Schema
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An RGB Schema defines, through coding, the necessary template for Genesis and embeds all the rules of available contract operation representing its business logic allowing for the related state to be updated.
As mentioned earlier, an RGB Schema is the analog of a class for an OOP language. Hence such a construction is used to define the various standards for RGB contracts and assets, for example: fungible assets, collectibles, digital identities, etc.
The issuer of an asset on RGB uses (and makes available to the public) a Schema in order to define the issuance properties encoded in the Genesis. This way, the contract can be supported by RGB wallets and become fully operational. Thus, when the users receive some information about an asset on RGB (data and contract) they must validate them against the Schema distributed by the issuer of that asset.
In fact, the Schema validation is the very first operation step that a user needsrepresents to undergo before interacting in any way with the contract (e.g. to perform the desired contract operations).
From a functional point of view, the Schema construct addresses the following questions:
What kinds of owned states and Assignments exist?
What kinds of Valences exist?
What Global State does the contract have?
How is Genesis structured?
What kind of State Transitions and State Extensions are possible?
What Metadata can contract operations have?
How state data are allowed to change within state transitions?
What sequences of transitions are allowed?
From a technical point of view, an RGB Schema is a functional declarative document that need to be compiled for effective usage inside RGB applications and wallets.
Among the most important properties, a Schema:
References an optional Root SchemaId from which a basic and customizable structure layout is derived.
Defines all the variable used in contract state and transition using a specific strict type system encoding. Of particular importance inside the Schema are specified the Types related to:
Metadata.
Owned state.
Global state.
Valences.
Contract Operation.
Defines all the data structure required for Genesis operation, which marks the first instantiation of the contract.
Allows for programmed updates to the contract without having to modify the infrastructure software, so that wallets and explorers can accept modified asset types without making any changes to their respective code.
Embeds the state validation script and the related functions for the client-side part of RGB. The scripts are executed through the AluVM engine which represent the most fundamental parts of the execution and validation of the business logic.
Even this architectural choice regarding Schema appears to be very different from blockchain-based contracts, for example, those implemented on Ethereum. Indeed in these latter systems, the contract itself is provided as an executable code that implements the rules for changing and implementing the state which is eventually directly stored into the blockchain. In contrast, in RGB the contract is encoded in a purely declarative way.
In every Contract Operation performed in the client-side validation phase, the Contract Schema is always referenced and checked against. In particular, after compilation, the Schema can provide all the necessary data structure to perform the issuance of the contract represented by the Genesis Operation.
As mentioned earlier, Schema clearly differentiates contract developers from issuers, who may know nothing about coding and programming. This kind of approach makes extensive use of contract templates which can be used promptly by issuers who may benefit from avoiding common programming mistakes in the implementation phase.
After compilation, the Schema is encoded in a .rgb binary file or in an .rgba armored binary file, which can be imported by the wallet software.
In the next subsection, we will provide an example of an actual Schema used for the issuance of a Non Inflatable Fungible Asset.
