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Key-Value Storage for contracts

An alternative way for storing state for contracts.


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 the following methods:

  1. SmartWeave.kv.put(key, value) - allows to store a value (e.g. a wallet balance)
  2. SmartWeave.kv.get(key) - allows to retrieve the latest value from the storage by its key
  3. SmartWeave.kv.del(key) - allows to delete the value from the storage by its key. The value is not actually deleted, but marked as such. That means that the value will be readable for contracts interacting on lower sortKey.
  4. SmartWeave.kv.keys(options: SortKeyCacheRangeOptions) - allows to fetch kv storage keys for a specified range
  5. SmartWeave.kv.kvMap(options: SortKeyCacheRangeOptions) - allows to fetch key value map for a specified range

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.


The KV storage transaction mechanism has changed. In order to meet the new requirements we introduced a new range key value access functionality. It is now possible to access keys and values by a specified range and limit. These changes are possible thanks to a new commit/rollback approach. Active transaction stores items directly to the underlying kv storage. This allows to use more advanced kv storage functionality. In case an interaction fails the rollback batch is executed to restore the storage state from before the failed interacion.

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 kv entries for a specified range


In order to take the full of advantage of the kv storage, consider writing view functions in your contract. This will enable an easy and fast access to the current kv state without any contract's state changes.

let partialSum = 0;
for await (let part of (await SmartWeave.kv.kvMap({ gte: 'pref.', lte: 'pref.\xff'})).values()) {
partialSum = partialSum + parseInt(part);

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');

Implementation details

  1. both 'put' and 'del' methods create new entries in kv storage under a current transaction sort key. In case of an error, entries inserted during active transaction are removed.
  2. all entries have the sortKey added to 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.