental feature (though already used by our own contracts), with some constraints (see below).


Imagine a PST contract with millions of entries in the 'balances' map. With the traditional way of storing a state - a change for one entry, requires passing to contract the whole state (the json) and then storing/caching it again - with only this one change. This is a big issue both from the storage perspective (we have to cache very similar, very big json's) and performance perspective - as the state needs to be deep-copied before each interaction - to assure a transactional processing (i.e. either all changes from a given interactions should be applied - or none, in case the contract would throw an Error).


An alternative way of storing a contract state - via a Key-Value storage (backed-up by default by the LevelDB) has been introduced. This allows to store and/or retrieve only required parts of the state within any given interaction - instead of having to process the whole BIG json each time.


The SmartWeave 'global' object has 2 new methods added:

  1. SmartWeave.kv.put(key, value) - allows to store a value (e.g. a wallet balance)
  2. SmartWeave.kv.get(key) - allows to retrieve a value from the storage by its key

The Contract interface has a new method added: getStorageValues(keys: string[]): Promise<SortKeyCacheResult<Map<string, any>>> - which allows to check the latest values for the passed array of keys.


All the changes in the KV storage done within one given interaction are atomic - either all will be applied, or none (in case of an ContractError) - this is handled by the commit/rollback mechanism.

Enabling KV storage for contract

In order to use a KV storage in the contract, either:

  1. the contract's manifest must be specified during deployment - with a useKVStorage option set to true:
await warp.deploy({
initState: JSON.stringify(initialState),
src: contractSrc,
evaluationManifest: {
evaluationOptions: {
useKVStorage: true
  1. the evaluationOptions used for contract evaluation must have the useKVStorage option set true, e.g.:
useKVStorage: true
}) as PstContract

With the KV Storage, the 'traditional', json-based state can be still used for storing some of the data (e.g. the name and the ticker of the PST)

Example usage in the contract

Examples of using the storage in the contract code (in a standard, PST transfer method):

  if (input.function === 'transfer') {
// NOTE: initial validations are skipped for code brevity
const target =;
const qty = input.qty;

// NOTE: getting a value from the KV storage
let callerBalance = await SmartWeave.kv.get(caller);
callerBalance = callerBalance ? callerBalance : 0;

if (callerBalance < qty) {
throw new ContractError(`Caller balance not high enough to send ${qty} token(s)!`);

callerBalance -= qty;
// NOTE: upserting a value in the KV storage
await SmartWeave.kv.put(caller, callerBalance);

let targetBalance = await SmartWeave.kv.get(target);
targetBalance = targetBalance ? targetBalance : 0;

targetBalance += qty;
await SmartWeave.kv.put(target, targetBalance);

return {state};

The full contract example is available here:

Example of retrieving the values via SDK

const result = (await contract.getStorageValues(['voo', 'doo'])).cachedValue;
const fooValue = result.get('voo');
const dooValue = result.get('doo');



Currently - when using KV storage - no interactions with foreign contracts are allowed. Such support will be added in the future PRs.

Implementation details

  1. the 'put' method adds an entry to the array. All entries from this array are 'committed' to the underneath storage only when SDK verifies that the result of the given interaction is successful.
  2. all entries have the sortKey added to the the stored key (the sortKey is taken from the SmartWeave._activeTx).
  3. The underlying storage is an implementation of the SDKs SortKeyCache interface. By default it is using the LevelDB (with a file based storage for node.js env and IndexedDB storage for browser env)
  4. Any storage compatible with SortKeyCache interface can be used (e.g. In order to configure custom storage - use the warp.useKVStorageFactory method, e.g.:
.useKVStorageFactory((contractTxId) => new LmdbCache({
dbLocation: `./cache/warp/kv/lmdb_2/${contractTxId}`

The cache for each contract has to be stored in a separate db file - that's why the useKVStorageFactory has the contractTxId as an argument.