A Guide on How to Write Function for Scalar DL

This document sets out some guidelines for writing functions for Scalar DL.

What is a function for Scalar DL ?

A Function (Smart Function) for Scalar DL is a Java program extending Function class written for implementing a single function of business logic, which mainly manages the data of an application that is deployed on top of Scalar DL. Before taking a look at this, it is recommended to check Getting Started in Scalar DL and Scalar DL v1 design document to understand what Scalar DL is and its basic terminologies. Understanding contact guide is also helpful to understand this guide properly.

Background

Assets managed by Contracts in Scalar DL are, as most of the DLT platforms including Blockchains, tamper-evident, and append-only so their data structure is limited in modeling wide variety of applications and they are not able to be deleted. Many of the DLT platforms deal with the issue by having another database such as an RDBMS in front of a ledger to handle the application's data in the database and write logs to the ledger as evidence. However, this scheme is not ideal since data consistency between the database and the ledger is not always preserved. There could be a case where applications don't have corresponding logs in the ledger due to a catastrophic failure, which defeats the purpose of writing logs to the ledger as evidence. Scalar DL resolves the issue with a little different approach by introducing Functions to manage applications' data and making Contracts and Functions execute atomically by utilizing underlining distributed ACID transactions.

Write a Function

Let's take a closer look at PaymentFunction to better understand how to write a function.

public class PaymentFunction extends Function {

  @Override
  public void invoke(
      Database database,
      Optional<JsonObject> functionArgument,
      JsonObject contractArgument,
      Optional<JsonObject> contractProperties) {
    JsonArray array = contractArgument.getJsonArray(ASSETS_ATTRIBUTE_NAME);
    int amount = contractArgument.getInt(AMOUNT_ATTRIBUTE_NAME);
    String fromId = array.getString(0);
    String toId = array.getString(1);

    Optional<Result> account1 = database.get(new Get(new Key(new TextValue("id", fromId))));
    Optional<Result> account2 = database.get(new Get(new Key(new TextValue("id", toId))));

    long balance1 = ((BigIntValue) account1.get().getValue("balance").get()).get();
    long balance2 = ((BigIntValue) account2.get().getValue("balance").get()).get();

    // Transfer
    balance1 -= amount;
    balance2 += amount;

    Put put1 =
        new Put(new Key(new TextValue("id", fromId)))
            .withValue(new BigIntValue("balance", balance1));
    Put put2 =
        new Put(new Key(new TextValue("id", toId)))
            .withValue(new BigIntValue("balance", balance2));

    database.put(put1);
    database.put(put2);
  }
}

As you can easily see, it is a money transfer application, which sends the specified amount of money between two accounts.

About the arguments

Similar to a Contract using Ledger object to manage assets, a Function uses Database object to manage records in the underlining database. Note that Database implements Scalar DB interface so that you can do CRUD operations base on the data model of Scalar DB.

JsonObject for a function argument is a JSON object and a runtime argument for the function specified by the requester. The argument is not digitally signed as opposed to the contract argument so that it can be used to pass data that is stored in the database but it might be deleted at some later point for some reason.

contractArgument and contractProperties are the corresponding contract's argument and properties. See the contract guide to understand what they are.

How to use Functions

The Function feature is enabled by default; thus, nothing needs to be configured in Ledger except for the following things. If you want to disable the feature, please set scalar.dl.ledger.function.enabled to false in the properties of Ledger.

Add an application-specific schema

Since Functions can read and write arbitrary records through the Scalar DB CRUD interface, Scalar DL can't define the database schema for the Function by itself. It is the applications' owner's responsibility to define such schema and apply it to the database by themselves or asking system admins to do it depending on who owns and manages the database.

Register a Function

You then need to register a Function to Ledger before used like you register a Contract.

register-function --properties client.properties --function-id test-function --function-binary-name com.example.function.TestFunction --function-class-file /path/to/TestFunction.class

Execute a registered Function

Functions that are being executed are specified in a contract argument in a JSON format with _functions_ as a key and an array of function IDs as a value as follows:

execute-contract --properties client.properties --contract-id test-contract --contract-argument '{..., "_functions_": ["test-function"]}' --function-argument '{...}'

Similar to a Contract, a Function can invoke another Function so multiple Functions (and multiple Contracts) can be grouped together. Scalar DL executes a group of Contracts and Functions in an ACID manner so that they can be done atomically and in a consistent, isolated, and durable manner.

How to use Contracts and Functions properly

Contracts and Functions should be properly used to make the scheme meaningful. As a basic principle, Contracts should be used to manage data that requires tamper-evidence, and Functions should be used to manage data that can be updated or deleted or that needs a more flexible data model. As a good practice, Functions are used to manage applications' data and Contracts are used to manage the logs of applications' execution as evidence. For example in a payment application, a Function manages the account balances of users and a Contract manages the evidence of payment between the users.

References