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Understanding Dictionaries

In a Casper network, you can now store sets of data under Keys. Previously, URefs were the exclusive means by which users could store data in global state. To maintain persistent access to these URefs, they would have to be stored within an Account or Contract context. In the case of Contracts, sustained and continuous use of URefs would result in the expansion of the associated NamedKeys structures.

Individual value changes to data stored within the NamedKeys would require deserializing the entire NamedKeys data structure, increasing gas costs over time and thus having a negative impact. Additionally, users storing large subsets of mapped data structures would face the same deep copy problem where minor or single updates required the complete deserialization of the map structure, also leading to increased gas costs.

As a solution to this problem, the Casper platform provides the Dictionary feature, which allows users a more efficient and scalable means to aggregate data over time.

Seed URefs

Items within a dictionary exist as individual records stored underneath their unique dictionary address, or seed URef, in global state. In other words, items associated with a specific dictionary share the same seed URef but are otherwise independent of each other. Dictionary items are not stored beneath this URef, it is only used to create the dictionary key.

As each dictionary item exists as a stand-alone entity in global state, regularly used dictionary keys may be used directly without referencing their seed URef.

Creating Dictionaries

Dictionaries are ideal for storing larger volumes of data for which NamedKeys would be less suitable.

Creating a new dictionary is fairly simple and done within the context of a Deploy sent to a Casper network. The associated code is included within the casper_contract crate. Creating a dictionary also stores the associated seed URef within the named keys of the current context.


Developers should always consider context when creating dictionaries. We recommend creating a dictionary within the context of a Contract.

While you can create a dictionary in the context of an Account and then pass associated access rights to a Contract, this approach can create potential security issues. If a third party uses the Contract, the initiating Account with access rights to the dictionary may be undesirable. To rectify this, you may send an additional Deploy removing those access rights, but it is better to create the dictionary within the context of the Contract.

The following code snippet shows the most basic example of creating a dictionary.


The following example includes the creation of a dictionary known as LEDGER within a contract's function. In this instance, the dictionary will be used to track donations made to a fundraising purse also created by the init function.

pub extern "C" fn init() {
let fundraising_purse = system::create_purse();
runtime::put_key(FUNDRAISING_PURSE, fundraising_purse.into());
// Create a dictionary track the mapping of account hashes to number of donations made.

Writing Entries into a Dictionary

After the creation of a dictionary, you may then add entries through the use of the following code:

storage::dictionary_put(dictionary_uref, key, value);

The dictionary_uref refers to the seed URef established during the dictionary creation process. The key is the unique identifier for this dictionary item, and the value is the data to be stored within the dictionary item.

As stated above, these dictionary items do not require the seed URef, and they exist as individual keys in global state. If you know an individual key's address, you do not need to go through the process of identifying the seed URef first.

The following function serves to add an entry to the dictionary. If the item already exists, the function will update the value stored and referenced by that key. In this case, the code is storing the number of donations made.

fn update_ledger_record(dictionary_item_key: String) {
// Acquiring the LEDGER seed URef to properly assign the dictionary item.
let ledger_seed_uref = *runtime::get_key(LEDGER)

// This identifies an item within the dictionary and either creates or updates the associated value.
match storage::dictionary_get::<u64>(ledger_seed_uref, &dictionary_item_key).unwrap_or_revert()
None => storage::dictionary_put(ledger_seed_uref, &dictionary_item_key, 1u64),
Some(current_number_of_donations) => storage::dictionary_put(
current_number_of_donations + 1u64,

Accessing a Dictionary Item

The Casper platform provides four means of looking up a dictionary item. These means are explained within the DictionaryIdentifier JSON-RPC type. In brief, they consist of:

  • AccountNamedKey lookup via an Account's named keys.
  • ContractNamedKey lookup via a Contract's named keys.
  • URef lookup via the dictionary's seed URef.
  • Dictionary lookup via the unique dictionary item key.