Home > Articles

This chapter is from the book

Defining Services and Interfaces

Before we can get to work building our services, we need to define what those services are and their interfaces. When we begin decomposing the services, one of the questions that comes to mind is, how big should our services be? Deciding on the number and size of services can be a frustrating task. There are no specific rules for the most optimal size. Some combination of techniques can be used to help determine if we should consider further decomposing a service. The number of files or lines of code can provide some indication that we might want to consider breaking a service up. Another reason might be that a service is providing too many responsibilities, making it hard to reason about, test, and maintain. Team organization, which could require changes to the service partition and the team structure alike, can be a factor. Mixed service types, such as handling authentication and serving web pages, might be another good reason to further decompose a service.

An understanding of the business domain will be important here. The more we know about the capabilities and features of the application, the better we can define the services and their interfaces used to compose that application.

In the end, we want a number of small, loosely coupled services with closely related behavior located together in a service. Loosely coupled services enable us to change one service without needing to change another. It’s very important that we are able to update our services independently. We need to take some care when planning integrations between services, so that we do not inadvertently introduce coupling between the services. Accidental coupling could be caused by factors like shared code, rigid protocols, models, shared database, no versioning strategy, or exposing internal implementation details.

Decomposing the Application

When approaching an existing or new application, we will need to determine where the seams are and how to decompose the application into smaller services—among the more challenging and important things to get right in a microservices architecture. We want to make sure to minimize coupling, and that we keep closely related things together. As with good component design, we strive for high cohesion and low coupling in our service design.

Coupling is the degree of interdependence, and low coupling means we have a very small number of interdependencies. This is important for maintaining an independent lifecycle for our services. To keep coupling low, we need to carefully consider how we partition the application, as well as how we integrate the services.

An important thing to consider with regard to a microservices architecture is that when we are discussing high cohesion, we generally mean functional cohesion. Cohesion is the degree by which something is related, and if the degree of cohesion is high, then it’s very closely related. Things that have very high cohesion will generally need to change together. If we are making a change to a service we want to be able to make that change in only the one service and release it. We should not have to coordinate changes and releases across multiple services. By keeping closely related things together in a service it can be much easier to accomplish this. This will also tend to reduce chattiness across the services. A key difference with a microservices architectures when compared to traditional SOA is the importance placed on functional over logical cohesion.

We need to identify the boundaries in the application that we will use to define the individual services and their interfaces. As we mentioned previously, those boundaries should ensure closely related things are grouped together and unrelated things are someplace else. Depending on the size and complexity of the application this can be a matter of identifying the nouns and verbs used in the application and grouping them.

We can use Domain-Driven Design (DDD) concepts to help us define the boundaries within our application that we will use to break it down in to individual services. A useful concept in DDD is the bounded context. The context represents a specific responsibility of the application which can be used in decomposing and organizing the problem space. It has a very well-defined boundary which is our service interface, or API.

When identifying bounded contexts in our domain, think about the business capabilities and terminology. Both will be used to identify and validate the bounded contexts in the domain. A deep dive into Domain-Driven Design (DDD) is out of the scope of this book, but there are a number of fantastic books in the market that cover this in great depth. I recommend “Domain-Driven Design” by Eric Evans and “Patterns, Principles, and Practices of Domain-Driven Design” by Scott Millet and Nick Tune. Defining these boundaries in a large complex domain can be challenging, especially if the domain is not well understood.

We can further partition components within a bounded context into their own services and still share a database. The partitioning can be to meet some nonfunctional requirements like scalability of the application or the need to use a different technology for a feature we need to implement. For example, we might have decided our product catalog will be a single service, and then we realized the search functionality has much different resource and scale requirements than the rest of the application. We can decide to further partition that feature into its own individual service for reasons of security, availability, management, or deployment.

Service Design

When building a service, we want to ensure our service does not become coupled to another team’s service, requiring a coordinated release. We want to maintain our independence. We also want to ensure we are not breaking our consumer when deploying updates, including breaking changes. To achieve this, we will need to carefully design the interfaces, tests, and versioning strategies, and document our services while we do so.

When defining the interfaces, we need to ensure we are not exposing unnecessary information in the model or internals of the services. We cannot make any assumptions of how the data being returned is used, and removing a property or changing the name of an internal property that is inadvertently exposed in the API can break a consumer. We need to be careful not to expose more than what is needed. It’s easier to add to the model that’s returned than it is to remove or change what is returned.

Integration tests can be used when releasing an update to identify any potential breaking changes. One of the challenges with testing our services is that we might not be able to test with the actual versions that will be used in production. Consumers of our service are constantly evolving their services, and we can have dependencies on services that have dependencies on others. We can use consumer-driven contracts, mocks, and stubs for testing consumer services and service dependencies. This topic is covered in Chapter 6, “DevOps and Continuous Delivery.”

There will come a time when we need to introduce breaking changes to the consumer and when we do this, having a versioning strategy in place across the services will be important.

There are a number of different approaches to versioning services. We could put a version in the header, query string, or simply run multiple versions of our service in parallel. If we are deploying multiple versions in parallel, be aware of the fact that this will involve maintaining two branches. A high-priority security update might need to be applied to multiple versions of a service. The Microsoft Patterns & Practices team has provided some fantastic guidance and best practices for general API design and versioning here: https://azure.microsoft.com/en-us/documentation/articles/best-practices-api-design/.

It’s also important that the services are documented. This will not only help services get started consuming the API quickly, but can also provide best practices for consuming and working with the API. The API can include batch features that can be useful for reducing chattiness across services, but if the consumer is not aware these batch features do not help. Swagger (http://swagger.io) is a tool we can use for interactive documentation of our API, as well as client SDK generation and discoverability.

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.


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.


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.


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.


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


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


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.


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.


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