[CQRS Journey] Journey 4 : Extending and Enhancing the Orders and Registrations Bounded Context

04 Jan 2020

Changes to the bounded context

topics
- Improvements to the way message correlation works with the RegistrationProcessManager class
- Implementing a record locator to enable a registrant to retrieve an order that she saved during a previous session
- Adding a countdown timer to the UI to enable a registrant to track how much longer they have to complete an order
- Supporting orders for multiple seat types simultaneously
- CQRS command validation

Working definitions for this chapter

Command
- A request for the system to perform an action that changes the state of the system
- A single recipient processes a command
- Sending a command is an asynchronous operation with no return value.
Event
- Event describes something that has happened in the system, typically as a result of a command
- Multiple subscribers can handle a specific event
Process manager
- Process manager coordinates the behavior of the aggregates in the domain
- The process manager does not contain any business logic, only logic to determine the next command to send

User stories

Implement a login using a record locator
Tell the registrant how much time remains to complete an order
Enable a registrant to create an order that includes multiple seat types

Architecture

Patterns and concepts

Record locators

The system needs to be able to retrieve order information quickly based on the registrant’s email address and access code

Querying the read side

To minimize the work that the queries on the read side must perform, these SQL views provide a denormalized version of the data

Storing denormalized views in a database

implementing the ViewRepository class

Using the IQueryable interface

var ordersummary = repository.Query<OrderSummary>().Where(LINQ query to retrieve order summary); 
var orderdetails = repository.Query<OrderDetails>().Where(LINQ query to retrieve order details);

Simplicity
- This approach uses a thin abstraction layer over the underlying database
Testability
- You can use LINQ to Objects for mocking

Using non-generic DAOs
```
var ordersummary = dao.FindAllSummarizedOrders(userId); 
var orderdetails = dao.GetOrderDetails(orderId);
var ordersummary = OrderSummaryDAO.FindAll(userId); 
var orderdetails = OrderDetailsDAO.Get(orderId);
```
- Simplicity
  - Dependencies are clearer for the client
- Flexibility
  - The Get and Find methods hide details
    - the partitioning of the data store
    - the data access methods such as an object relational mapping
    - This makes it easier to change these choices in the future
- Performance
  - You can easily optimize the queries that the Find and Get methods run
- Testability
  - It is easier to specify unit tests for the Find and Get methods than to create suitable unit tests for the range of possible LINQ queries that a client could specify
- Maintainability
  - All of the queries are defined in the same location, the DAO classes, making it easier to modify the system consistently

CQRS command validation

error
- retrying the operation
business failure
- A business failure should have a predetermined business response
- If the system cannot reserve a seat because there are no seats left, then it should add the request to a wait list

kjgleh's