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Aggregate size and granularity
When you are creating a model for your domain one of the major decisions to decide on is the size you will create each of the aggregates. On the other hand the ideal aggregate size is smaller. Since updates to each aggregate are serialized more fine-grained aggregates can increase the amount of concurrent requests the application is able to handle, thereby increasing scalability.

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This will also enhance the user experience since it decreases the likelihood of two users trying simultaneous updates from one aggregate. In contrast as an aggregate defines the scope of the transaction, you could have to create a larger aggregate to make a specific update one-time.
For instance, I discussed the fact that the FTGO application’s domain model Order and Consumer are separate aggregates. Another option could be to include Order one of aggregate Consumer. Locals email lists.

One benefit of this greater consumer aggregate is the fact that the program can completely update a consumer and some or all of the orders. One drawback to this approach is that it restricts the possibility of scaling. Transactions that update multiple orders of the same customer will be serialized. In the same way, two users could have a conflict if they attempt to modify orders that differ on the behalf of the exact same user.

Another issue with this model within a microservice framework is that it creates an obstruction to decomposition. The logic that runs orders and Consumers for example should be centralized in this same system, making the service more extensive. Due to these reasons making aggregates as fine-grained as is possible is the best option.
Business logic that is designed using aggregates

In the typical (micro)service the bulk part of the logic for business is comprised of aggregates. Locals email lists. The remainder portion of the logic is located within the domain services and sagas. The sagas play out patterns of local transactions to ensure data consistency. These services are entry points to the business logic, and are invoked by adapters inbound. A service makes use of an aggregate repository to pull aggregates in the database, or store aggregates in the database. Each repository is designed with an outbound adapter which connects to the database. Figure 5.9 illustrates the aggregate-based structure for the logic used for Order Service. Order Service.

The business logic is comprised of the Order aggregate the OrderService service class, the OrderRepository as well as one or two stories. The OrderService uses the Order-Repository service to store and load orders. For simple requests specific to the particular service it update an Order aggregate. If an update request involves several services that the OrderService can also start the saga explained in chapter 4. Locals email lists.

We’ll examine the code first. But first, let’s explore the concept closely connected to aggregates domain events.

What is an event of the domain?

Domain events are an event that has a name created by a past-participle verb. It’s got proper-ties that describe the events. Each property can be described as an initial value or a value object. For example, an order created event class contains the property orderId.
A domain event usually contains metadata, including the event’s ID as well as a time stamp. Locals email lists. It may also contain the name of the person who initiated the change since it’s helpful to audit. The metadata may belong to the object that hosts events, or specified as the superclass. The event metadata could be contained inside an envelope object that is wrapped around the object that is being used. Its ID for the event that generated the event may be a part of the envelope, rather than being an event-specific property.

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This OrderCreated incident is one example of a domain-related event. It does not have any fields since the ID of the order is included in the envelope for the event. The following table is the OrderCreated class as well as the DomainEventEnvelope class.

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local email ids

Event enrichment

Imagine, for instance you’re writing an event consumer which processes orders. The OrderCreated class of events that we have depicted earlier provides the essentials of what occurred. However, your event’s user may require the details of an order for processing for an OrderCreated event. Another option is to obtain the information via the Order-Service. The disadvantage of an event-related consumer contacting the service for aggregate information is that it will incur the costs of an order for service.

A different approach, known as event enrichment is to allow events to provide information that people require. This makes it easier for event attendees since they don’t have to seek out the data from the website that hosted the event. When the event is an OrderCreated event the Order aggregate may add value to the event by incorporating the details of an order. The following list shows how to create an enhanced event. Locals email ids.
Even though event enrichment makes it easier for consumers, it has a drawback that it could make class less stable. A class of events may need to be modified whenever the needs of its customers shift. This could result in a decrease in maintainability since the changes could be detrimental to multiple aspects in the system. Achieving every consumer’s satisfaction can be futile. In many cases, it’s pretty obvious what properties should be included in the event.

Identifying domain-specific events

There are many different methods to identify domain-specific events. The require-ments typically describe scenarios in which notifications are needed. The requirements could contain language like “When the event occurs, do X.” For instance an example of a requirement in the FTGO application is “When an order is placed send the user an email.” The need for a notification implies an existence domain-related event. Locals email ids.

Another method that is gaining popularity is the use of event storming. Event storming is a type of workshop format to help you understand an intricate area. It involves gathering experts from the domain in a space, lots of sticky notes and an enormous whiteboard or roll of paper to stick notes on. The outcome of this process is an event-centric model of domain composed of aggregates and event.

Event storming consists of three major steps:

Brainstorming events: Ask the experts from the domain to brainstorm domain-specific events. The domain events can be represented with sticky notes of orange, placed in rough time on the model surface.

Determine event triggers. Ask the experts in the domain to determine the trigger for each event and which or more of these:

User actions, presented by an orange sticky note

External system, illustrated by a sticky purple note

Another domain-related event

Time passing

Find aggregates–Ask experts in the domain to determine the aggregate that combines each command and produces the appropriate event. Aggregates are represented by sticky yellow notes. Locals email ids.

Figure 5.10 depicts the outcome of an event-storming exercise. In just a few hours, participants discovered many domain events, commands and aggregates. This was a great start in the process of creating a model for domains.

A Policy for Event Command and Aggregate Command

Figure 5.10 The results of an event-storming workshop which took a few hours. Locals email ids.The sticky notes represent events that are laid out on the timeline; commands that represent user actions and aggregates that emit events when triggered by an instruction.

Event storming is an effective method for rapidly creating an effective domain model.

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After having covered the fundamentals about domain-specific events, we can take a look at the process of creating as well as publishing these.

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local email address lists

Business logic design in microservices architecture

Publishing and generating domain-related events

Domain events are an example of asynchronous messaging that is covered in chapter 3. However, before business logic can send them to a message broker, it needs to create them. Let’s examine how to accomplish that.


Theoretically Domain events are reported by aggregates. An aggregate can tell what state it is in and therefore the event it will publish. A aggregate can invoke the mes-sending API directly. The disadvantage of this method is that since aggregates cannot utilize dependency injection. Therefore, the mes-saging API will have to be distributed as an argument to a method. It would also intertwine concerns about infrastructure as well as business-related logic. This is inconvenient. Locals email address lists.

The best approach is to divide accountability between both the aggregate as well as the class (or similar class) that calls it. Service can make use of dependency injection to gain access towards the message API, and then easily publishing events. The aggregate produces events every time its state changes , and then sends them back in the form of a service. There are several possible ways for an aggregate to bring events back into the system. One possibility is for returning the value from the aggregate method to contain the events that occurred. For instance, the below table shows how a ticket aggregate’s Accept() method could return an event called Ticket-AcceptedEvent back to the client.

This accept() method initially calls the TicketRepository to download the Ticket from the database. The Ticket is then updated using the Accept(). KitchenService then pub-lishes events returned by Ticket by calling DomainEventPublisher.publish(), described shortly. Locals email address lists.

This method is very simple. Methods that normally would be void return types will now return a List . The only possible drawback is that the return method of non-void methods is more complicated. They have to return an object that contains the return value as well as List . There will be an example of this technique in the near future.

Another alternative is for an aggregate root gather events within the field. Locals email address lists. The service then retrieves events and then publishes the events. For instance, the following list shows an alternative to the Ticket class which works by this method.

Ticket extends AbstractAggregateRoot, which defines a registerDomainEvent() method that records the event. A service would call AbstractAggregateRoot.get-DomainEvents() to retrieve those events.

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My preference is the first choice which is to return instances to the services.

However, the idea of accumulating events within the aggregate root could be an alternative. In fact, the Spring Data Ingalls release train (https://spring.io/blog/2017/01/30/what-s-new-in-spring-data-release-ingalls) implements a mechanism that automatically publishes events to the Spring ApplicationContext. The main drawback is that to reduce code duplication, aggregate roots should extend a superclass such as AbstractAggregate-Root, which might conflict with a requirement to extend some other superclass.

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Another issue is that although it’s easy for the aggregate root’s methods to call register-DomainEvent(), methods in other classes in the aggregate would find it challenging. They’d likely have to transfer the events on to the root of aggregate.


Chapter 3 discusses the best way to send messages with confidence in an in-depth local database transaction. Domain events aren’t any different. A service needs to utilize transactional messaging for publishing events to ensure they are published in the context of a transaction that changes the total in the database. This framework, called the Eventuate Tram framework, described in chapter 3, is mechanism. It inserts instances into the OUTBOX table as part of an ACID transaction which is responsible for updating the database. Once the transaction is committed the events entered into the OUTBOX table are published in the messaging broker. Locals email database lists.

The Tram framework has the DomainEventPublisher interface as illustrated in the following list. It defines a number of overloaded publishing() method that accept the aggregate type and the ID parameter as inputs, as well as the list of domain-specific events.

A service can call its DomainEventPublisher Publisher directly. One disadvantage to doing this is that it does not guarantee that the service will only publish legitimate events. KitchenService is one example. KitchenService must exclusively publish events that use the Ticket-DomainEvent interface used to mark events in the Ticket aggregate’s event. A better option is for services to implement a subclass of AbstractAggregateDomainEvent-Publisher, which is shown in listing 5.7. AbstractAggregateDomainEventPublisher is an abstract class that provides a type-safe interface for publishing domain events. It’s a generic type of class that contains two type parameters: A, which is the aggregate type and E, which is the interface type used to mark the events in the domain. A service is able to publish events using”publish() method with two parameters that are an aggregate of type A, and a list of events that fall under the type of E. Locals email database lists.

Consuming domain events

Domain events are then broadcast via messages that are sent out to messaging broker, like Apache Kafka. Consumers can access the broker’s API for clients directly. But it’s more convenient to use a higher-level API such as the Eventuate Tram framework’s Domain-EventDispatcher, described in chapter 3. A DomainEventDispatcher dispatches domain events to the appropriate handle method. The figure 5.9 provides an example of an event handler class. KitchenServiceEventConsumer subscribes to events published by Restaurant Service whenever a restaurant’s menu is updated. It’s accountable for keeping Kitchen Service’s version of the information current.

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Kitchen Service business logic

The first instance can be seen in Kitchen Service, which enables the restaurant to handle their customers’ orders. The two principal aggregates in these services are Ticket aggregate and the Restaurant aggregate. The Restaurant aggregate is aware of the menus of restaurants and their opening hours, and is able to validate orders. Tickets represent an order that the restaurant has to prepare to be picked up by courier. Figure 5.11 shows these aggregates as well as other important elements of the business logic and the adapters of the service.

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In addition to the aggregates, the other main parts of Kitchen Service’s business logic are KitchenService, TicketRepository, and RestaurantRepository. Kitchen-Service is the logic of business’s entry point. It defines the methods used for creating and updating aggregates of tickets and restaurants. TicketRepository and RestaurantRepository describe methods for storing Restaurants and Tickets, respectively.
Kitchen Service Kitchen Service service has three adapters inbound:

The ticket aggregate

The Ticket is among the aggregates that comprise Kitchen Service. As explained in chapter 2, talking about the notion of Bounded Context this aggregate is the restaurant kitchen’s perception on an ordered. It doesn’t include any information about the person ordering like their name, address as well as delivery information or payment information. It’s primarily focused on allowing the kitchen of a restaurant for preparing an Order for pick-up. Furthermore, Kitchen-Service doesn’t generate a unique ID for this particular aggregate. Instead, it utilizes the ID that is supplied by OrderService. Buy Locals email id lists.

Let’s begin by looking at the basic structure of this class. Then, we’ll look at its methods.

The following table provides an excerpt from the code used for this class. The class Ticket is similar to a conventional domain class. The major distinction is that all references to other aggregates occur through the principal key.

This class is a part of JPA and is linked to the Table of Tickets. Buy Locals email id lists. The restaurantId field can be described as a Long instead of an object reference to a restaurant. The readyBy field is used to store an estimate of what time the food order is ready to pick up. The ticket class includes a variety of fields that keep track of the sequence of the order, including acceptTime and preparing-Time and pickupTime. Let’s take a look at the method of the class.

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The Ticket aggregate is composed of several methods. As you can see it uses an static generate() method that is an industrial method that produces tickets.

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local email directory

There are also meth-ods which are invoked whenever the restaurant changes the status that the customer has placed an order

Accept the order ()–The place has taken the reservation.

The restaurant is preparing ()–The restaurant is currently making the order, which means that the order is unable to longer be cancelled or changed.

readyForPickup()–The order can now be picked up. Buy Locals email directory.

Create() method generates Tickets. The making() procedure is used at the time the restaurant is beginning to prepare the food. It alters the status for the meal to PREPARING and records the time and then creates an event. The cancellation() technique is used when a user wants to cancel the order. If the cancellation is permitted this method alters the status of the order and produces an event. If not, it throws an error. They are called when responding to REST API requests along with events and messages from com-mand. Let’s take a look at the classes that call the aggregate’s method.


KitchenService is called by the inbound adapters of the service. It defines a variety of methods to alter the state of an order. These include accept() reject(), accept()() and preparing() and many more. Every method load the specified aggregate, then calls the corresponding method on the aggregate root and then publishes any events of the domain. The following list demonstrates the acceptance() method. Buy Locals email directory.

This accept() technique is called when the restaurant is ready to accept an order that is new. It is a method that has two parameters:

orderId–ID of the purchase order that you accept

readyBy–Estimated date by which the order will be available for pick-up

This method retrieves the aggregated Ticket and calls the acceptance() process. It announces all events that are generated.

We’ll now examine the class that manages the asynchronous commands.

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The KitchenServiceCommandHandler class is an adapter that’s responsible for handling command messages sent by the various sagas implemented by Order Service. This class specifies the handler method that is used for each command, and calls KitchenService for creating or update a ticket. This table provides an example of this class.

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locals email outlook

Service business logic Service business logic

As discussed in previous chapter, Order Service provides an API for creating, updating or cancelling orders. The API is used primarily by the user. Figure 5.12 depicts the basic structure for the API. It is known as the Order aggregate. This represents the primary aggre-gate to Order Service. However, there’s also a Restaurant aggregate that’s a partial duplicate of the information that is owned through Restaurant Service. It allows Order Service to validate and price line items on an Order. Buy Locals email outlooks online.

In addition to the Order and Restaurant aggregates, the business logic consists of OrderService, OrderRepository, RestaurantRepository, and various sagas such as the CreateOrderSaga described in chapter 4. OrderService is the principal entry point to the business logic. It outlines ways to create and update Orders along with Restaurants. OrderRepository provides methods to persist orders, while RestaurantRepository provides methods for storing Restaurants. Order Service has several inbound adapters.

The Order Aggregate

The Order aggregate is an order that is placed by consumers. Let’s first examine how the Order aggregate is constructed Order aggregate, and then find out the ways it works.

The structure of the order AGGREGATE

Figure 5.13 illustrates how the aggregate of orders is constructed. It is the Order class that forms at the heart of the order aggregate. The Order aggregate also includes value objects like Order-LineItem, DeliveryInfo, and PaymentInfo. Buy Locals email outlooks online.

The class Order contains an array of OrderLineItems. Since an Order’s Consumer and Restaurant are other aggregates, they are referred to using the their primary key value. The Order class contains the DeliveryInfo class that holds the delivery address as well as the delivery time desired and a PaymentInfo that stores the information about payment. The following listing reveals the code.

This class is saved using JPA and is linked to the ORDERERS table. The ID field serves as the key. Version fields are utilized to lock optimistically. Buy Locals email outlooks online. The status of an order is represented in the OrderState Enumeration. The fields of DeliveryInformation and Pay-Information are converted with the @Embedded annotation. They are stored as col-umns within the table ORDERS. The field orderLineItems contains an embedded object that holds the items of an order line. The Order aggregate is made up of more than fields. It also incorporates business logic that can be explained by the state machine. Let’s examine the machine known as the state.

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To create or modify orders, Order Service must collaborate with other service providers through sagas. In either case, the first stage of the saga invokes an Order procedure that confirms that the operation is completed and then changes the status for an Order to a pending status.

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A state that is pending as discussed in chapter 4, is an illustration of a semantic lock countermeasure which ensures that sagas are not in conflict with each other. In the end, after the saga has invoked participant services, it changes it’s Order in order to show the results. For instance, as explained in chapter 4. Create Order Saga has multiple participants’ services, which include Consumer Service, Accounting Service as well as Kitchen Service. OrderService initially creates an order in the APPROVAL_PENDING status, and later alters its state to either APPROVED OR REJECTED. The behaviour of an order is able to be described .

In the same way, other Order Service operations like revision() or cancel() initially change an Order to a waiting state and then use a saga confirm that the operation can be carried out. After the saga has confirmed the operation is able to be executed then the Order into a new state that is reflective of the outcome from the procedure. If the verification operation is unsuccessful and the Order returns to its previous state. Buy Locals email outlooks online.

For instance cancellation() operation is the first to transition the Order into the CANCEL_PENDING condition. If the order is able to be cancelled it will be cancelled. Cancel Order Saga changes the status of the order to the CANCELLED status. In the event that an cancellation() action is denied due to, for instance it’s not time for the cancellation the Order is reverted in its APPROVED state.

Let’s take a look at ways in which the Order aggregate works to implement that state machine.


The Order class is comprised of various methods each one of which is related to the epic saga. Within each group, one procedure is used at beginning of the story, while the other methods are invoked at the conclusion. In this article, I’ll discuss the business logic behind an Order. Then we’ll examine the process of updating an Order. This table shows the methods of an Order which are used in the process of making an order. Buy Locals email outlooks online.

The initial status of the Order is approval_pending. After the CreateOrderSaga concludes it will the note Approved() or noteRejected(). Note-Approved() technique is called when the credit card used by the consumer is authorized successfully. NoteRejected() technique is invoked in the event that one service rejects the request or the authorization fails. As you will see, the state of the Order aggregate is what determines the behaviour of many the methods it employs. Similar to the Ticket aggregate it also produces events.

In addition to creatingOrder() In addition to createOrder(), the Order class provides a number of updates methods. For instance Revise Order Saga Revise Order Saga revises an order first by calling its revise() method, and then after confirming that the revision is completed, it calls the confirm-revised() process. This table outlines the methods used. Buy Locals email outlooks online.

This revise() technique is utilized to begin the revision process to an ordered. In addition, it ensures that the revised order does not violate the minimum requirements for orders and also changes the status that the purchase is in REVISION_PENDING. After Revise Order Saga has successfully changed the settings of the Kitchen Service and Accounting Service It then will call verifyRevision() to finish the revision.
These methods are used by OrderService. Let’s look at this class.

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The class OrderService

The OrderService class defines the methods used for creating and updating orders. It is the primary entry point to the business logic, and is called by various inbound adapters like the REST API. Its methods generally create an saga that orchestrates the creation and updates of aggregates for orders.

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In the end, this service is more complex in comparison to the KitchenService class that was discussed earlier. This table shows an excerpt from this class. OrderService is injected with various dependencies, including OrderRepository, OrderDomainEventPublisher, and several saga managers. It provides a variety of methods, such as the creation of orders() or reviseOrder().

This createOrder() method initially creates and keeps an order aggregate. Then, it publishes the domain events generated from the aggregate. In the end, it generates an Order-Saga called a CreateOrder. ReviseOrder() reviseOrder() will retrieve the Order and creates an ReviseOrderSaga. Buy Locals email address database online.

In many ways the business logic of microservices-based apps isn’t as different from an application that is monolithic. It’s made up of classes, such as ser-vices, JPA-backed entities and repositories. There are some differences however. A domain model is structured as a collection of DDD aggregates which impose various design restrictions. In contrast to a conventional object model, the references between classes within different aggregates are determined by the primary key value, not references to objects. A transaction is also able to only be used to create or update an aggregate. It is also beneficial for aggregates to release domain events whenever their state alters.

A further major distinction is that many services use sagas to ensure data consistency across various services. For instance, Kitchen Service merely participates in sagas, but it does not initiate them. However, Order Service relies heavily on sagas to create and changing orders. It’s because orders have to be transactionally compatible with other data services. This is why the majority of OrderService techniques create a mess instead of updating an Order directly. Buy Locals email address database online.

This chapter has discussed the ways to integrate business logic using the traditional method of persistence. This has included the integration of messaging and event publishing with transaction management in databases. The code for publishing events is integrated with an enterprise logic. The next chapter focuses on the event sourcing process, which is an approach to writing business logic that ensures that event generation is an integral part of the business logic instead of being added on.

This procedure-based Transaction scripting pattern typically the best method to implement basic business logic. However, when you are implementing more complicated business logic, you should think about using an object-oriented Domain Model pattern. Buy Locals email address database online.

An effective way to arrange the business logic of a service is by putting it in a set of DDD aggre-gates. DDD aggregates are beneficial as they allow you to simplify the domain model, remove the possibility of object references between different services and guarantee that every ACID transaction is within the service.

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An aggregate should be able to publish domain events whenever it’s made or modified. Domain events are a broad range of applications. Chapter 4 examines how they can be utilized to implement choreography-based stories. In chapter 7 I discuss how you can use domain events to refresh replicated data. Domain event participants can inform users and other applications and also publish WebSocket messages to the user’s browser.

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A few years ago, Mary had learned about events sourcing, an approach to writing event-specific business logic as well as persistent domain objects. She was fascinated by its many benefits, like the fact that it records the entire history of changes made to an aggregate. However, it was a mystery. Because of the significance to domain event events as part of a microservices, she’s now wondering if it’s beneficial to investigate the use of event sourcing in the FTGO application. Event sourcing eliminates a cause of programming mistakes by ensuring that events is published every time an aggregate is made or modified.

This chapter begins by explaining how event sourcing is used as well as how to utilize it to create business logic. I will explain how event sourcing records each event as a sequence of events into what’s called the event storage. I review the advantages and drawbacks of using event sourcing, and also explain the process of implementing events into an existing store. I present a simple framework to write business logic based on event sourcing. Then, I will explain how event sourcing can be an excellent foundation for the development of stories. Let’s look at ways to build business logic by using event source. Buy Locals email directory database online.
Event sourcing is an alternative method of structuring business logic and persisting aggregates. It keeps an aggregate in an event sequence. Each event represents a modification for the entire aggregate. The application recreates the present condition of an aggregate by repeating the events.

Event sourcing offers a number of significant advantages. It is for instance, it keeps the history of aggregates which is beneficial in the auditing and regulatory context. Additionally, it is able to reliably publish domain-specific events, which are particularly beneficial when using a microservice design. However, it has its own drawbacks. It requires a steep learning curve due to the fact that it’s a different method of writing an enterprise logic. Additionally, querying the event store is usually challenging, and requires you to follow CQRS. CQRS pattern, which is described in chapter 7. Buy Locals email directory database online.

This section begins by discussing the shortcomings of the traditional persistence. I then discuss the event sourcing process in detail , and then discuss how it can overcome those limitations. Then, I demonstrate what you can do to make the Order aggregate by using event source. In the final part, I discuss the advantages and disadvantages of using event sourcing.

The issue with traditional persistence

The conventional method of persistence is to map classes to databases the fields of these classes to columns in tables, and instances of the classes to rows within these tables. As an example the figure 6.1 illustrates exactly how an Order aggregate, as described in the chapter 5 is assigned onto the table ORDER. The OrderLineItems of the aggregate are linked onto the ORDER_LINE_ITEM database. Buy Locals email directory database online.
The application stores an order instance in rows within the ORDER AND ORDER_LINE_ITEM table. It could do this using an ORM framework like JPA or a lower-level framework like MyBATIS.
This method is clearly effective because the vast majority of enterprise software stores data in this manner. However, it comes with a few negatives and drawbacks:

The impedance of the object is not in line with the expected value.

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Insufficient history of the aggregate.

The process of implementing audit logs is time-consuming and error-prone.

Event publishing is connected to the business process.

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Let’s take a look at each of these issues beginning with the Object-Relational imperceptibility mismatch issue.


An old problem is known as the Object-Relational impedance mismatch issue. There is a fundamental conceptual gap between the tabular schema of a relational schema as well as the structure and graph of a complex domain model, with its intricate connections. Certain aspects of this issue can be seen in the heated discussions about the viability of Object/Relational Mapping (ORM) models. For example, Ted Neward has said that “Object-Relational mapping is the Vietnam of Computer Science” (http://blogs .tedneward.com/post/the-vietnam-of-computer-science/). To be honest I’ve utilized Hibernate successfully to create applications where the schema of the database is derived from the object model. But the issues are much more beyond the limitation of any one particular ORM framework. Buy Locals email and contact lists online.


Another drawback of the traditional persistence model is that it can only store what is happening to an aggregate. After an aggregate is modified, the previous state disappears. If an application has to keep its history for an aggregate, possibly for purposes of regulation, developers need to implement the mechanism for themselves. It takes time to implement an aggregate history feature and requires duplicating code that has to be in sync to the logic of business.

Implementing AUDIT LOGGING is a tense AND AN ERROR PRONE

Another concern is audit logs. A lot of applications require an audit log, which records the users who have altered and in what overall. Certain applications require auditing for reasons of security or for regulatory reasons. In other applications the log of actions taken by users is an essential aspect. For instance issue trackers as well as task management applications like Asana and JIRA provide the history of any changes made to issues and tasks. The challenge of the implementation of auditing is that, in addition to being a time-consuming process the auditing logging software and business logic may differ, which can lead to problems. Buy Locals email and contact lists online.

The event’s publication is pinned to the LOGIC OF BUSINESS

Another issue with conventional persistence is the fact that it typically does not support publishing domain-specific events. Domain events, as discussed in chapter 5 can be defined as events released through an aggregate every time its status changes. They’re a great way of sending out notifications and synchronizing data within microservice architecture. Certain ORM frameworks, like Hibernate are able to invoke application-provided calling backs when objects in the data change. 

However, there is no way to publishing messages automatically as part of the process that update the data. Buy Locals email and contact lists online. As with auditing and history developers are required to incorporate events-generation logic that is not being synchronized with business logic. However, there is an answer to these problems and that’s event source.

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An overview of event sourcing

Event source is an event-based method to implement business logic and per-sisting aggregates. The aggregate is saved within the database in the form of a set of events. Each event is a alteration in the aggregate. The business logic of an aggregate is structured around the need to create as well as consume the events. Let’s take a look at how this works.

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In earlier posts in section 6.1.1 I explained the way conventional persistence maps aggregates onto tables as well as its fields into columns in addition to their rows for instances. Event sourcing is a different method of persistent aggregates, based upon the concept of domain-specific events. It stores each aggregate as a set of events in a database, also known as an “event store.

Take, for instance the Order aggregate. As the figure 6.2 illustrates, rather than keep each Order in an individual column in the ORDER table event sourcing records each Order aggregate in one or more rows within An EVENTS table. Each row represents a specific domain event, like Order Created, Order Approved and Order Shipped and so on. Locals email and contact lists database.

If an app creates, or edits the aggregate it adds events created through theaggregate into its table EVENTS. A program loads an aggregate from an event store by taking its events, and then replaying the events. Particularly loading an aggregate comprises of three steps:

Then load the events to calculate the sum.

Make an instance of aggregate using the default constructor.

Then, iterate through the events by applying the call().

For instance for example, The Eventuate Client framework, covered in the section 6.2.2 is based on code that is similar to the following to calculate an aggregate:

A class aggregate class = …;

Aggregate aggregate = aggregateClass.newInstance(); for (Event event : events) {

aggregate = aggregate.applyEvent(event);


// use aggregate…

The class is created by creating an instance the class and then iterates through the events by calling appendEvent() method. If you’re familiar in functional programming, then you could identify this as a fold or reduce operation. Locals email and contact lists database.

It could be a bit odd and unaccustomed to recreate the state of memory of an aggre-gate through loading events and replaying the events. In some ways, it’s much different than how an ORM framework like JPA and Hibernate load an object. A ORM framework load an object using some or all of the SELECT commands in order to get the state that is currently stored by instantiating objects using their default constructors. It utilizes reflection to initialize these objects. What makes event sourcing different is that it reconstructs the memory state is done by using events.

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Chapter 5 describes the concept of domain events, which is a method to inform subscribers of the changes in aggregates. Events may contain only minimal information, like one ID for aggregates, or be enhanced to include data that is useful to the common user.

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For instance an event from an event from the Order Service can publish an OrderCreated Event when an order is made. The OrderCreated event could only contain the orderId. Or, it could include the entire order so customers of the event do not need to retrieve the information through Order Service. Order Service. The way events are announced and what they contain is determined by the demands of the people who attend. When it comes to event sourcing, however it’s the aggregate that determines the nature of the events and their organization.

Events aren’t a requirement when you use event sources. Every state change in an aggregate, even the creation of it, can be represented as an event in the domain. When the aggregate’s state changes, it will release an event. For instance the case of an Order aggregate has to emit an OrderCreated event whenever it’s first created, and then it must emit an Order* event every time it’s changed. This is a stricter requirement than prior the time when aggregates was only able to emit events relevant to consumers. Locals email address and contact lists database.
Additionally the event should contain the data the aggregate requires in order to make the state change. An aggregate’s status is composed of the values that are in those fields in the object that make up the aggregate. A state change could be as easy as altering the value in the field on an object, like Order.state. Alternately, a state change could be a matter of adding or removing objects, for example, making changes to an Order’s line items.

Let’s say, as the figure 6.3 illustrates, that the state currently in the overall aggregate S while the state that is to be changed will become S’. The event E that symbolizes the state change has to contain the information that when an order is in the state S and you want to apply it, calling order.apply(E) will bring an Order’s status to S’. In the following section, you’ll be able to see that applying() is an application method that executes the state change reflected through an incident. Locals email address and contact lists database.

Certain events, like those that are part of the Order Shipped event, contain minimal or no data, and simply depict the state change. Apply() method manages the Order Shipped event by changing the status field of the order to SHIPPED. However, other events have many details. An OrderCreated Event, for example, has to include all the information required to be used by using the Apply() method used to create an order, which includes line items as well as payment information, delivery information and more. Since events are designed to keep an aggregate of data so you don’t have the possibility of using a minimum OrderCreated Event that has the orderId.

The business logic takes care of the need to update an aggregate using an command method from the root of the aggregate. In traditional applications the command method tests its arguments and changes one or more fields in the aggregate. Command methods employed in an application that uses event sourcing work as they create events. As the figure 6.4 illustrates, the result of a com-mand method is a series of events that reflect the state changes that need to be implemented. These events are stored in the database, and then applied to the aggregate in order to change its state. Locals email address and contact lists database.

The requirement to generate events and apply them requires a restructuring–albeit mechanical–of the business logic. Event sourcing transforms an operation into two or different methods. The first method uses an object in the command, which is the source of the request and decides on the state changes that must be carried out. It then validates the arguments and, without altering the status of the overall, it it returns an event list that reflects the changes in state. This approach generally throws an error if the command is not able to be executed.
The other methods require a specific event type as a parameter. They then adjust the total. There’s a method for every event. It is important to remember that these methods cannot be successful, as an event can be described as a state change that occurred. Each method is able to update the total in response to the incident.

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This framework, the Eventuate Client framework, an event-sourcing framework, which is explained in more depth at section 6.2.2, is the name of these methods processing() as well as use(). Process() method is the command object that contains the arguments for the update request as a parameter, and returns the list of events.

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The apply() method uses events as parameters, and returns null. An aggregate can define several overloaded versions of these methods that include One process() technique for each Command class , and the one application() technique for each type of event emitted through the aggregate. Figure 6.5 illustrates an illustration.

{public class Order Public Class Order

Public List revise(OrderRevision orderRevision) {(state) switch (state) {•

case that has been authorized:

LineItemQuantityChange change =

orderLineItems.lineItemQuantityChange(orderRevision);  buy best locals email lists online.

if (change.newOrderTotal.isGreaterThanOrEqual(orderMinimum)) { throw new OrderMinimumNotMetException();


this.state = REVISION_PENDING;

Return …;


throw new UnsupportedStateTransitionException(state);



{public class Order Public Class Order

Public List process(ReviseOrder command) { OrderRevision orderRevision = command.getOrderRevision(); switch (state) {

case that has been authorized:

LineItemQuantityChange change =

orderLineItems.lineItemQuantityChange(orderRevision); if (change.newOrderTotal.isGreaterThanOrEqual(orderMinimum)) {

throw new OrderMinimumNotMetException(); buy best locals email lists online.


return singletonList(

new OrderRevisionProposed(

orderRevision, change.currentOrderTotal, change.newOrderTotal));


throw new UnsupportedStateTransitionException(state); buy best locals email lists online.



{public class Order public class Order

public void apply(OrderRevisionProposed event) { this.state = REVISION_PENDING;


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In this instance the reviseOrder() method is replaced with the process() method as well as the apply() procedure. Process() method accepts an ReviseOrder command as an argument. This command class is defined by applying Introduce Parameter Object refactor-ing (https://refactoring.com/catalog/introduceParameterObject.html) to the revise-Order() method.

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The process() method either returns an OrderRevisionProposed event, or throws an exception if it’s too late to revise the Order or if the proposed revi-sion doesn’t meet the order minimum. The apply() method for the OrderRevision-Proposed event changes the state of the Order to REVISION_PENDING.

An aggregate is made by one of the steps below:

Instantiate aggregate root using its default constructor.

Invoke the process() to create the events that are new. buy best locals email lists online.

Refresh the total by looping through the new events making it a call to its application().

Keep the latest events in the store for events.

A new aggregate is created by one of the steps below:

Access aggregate’s events through the store for events.

Create the root of aggregate with the default constructor.

Then iterate through the loaded events and then call the apply() to the root of the entire sequence.

Use its() procedure to create new events. buy best locals email lists online.

Refresh the total by going through the events that have changed, applying().

Keep the latest events in the store for events.

To witness the process in action we’ll now take a take a look at the event-sourcing model of Order aggregate.

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Listing 6.1 lists the fields in the Order aggregate as well as the processes responsible for creating it. The event-sourcing version of Order aggregate shares some similarities with the version based on JPA that is shown in chapter 5.

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The fields are similar and it also emits similar events. The difference is that its operation logic operates using pro-cessing command lines that generate events and then applying the events to update the state of the aggregate. Every method that updates or creates the aggregate based on JPA such as the createOrder() as well as reviseOrder() is substituted in the event-sourcing version using processes() as well as use() techniques.

Listing 6.1 The fields in the Order aggregate and the methods it uses to initialize an instance

{public class Order public class Order

private OrderState state

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private Long restaurantId

Private OrderLineItems orderLineItems;

private DeliveryInformation deliveryInformation; private PaymentInformation paymentInformation; private Money orderMinimum;

Public Order to the public() {is a valid command and
returns|The event is returned as|Returns} an OrderCreatedEvent
Public List process(CreateOrderCommand command) {
… verify request …
return events(new OrderCreatedEvent(command.getOrderDetails())); buy