A factory is a smart contract that stores a compiled contract on itself, and automatizes deploying it into sub-accounts.

This particular example presents a factory of donation contracts, and enables to:

1. Create a sub-account of the factory and deploy the stored contract on it (create_factory_subaccount_and_deploy).
2. Change the stored contract using the update_stored_contract method.

#[payable]
    pub fn create_factory_subaccount_and_deploy(
        &mut self,
        name: String,
        beneficiary: AccountId,
        public_key: Option<PublicKey>,
    ) -> Promise {
        // Assert the sub-account is valid
        let current_account = env::current_account_id().to_string();
        let subaccount: AccountId = format!("{name}.{current_account}").parse().unwrap();
        assert!(
            env::is_valid_account_id(subaccount.as_bytes()),
            "Invalid subaccount"
        );

        // Assert enough tokens are attached to create the account and deploy the contract
        let attached = env::attached_deposit();

        let code = self.code.clone().unwrap();
        let contract_bytes = code.len() as u128;
        let minimum_needed = NEAR_PER_STORAGE.saturating_mul(contract_bytes);
        assert!(
            attached >= minimum_needed,
            "Attach at least {minimum_needed} yⓃ"
        );

        let init_args = near_sdk::serde_json::to_vec(&DonationInitArgs { beneficiary }).unwrap();

        let mut promise = Promise::new(subaccount.clone())
            .create_account()
            .transfer(attached)
            .deploy_contract(code)
            .function_call(
                "init".to_owned(),
                init_args,
                NO_DEPOSIT,
                TGAS.saturating_mul(5),
            );

        // Add full access key is the user passes one
        if let Some(pk) = public_key {
            promise = promise.add_full_access_key(pk);
        }

        // Add callback
        promise.then(
            Self::ext(env::current_account_id()).create_factory_subaccount_and_deploy_callback(
                subaccount,
                env::predecessor_account_id(),
                attached,
            ),
        )
    }

Install cargo-near and run:

cargo near build

cargo test

Deployment is automated with GitHub Actions CI/CD pipeline. To deploy manually, install cargo-near and run:

cargo near deploy <account-id>

In this example we will be using NEAR CLI to intract with the NEAR blockchain and the smart contract

If you want full control over of your interactions we recommend using the near-cli-rs.

create_factory_subaccount_and_deploy will create a sub-account of the factory and deploy the stored contract on it.

near call <factory-account> create_factory_subaccount_and_deploy '{ "name": "sub", "beneficiary": "<account-to-be-beneficiary>"}' --deposit 1.24 --accountId <account-id> --gas 300000000000000

This will create the sub.<factory-account>, which will have a donation contract deployed on it:

near view sub.<factory-account> get_beneficiary
# expected response is: <account-to-be-beneficiary>

update_stored_contract enables to change the compiled contract that the factory stores.

The method is interesting because it has no declared parameters, and yet it takes an input: the new contract to store as a stream of bytes.

To use it, we need to transform the contract we want to store into its base64 representation, and pass the result as input to the method:

# Use near-cli to update stored contract
export BYTES=`cat ./src/to/new-contract/contract.wasm | base64`
near call <factory-account> update_stored_contract "$BYTES" --base64 --accountId <factory-account> --gas 30000000000000

This works because the arguments of a call can be either a JSON object or a String Buffer

Factories are an interesting concept, here we further explain some of their implementation aspects, as well as their limitations.

NEAR accounts can only create sub-accounts of themselves, therefore, the factory can only create and deploy contracts on its own sub-accounts.

This means that the factory:

1. Can create sub.factory.testnet and deploy a contract on it.
2. Cannot create sub-accounts of the predecessor.
3. Can create new accounts (e.g. account.testnet), but cannot deploy contracts on them.

It is important to remember that, while factory.testnet can create sub.factory.testnet, it has no control over it after its creation.

The update_stored_contracts has a very short implementation:

#[private]
    pub fn update_stored_contract(&mut self) {
        self.code.set(env::input());
    }

On first sight it looks like the method takes no input parameters, but we can see that its only line of code reads from env::input(). What is happening here is that update_stored_contract bypasses the step of deserializing the input.

You could implement update_stored_contract(&mut self, new_code: Vec<u8>), which takes the compiled code to store as a Vec<u8>, but that would trigger the contract to:

1. Deserialize the new_code variable from the input.
2. Sanitize it, making sure it is correctly built.

When dealing with big streams of input data (as is the compiled wasm file to be stored), this process of deserializing/checking the input ends up consuming the whole GAS for the transaction.

• cargo-near - NEAR smart contract development toolkit for Rust
• near CLI-rs - Iteract with NEAR blockchain from command line
• NEAR Rust SDK Documentation
• NEAR Documentation
• NEAR StackOverflow
• NEAR Discord
• NEAR Telegram Developers Community Group
• NEAR DevHub: Telegram, Twitter