Home > Articles > Programming

Implementing Domain-Driven Design: Aggregates

  • Print
  • + Share This
Clustering Entities and Value Objects into an Aggregate with a carefully crafted consistency boundary may at first seem like quick work, but among all DDD tactical guidance, this pattern is one of the least well understood. Vaughn Vernon, author of Implementing Domain-Driven Design, explains how crucial it is to follow the Aggregate Rules of Thumb when designing Aggregates.

Read Implementing Domain-Driven Design and more than 24,000 other books and videos on Safari Books Online. Start a free trial today.



This chapter is from the book

This chapter is from the book

  • The universe is built up into an aggregate of permanent objects connected by causal relations that are independent of the subject and are placed in objective space and time.
  • –Jean Piaget

Clustering Entities (5) and Value Objects (6) into an Aggregate with a carefully crafted consistency boundary may at first seem like quick work, but among all DDD tactical guidance, this pattern is one of the least well understood.

To start off, it might help to consider some common questions. Is an Aggregate just a way to cluster a graph of closely related objects under a common parent? If so, is there some practical limit to the number of objects that should be allowed to reside in the graph? Since one Aggregate instance can reference other Aggregate instances, can the associations be navigated deeply, modifying various objects along the way? And what is this concept of invariants and a consistency boundary all about? It is the answer to this last question that greatly influences the answers to the others.

There are various ways to model Aggregates incorrectly. We could fall into the trap of designing for compositional convenience and make them too large. At the other end of the spectrum we could strip all Aggregates bare and as a result fail to protect true invariants. As we’ll see, it’s imperative that we avoid both extremes and instead pay attention to the business rules.

Using Aggregates in the Scrum Core Domain

We’ll take a close look at how Aggregates are used by SaaSOvation, and specifically within the Agile Project Management Context the application named ProjectOvation. It follows the traditional Scrum project management model, complete with product, product owner, team, backlog items, planned releases, and sprints. If you think of Scrum at its richest, that’s where ProjectOvation is headed; this is a familiar domain to most of us. The Scrum terminology forms the starting point of the Ubiquitous Language (1). Since it is a subscription-based application hosted using the software as a service (SaaS) model, each subscribing organization is registered as a tenant, another term of our Ubiquitous Language.

From these they envisioned a model and made their first attempt at a design. Let’s see how it went.

First Attempt: Large-Cluster Aggregate

The team put a lot of weight on the words Products have in the first statement, which influenced their initial attempt to design Aggregates for this domain.

This design is shown in the following code, and as a UML diagram in Figure 10.1:

public class Product extends ConcurrencySafeEntity  {
    private Set<BacklogItem> backlogItems;
    private String description;
    private String name;
    private ProductId productId;
    private Set<Release> releases;
    private Set<Sprint> sprints;
    private TenantId tenantId;
    ...
}
Figure 10.1

Figure 10.1. Product modeled as a very large Aggregate

The big Aggregate looked attractive, but it wasn’t truly practical. Once the application was running in its intended multi-user environment, it began to regularly experience transactional failures. Let’s look more closely at a few client usage patterns and how they interact with our technical solution model. Our Aggregate instances employ optimistic concurrency to protect persistent objects from simultaneous overlapping modifications by different clients, thus avoiding the use of database locks. As discussed in Entities (5), objects carry a version number that is incremented when changes are made and checked before they are saved to the database. If the version on the persisted object is greater than the version on the client’s copy, the client’s is considered stale and updates are rejected.

Consider a common simultaneous, multiclient usage scenario:

  • Two users, Bill and Joe, view the same Product marked as version 1 and begin to work on it.
  • Bill plans a new BacklogItem and commits. The Product version is incremented to 2.
  • Joe schedules a new Release and tries to save, but his commit fails because it was based on Product version 1.

Persistence mechanisms are used in this general way to deal with concurrency.1 If you argue that the default concurrency configurations can be changed, reserve your verdict for a while longer. This approach is actually important to protecting Aggregate invariants from concurrent changes.

These consistency problems came up with just two users. Add more users, and you have a really big problem. With Scrum, multiple users often make these kinds of overlapping modifications during the sprint planning meeting and in sprint execution. Failing all but one of their requests on an ongoing basis is completely unacceptable.

Nothing about planning a new backlog item should logically interfere with scheduling a new release! Why did Joe’s commit fail? At the heart of the issue, the large-cluster Aggregate was designed with false invariants in mind, not real business rules. These false invariants are artificial constraints imposed by developers. There are other ways for the team to prevent inappropriate removal without being arbitrarily restrictive. Besides causing transactional issues, the design also has performance and scalability drawbacks.

Second Attempt: Multiple Aggregates

Now consider an alternative model as shown in Figure 10.2, in which there are four distinct Aggregates. Each of the dependencies is associated by inference using a common ProductId, which is the identity of Product considered the parent of the other three.

Figure 10.2

Figure 10.2.Product and related concepts are modeled as separate Aggregate types.

Breaking the single large Aggregate into four will change some method contracts on Product. With the large-cluster Aggregate design the method signatures looked like this:

public class Product ... {
    ...
    public void planBacklogItem(
        String aSummary, String aCategory,
        BacklogItemType aType, StoryPoints aStoryPoints) {
            ...
    }
    ...
    public void scheduleRelease(
        String aName, String aDescription,
        Date aBegins, Date anEnds) {
        ...
    }

    public void scheduleSprint(
        String aName, String aGoals,
        Date aBegins, Date anEnds) {
        ...
    }
    ...
}

All of these methods are CQS commands [Fowler, CQS]; that is, they modify the state of the Product by adding the new element to a collection, so they have a void return type. But with the multiple-Aggregate design, we have

public class Product ... {
    ...
    public BacklogItem planBacklogItem(
        String aSummary, String aCategory,
        BacklogItemType aType, StoryPoints aStoryPoints) {
        ...
    }

    public Release scheduleRelease(
        String aName, String aDescription,
        Date aBegins, Date anEnds) {
        ...
    }

    public Sprint scheduleSprint(
        String aName, String aGoals,
        Date aBegins, Date anEnds) {
        ...
    }
    ...
}

These redesigned methods have a CQS query contract and act as Factories (11); that is, each creates a new Aggregate instance and returns a reference to it. Now when a client wants to plan a backlog item, the transactional Application Service (14) must do the following:

public class ProductBacklogItemService ... {
    ...
    @Transactional
    public void planProductBacklogItem(
        String aTenantId, String aProductId,
        String aSummary, String aCategory,
        String aBacklogItemType, String aStoryPoints) {

        Product product =
            productRepository.productOfId(
                    new TenantId(aTenantId),
                    new ProductId(aProductId));

        BacklogItem plannedBacklogItem =
            product.planBacklogItem(
                    aSummary,
                    aCategory,
                    BacklogItemType.valueOf(aBacklogItemType),
                    StoryPoints.valueOf(aStoryPoints));

        backlogItemRepository.add(plannedBacklogItem);
    }
    ...
}

So we’ve solved the transaction failure issue by modeling it away. Any number of BacklogItem, Release, and Sprint instances can now be safely created by simultaneous user requests. That’s pretty simple.

However, even with clear transactional advantages, the four smaller Aggregates are less convenient from the perspective of client consumption. Perhaps instead we could tune the large Aggregate to eliminate the concurrency issues. By setting our Hibernate mapping optimistic-lock option to false, we make the transaction failure domino effect go away. There is no invariant on the total number of created BacklogItem, Release, or Sprint instances, so why not just allow the collections to grow unbounded and ignore these specific modifications on Product? What additional cost would there be for keeping the large-cluster Aggregate? The problem is that it could actually grow out of control. Before thoroughly examining why, let’s consider the most important modeling tip the SaaSOvation team needed.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020