Event-Driven Architecture in Golang/

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Connecting the Domain Events with the Read Model

Connecting the Domain Events with the Read Model

Understand how to use middleware to publish domain events.

We'll cover the following...

Running the application now with all the changes in place for the Store Management module, we will see the following appear in the logs when we add a new product:

started mallbots application
web server started; listening at http://localhost:8080
rpc server started
INF --> Stores.GetStores
INF <-- Stores.GetStores
INF --> Stores.AddProduct
INF <-- Stores.AddProduct
The logs after adding a new product

Assuming everything is wired up correctly, and the handler access logging is set to log all HandleEvent() calls, this might seem a little confusing. There should be some additional lines in there that show the HandleEvent() method on CatalogHandlers was accessed. What we should be seeing is this:

started mallbots application
web server started; listening at http://localhost:8080
rpc server started
INF --> Stores.GetStores
INF <-- Stores.GetStores
INF --> Stores.AddProduct
INF --> Stores.Catalog.On(stores.ProductAdded)
INF <-- Stores.Catalog.On(stores.ProductAdded)
INF <-- Stores.AddProduct
The logs of CatalogHandlers implementation

There is a simple explanation for why we do not see the event being handled. The reason we do not see the extra lines signaling to us that the CatalogHandlers implementation received the stores.ProductAdded event is that by the time the domain event publisher gets a hold of the product aggregate, the events have already been committed and cleared from it. Here are the important lines from the AddProduct command handler:

// ...
if err = h.products.Save(ctx, product); err != nil {
    return err }
if err = h.domainPublisher.Publish(ctx, product.GetEvents()...,); err != nil {
    return err
}
// ...
The important lines from the AddProduct command handler

Recall that the third step for the Save() method on AggregateRepository is to update the aggregate version and clear the events using the aggregate.CommitEvents() ...