A event-driven framework for writing reliable and scalable system.

At a high level, it provides a few major components:

• Tools for core components with traits,
• Macros for processing events and commands

A Tour of Event-Driven-Library

Event-Driven-Library consists of a number of modules that provide a range of functionality essential for implementing messagebus-like applications in Rust. In this section, we will take a brief tour, summarizing the major APIs and their uses.

Command & Event

You can register any general struct with Command Derive Macro as follows:

#[derive(Command)]
pub struct MakeOrder {
    pub user_id: i64,
    pub items: Vec<String>,
}

As you attach Command derive macro, MessageBus now is going to be able to understand how and where it should dispatch the command to.

Likewise, you can do the same thing for Event:

#[derive(Serialize, Deserialize, Clone, Message)]
#[internally_notifiable]
pub struct OrderFailed {
    #[identifier]
    pub user_id: i64,
}

#[derive(Serialize, Deserialize, Clone, Message)]
#[internally_notifiable]
pub struct OrderSucceeded{
    #[identifier]
    pub user_id: i64,
    pub items: Vec<String>
}

Note that use of internally_notifiable(or externally_notifiable) and identifier is MUST.

• internally_notifiable is marker to let the system know that the event should be handled within the application
• externally_notifiable is to leave OutBox.
• identifier is to record aggregate id.

Initializing Command Handlers

Command handlers are responsible for handling commands in an application, the response of which is sent directly to clients. Commands are imperative in nature, meaning they specify what should be done.

use event_driven_library::prelude::{init_command_handler, init_event_handler};


init_command_handler!(
{
   MakeOrder: OrderHandler::make_order,
   CancelOrder: OrderHandler::cancel_order
}
);

In the example above, you see MakeOrder is mapped to OrderHandler::make_order, handler in application layer.

At this point, imagine you want to handle both success/failure case of the MakeOrder command processing. Then you have to think about using event handlers.

Registering Event

Event is a side effect of Command or yet another Event processing. You can register as many handlers as possible as long as they all consume same type of Event as follows:

Example

init_event_handler!(
{
   OrderFaild: [
           NotificationHandler::send_mail,
           ],
   OrderSucceeded: [
           DeliveryHandler::checkout_delivery_items,
           InventoryHandler::change_inventory_count
           ]
}
);

In the MakeOrder Command Handling, we have either OrderFailed or OrderSucceeded event with their own processing handlers. Events are raised in the handlers that are thrown to MessageBus by Context. MessageBus then loops through the handlers UNLESS StopSentinel is received.

Handler API Example

Handlers can be located anywhere as long as they accept two argument:

• msg - either Command or Event
• context - [AtomicContextManager]

Example

pub async fn make_order(
    cmd: MakeOrder,
    context: AtomicContextManager,
) -> Result<ServiceResponse, ServiceError> {
    let mut uow = UnitOfWork::<Repository<OrderAggregate>, TExecutor>::new(context).await;

    let mut order_aggregate = OrderAggregate::new(cmd);
    uow.repository().add(&mut task_aggregate).await?;

    uow.commit::<ServiceOutBox>().await?;

    Ok(().into())
}

But sometimes, you may want to add yet another dependencies. For that, Dependency Injection mechanism has been implemented. So, you can also do something along the lines of:

pub async fn make_order(
    cmd: MakeOrder,
    context: AtomicContextManager,
    payment_gateway_caller: Box<dyn Fn(String, Value) -> Future<(), ServiceError> + Send + Sync + 'static> //injected dependency
) -> Result<ServiceResponse, ServiceError> {
    let mut uow = UnitOfWork::<Repository<OrderAggregate>, TExecutor>::new(context).await;

    let mut order_aggregate = OrderAggregate::new(cmd,payment_gateway_caller);
    uow.repository().add(&mut task_aggregate).await?;

    uow.commit::<ServiceOutBox>().await?;

    Ok(().into())
}

How is this possible? because we preprocess handlers so it can allow for DI container.

Dependency Injection

You can simply register dependencies by putting attribute on top of free function.

Example

#[dependency]
pub fn payment_gateway_caller() -> Box<dyn Fn(String, Value) -> Future<(), ServiceError> + Send + Sync + 'static> {
    if cfg!(test) {
        __test_payment_gateway_caller()  //Dependency For Test
    } else {
        __actual_payment_gateway_caller() //Real Dependency
    }
}

This is great as you can take your mind off static nature of the language.

MessageBus

At the core is event driven library is MessageBus, which gets command and gets raised event from UnitOfWork and dispatch the event to the right handlers. As this is done only in framework side, the only way you can 'feel' the presence of messagebus is when you invoke it. Everything else is done magically.

Example

#[derive(Command)]
pub struct MakeOrder { // Test Command
    pub user_id: i64,
    pub items: Vec<String>
}

async fn test_func(){
    let bus = MessageBus::new(command_handler(), event_handler())
    let command = MakeOrder{user_id:1, items:vec!["shirts","jeans"]}
    match bus.handle(command).await{
        Err(err)=> { // test for error case }
        Ok(val)=> { // test for happy case }
    }
    }
    }   
}

Error from MessageBus

When command has not yet been regitered, it returns an error - BaseError::NotFound Be mindful that bus does NOT return the result of event processing as in distributed event processing.