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Metis-MCCI architecture#

  • M : Data model. Most contracts read and write contract world states. These states map to data models, each associated with only one component.
  • C : component. A component is a reusable, independent implementation unit that encapsulates data and methods and maintains orthogonality with other components.
  • C : controller. The controller coordinates the components and implements the contract interface.
  • I : interface. The interface is the user interface of the contract. The interface defines the behavior of the contract and, to some extent, defines metadata.
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As shown in the figure above, under the MCCI architecture, the contract is divided into a series of reusable components. The contract behavior is implent through the collaboration of components, and the contract behavior is clearly defined by interface and controller.

The contract's interface defines the contract's behavior, including:

  • constructor
  • message
  • event

The user of the contract interacts with the contract based on these three elements. In fact, these three elements also constitute ink! The main part of the contract metadata.

For a contract, these three things are guaranteed to be deterministic, unambiguous, and easy to understand. Therefore, the interface code of the contract code should be as cohesive as possible.

The contract controller is responsible for integrating the components. We break the main logic of the contract down into a series of reusable components, which can extend and compose based on other components.

A data model is the encapsulation of the state of a contract. A component of a contract needs a contract to meet the requirements of its data model. For a contract, its state will be represented as a combination of a series of data models.

In generally, the data model can also be considered as part of the contract behavior, and thus as part of the contract interface, but in most scenarios, external applications and users can not directly use the state of the chain, so the external encapsulation of the data model is not emphasized here.

Inheritance Vs Composition#

In contract development, we focus more on auditability of contracts, and the use of inheritance in solidity contract development will increase the difficulty of contract audit: The behavior logic of the contract is spread out in multiple files, even in different projects. Therefore, in Metis, we do not inherit the interface and implementation of the contract. The components and data model are introduced into the contract in a combined way.

Each component impl a series of functions, include the impl of messages and apis. Components can extend and compose based on other components.

For most of components, will like this:

/// The `EventEmit` impl the event emit api for ownable trait EventEmit<E: Env>: EnvAccess<E> {    /// Emit OwnershipTransferred event    fn emit_event_ownership_transferred(        &mut self,        previous_owner: Option<E::AccountId>,        new_owner: Option<E::AccountId>,    );}
/// The `Impl` define ownable component impl funcspub trait Impl<E: Env>: Storage<E, Data<E>> + EventEmit<E> {    /// init Initializes the contract setting the deployer as the initial owner.    fn init(&mut self) {        // logic    }
    /// Message impl     fn one_message_impl(&mut self) -> Result<()> {        // msg impl which will call by ```xxx::Impl::one_message_impl(self)```
        // use the hook        self.hook(xxx)?
        Ok(())    }
    /// Message for Query impl    fn one_query_impl(& self, param_acc: &E::AccountId) -> Data {        Data::default()    }
    /// API for other message    fn check_xxx(&self, owner: &E::AccountId) {    }
    // Hook which need impl by contract    fn hook(&mut self, params: &E::Balance) -> Result<()>;}

Some component contains a default implementation:

// a default impl, each contract which impl storage and event emitter can be componentimpl<E: Env, T: Storage<E, Data<E>> + EventEmit<E>> Impl<E> for T {}

To use this component, we can import this to contract:

#![cfg_attr(not(feature = "std"), no_std)]
#[metis_lang::contract] // use `metis_lang::contract`pub mod contract {    // use the component: xxx1 and xxx2    use metis_component_xxx1 as xxx1;    use metis_component_xxx2 as xxx2;
    // use `import` and `metis` marco    use metis_lang::{        import,        metis,    };
    #[ink(storage)]    #[import(xxx1, xxx2)] // import the component    pub struct Contract {        // add data to storage, which use Contract as Env to Data        xxx1: xxx1::Data<Contract>,        xxx2: xxx2::Data<Contract>,    }
    /// add event for component    /// in emit it will be emit_event_ownership_transferred    #[ink(event)]    #[metis(xxx1)] // event for xxx1    pub struct OwnershipTransferred {        /// previous owner account id        #[ink(topic)]        previous_owner: Option<AccountId>,        /// new owner account id        #[ink(topic)]        new_owner: Option<AccountId>,    }
    /// Event emitted when payee withdraw    #[ink(event)]    #[metis(xxx2)] // event for xxx1    pub struct OtherEvent {        #[ink(topic)]        pub payee: AccountId,        pub amount: Balance,    }
    impl xxx1::Impl<Contract> for Contract {        fn hook(            &mut self,            params: &E::Balance        ) -> Result<()> {            // some logic
            Ok(())        }    }
    // impl    impl Contract {        #[ink(constructor)]        pub fn new() -> Self {            // impl for default            let mut instance = Self {                xxx1: xxx1::Data::new(),                xxx2: xxx2::Data::new(),            };
            // init call            xxx1::Impl::init(&mut instance);            xxx2::Impl::init(&mut instance);
            // return instance            instance        }
        /// commits for one_message_impl        #[ink(message)]        pub fn one_message_impl(&mut self) -> Result<()> {            // some other check            xxx2::Impl::do_some_check(self);            xxx1::Impl::one_message_impl(self)        }
        /// commits for one_query_impl        #[ink(message, payable)]        pub fn one_query_impl(&self, payee: AccountId) {            xxx1::Impl::one_query_impl(self, payee)        }
        /// commits for other_message_impl        #[ink(message)]        pub fn other_message_impl(&mut self, payee: AccountId) {            xxx1::Impl::check_xxx(self)            // other logic        }    }
    #[cfg(test)]    mod tests {        // test for contract    }}