# Chapter 5. The Application

Evolve or die.

Up to this point we have discussed the tools that you can use to build Web APIs. We have discussed the fundamentals of the HTTP protocol, the basics of using ASP.NET Web API, and how the architectural pieces fit together. This is essential knowledge, but not the only objective of this book. This book is also about how to build evolvable Web APIs. This chapter is where we begin to talk about how to create a Web API that can evolve over a period of years—a long enough span of time that business concerns and technology will change.

Rather than discuss the issues in abstract scenarios, we will walk the walk and build an API that demonstrates the concepts we wish to convey. This API will concern a domain that should be familiar to every developer and is sufficiently realistic that it could be adopted in real-world scenarios.

Before delving into the details of the domain, we must ensure that evolvability is something we are truly prepared to pay for. It does not come for free. In order to achieve evolvability, where appropriate, we will apply the constraints of the REST architectural style. It is critical to recognize that we are not attempting to create a "RESTful API." REST is not the objective, but rather a means to an end. It is quite possible that we will choose to violate some of the REST constraints in certain scenarios. Once you understand the value and costs of an architectural constraint, you can make an informed decision about whether that constraint should be applied to achieve your goal. Evolvability is our goal.

Before we can begin designing our Web API, we need to define the building blocks for our application domain. This is part of the process that is so often ignored when people attempt to build distributed systems today. When you are building a classic browser-based application, the solution architecture is largely already defined. HTTP, HTML pages, CSS, and JavaScript all form the components of the solution. However, with a Web API, you are no longer constrained by the choices of the web browser. A web browser may be only one of many different client components in the distributed system.

With the building blocks defined, Chapter 7 will provide a sample API that assembles these components into a Web API. These component pieces will be essential knowledge for building a client application in Chapter 8. By defining the reusable components independently of the API, we can build the client with the capability to interact with these components without needing to know the precise shape of the Web API. Hypermedia will allow the client to discover the shape of the API and adapt to changes as it evolves.

## Why Evolvable?

What exactly do we mean by an evolvable API? To an extent, evolve is just a fancy word for change. However, evolution implies a continuous set of small changes that can, over many iterations, cause the end solution to look unrecognizable to the starting point.

There are a variety of ways that a Web API may need to evolve over the course of its life:

• Individual resources may need to hold more or less information.
• The way individual pieces of information are represented may change. A name or a data type may be changed.
• Relationships between resources can be added, can be removed, or can have their cardinality changed.
• Completely new resources may be added to represent new information in the API.
• Resources may need new representations to support different types of clients.
• New resources may be created to provide access to information with a finer or coarser granularity.
• The flow of processes supported by the API may change.

For many years, the software development industry attempted to follow the lead of more traditional engineering practices when it came to managing change. Conventional wisdom stated that change early in a product’s development was many times less expensive than a change that occurred at later stages in its development. The solution was often to strictly manage change and attempt to limit it by performing extensive up-front planning and design work. In recent years, the rise of agile software development practices has taken us on a different path, where change is accepted as part of the process. Embracing change in small iterations allows a software system to evolve to meet the needs of its users. It is not uncommon to hear of websites that release new versions of their software many times a day.

However, with Web APIs, where the plan is to allow external teams to develop their own software to consume the API, our industry has regressed to its old habits. Very often the mindset is that we need to plan and design the API so we can get it right, because if we make changes to the API that break the software, our customers will be unhappy. There are many examples of well-known API developers who have introduced a v2 API that required client applications to do lots of rework to migrate to the new version.

### Barriers to Evolution

There are a few factors that impact the difficulty of dealing with change. One of the biggest factors is who is impacted by the change. It is very common for the consumers of the API to be a different team than the producers of the API. In addition, the consuming team often belongs to a different company than the producing team. In fact, it is likely that there are many consumers from many different companies. Implementing a breaking change can make lots of customers unhappy.

Even when consumer and producer applications are managed by the same team, there can be constraints that prevent the client and server from being deployed simultaneously. When client code is deployed onto a large number of clients, it can be tricky to synchronize the updates of clients and servers. When clients are installed on locked-down machines, there can be additional complications. Sometimes forcing client updates just creates a bad user experience. If a user wishes to use a client application but before he can he must apply an update, he might get frustrated. Most popular auto-updating client software applications download new versions in the background while they are running and apply on the next restart. This requires the server software to be able to continue to support the old version of the client for at least a short period of time.

One of the challenging things about changing software is that some changes are really easy and have a very low impact, whereas other changes can affect many parts of the system significantly. Identifying what kind of change you are trying to perform is critical. Ideally, we should be able to compartmentalize our software so that we can separate the pieces that can be changed with minimal impact from those that have the potential for significant impact. As a simple example, consider making a change to the HTML specification versus adding a new page to a website. Changing HTML might require everyone to update their web browser, which is a mammoth endeavor, while adding a new web page to a site will not have any impact on web browsers. The web architecture has intentionally made this division to allow the world of websites (servers) to evolve without users having to continually update the web browser (clients).

Change needs to be managed. It requires control and coordination. Any steps we can take to facilitate change over the long term will easily pay for themselves many times over. However, building evolvable systems is not free. There are definitely costs involved.

### What Is the Cost?

In order to achieve evolvability, the REST constraints prevent our client applications from making certain assumptions. We cannot allow the client to know in advance about the resources that are available on the server. They must be discovered at runtime based on a single entry point URL.

Once the client has discovered the URL for a resource, it cannot make any assumptions about what types of representations might be returned. The client must use metadata returned in the response to identify the type of information that is being returned.

These limitations require building clients that are far more dynamic than a traditional client/server client. Clients must do a form of "feature detection" to determine what is possible, and they must be reactive to the responses that are returned.

A simple example of this is a server API that returns an invoice. Assuming a client knows how to render plain-text, HTML, and PDF documents, it is not necessary for the server to define in advance what format it will return. Maybe in the first version of the software a simple plain-text invoice is returned. Newer versions of the server might implement the ability to return the invoice as HTML. As long as a mechanism exists to allow the client to parse the response and identify the returned type, the client can continue working unchanged. This notion of messages that contain metadata that identifies the content of the message is called self-descriptive messaging.

Additionally, when defining API specifications you should specify the minimum detail necessary, not every detail. Apply Einstein’s wisdom of "Everything should be as simple as possible, but not simpler." Overconstraining an API will increase the chances of change impacting larger parts of the system.

Examples of overspecifying include:

• Requiring pieces of data to be provided in a certain order, when the order is not important to the semantics of the message
• Considering data as required when it is needed only under certain contexts
• Constraining interactions to specific serializations of platform-defined types

The following two sidebars describe real-life scenarios that demonstrate how being overly prescriptive can have a negative impact on the outcome.

In the following example, instead of simply stifling creativity and producing an inferior product, the result completely fails to satisfy the original requirements.

We often see similar results in software project management, where customers and business analysts try to use requirements to precisely define their envisioned solution, instead of capturing their true intent and letting the software professionals do what they do best. The contracts we use when building distributed systems must capture the intent of our business partners if they are going to survive over the long term.

The combination of runtime discovery, self-descriptive messaging, and reactive clients is not the easiest concept to grasp or implement, but it is a critical component of evolvable systems and brings flexibility that far outweighs the costs.

### Why Not Just Version?

The traditional approach to dealing with breaking changes in APIs is to use the notion of versioning.

From the perspective of developing evolvable systems, it is useful to consider versioning as a last resort, an admission of failure. Assigning a v1 to your initial API is a proclamation that you already know it cannot evolve and you will need to make breaking changes in your v2. However, sometimes we do get it wrong. Versioning is what we do when we have exhausted all other options.

The techniques discussed in this book are to help you avoid the need to create versions of your API. However, if and when you do need to version some piece of your API, try to limit the scope of that declaration of failure.

Do not interpret this guidance as saying you must do "big design up front" and get everything right the first time. The intent is to encourage an attitude of minimalism. Don’t specify what you don’t need to; don’t create resources you don’t need. Evolvable APIs are designed to change so that when you identify new things that must be added you can do it with minimum effort.

Versioning involves associating an identifier to a snapshot of an API, or some part of an API. If changes occur to the API, then a new identifier is assigned. Version numbers enable coordination between client and server so that a client can identify whether it can communicate with the server. Ideas like semantic versioning have created ways to distinguish between changes that are breaking and those that are nonbreaking. There are several ways to use versioning within evolvable systems with varying degrees of impact severity.

Versioning can be done:

• Within the payload (e.g., XML, HTML)
• With the payload type (e.g., application/vnd.acme.foo.v2+xml)
• At the beginning of the URL (e.g., /v2/api/foo/bar)
• At the end of the URL (e.g., /api/foo/bar.v2)

One of the most important characteristics of the web architecture is that it promotes the concept of payload format into a first-class architectural concept. In the world of RPC, parameters are simply an artifact of the procedural signature and do not stand alone. An example of this is HTML. HTML is a largely self-contained specification that describes the structure of a document. The specification has evolved hugely over the years. The HTML-based Web does not use URI-based versioning or media type versioning. However, the HTML document does contain metadata to assist the parser in interpreting the meaning of the document. This type of versioning helps to limit the impact of version changes to the media type parser codebase. Parsers can be created to support different versions of wire formats, making support for older document formats fairly easy. It remains a challenge to ensure that new document formats don’t break old parsers.

#### Versioning the media type

In recent years, the idea of versioning the media type identifier has become popular. One advantage of this approach is that a user agent can use the Accept header to declare which versions of the media type it supports. With well-behaving clients you can introduce breaking changes into a media type without breaking existing clients. The existing clients continue asking for the old version of the media type, and new clients can ask for the new version.

A downside to using media types to version is that it exacerbates one of the existing problems with server-driven negotiation. It is not unlikely that a service may use many different media types to expose a wide variety of content. Requiring that a client declare, on every request, all of the media types that it is capable of rendering adds a significant amount of overhead to each request. Adding versions into the mix compounds that problem. If a client supports v1,v2, and v3 of a particular media type, should all of them be included in an Accept header in case the server is only capable of rendering an old version? Some user agents have started to take the approach where they only send a subset of media types in the Accept header, depending on the link relation being accessed. This does help reduce the size of the Accept header, but it introduces an additional complexity where the user agent must be able to correlate link relations to appropriate media types.

#### Versioning in the URL

Versioning in the URL is probably the most commonly seen approach in public APIs. To be more precise, it is very common to put a version number in the first segment of a URL (e.g., http://example.org/v2/customers/34). It is also the approach most heavily criticized by REST practitioners. The objection is usually related to the fact that by adding URLs with a new version number, you are implicitly creating a duplicate set of resources with the new version number while many of those resources may not have changed at all. If URLs have previously been shared, then those URLs will point to the old version of the resource rather than the new version. The challenge is that sometimes this is the desired behavior and other times it is not. If a client is capable of consuming the new version, it would prefer the new one instead of the old version that it bookmarked. If the resource with the new version is actually identical to old version, then it introduces a new problem where there are two distinct URLs for the same resource. This has numerous downsides when you are trying to take advantage of HTTP caching. Caches end up storing multiple copies of the same resource, and cache invalidation becomes ineffective.

An alternative approach to URL versioning is appending the version number near the end of the URL (e.g., http://example.org/customer/v2/34). This allows individual resources to be versioned independently within an API and eliminates the problem of creating duplicate URLs for identical resources. Clients that construct URLs have a much more difficult time consuming this type of versioned URL, because it is not as simple as just changing the first path segment in all request URLs. Hypermedia-driven clients can’t even take advantage of this type of versioning because URIs are opaque to them. New resource versions must be identified via a versioned link relation to be accessible to this type of client.

Versioning of APIs is a difficult topic that has many pitfalls. Making the effort to avoid versioning as much as possible will likely provide many long-term benefits.

### Walking the Walk

So far in this chapter, we have discussed in general the pros and cons to developing evolvable applications. At many points in the development of an evolvable distributed application, you’ll have choices to make and the right answer is frequently "it depends." There is no way, in any reasonable number of pages, to consider every possibility and every outcome; however, there is value in demonstrating a particular set of choices to a particular set of circumstances. In the remainder of the chapter, we will focus on a specific application that presents common challenges, and we will consider the options and make a decision. In no way are we suggesting the choices we will make in the following chapters are the best for every scenario, but they should be illustrative of the types of decisions and choices that you will need to make while building an evolvable API.

## Application Objectives

It is always challenging to pick a domain for a sample application for demonstration purposes. It should be realistic but not to the point of getting bogged down in domain details. It should be complex enough to provide a wide range of scenarios but not so large that the architectural guidance gets lost in the implementation details. To eliminate the need for you to learn domain specifics, we chose a domain that software developers are already familiar with: issue tracking. It is an area in which we have all seen a variety of implementations of both client and server. It is naturally distributed because its primary focus is communicating and sharing information among different team members. There are many use cases that surround the lifecycle of an issue. There are many different types of issues that have their own set of distinct states. Issues have a wide variety of metadata associated with them.

### Goals

We want to define the types of information that we believe fall within the domain of issue tracking. We wish to:

• Define the minimal set of information required to represent an issue.
• Define a core set of information that is commonly associated with issues.
• Identify relationships between the information in our system.
• Examine the lifecycle of an issue.
• Identify the behavior associated with an issue.
• Classify the types of aggregations, filtering, and statistics that are done on issues.

This is not an attempt to exhaustively define the ultimate, all-encompassing issue schema. Our objective is not to define functionality for every possible scenario in the domain of issue tracking, but to identify a common set of information and terminology that delivers value to people who wish to build some form of issue tracking application, without limiting the scope of those applications. What we are defining will evolve.

When you look at different applications that attempt to address the same domain, you will often find they have taken slightly different approaches to the same problem. We are looking to identify those differences where it doesn’t matter which option is chosen; let’s just pick one, or enable both. When we eventually distill this domain down into media type specifications, link relations, and semantic profiles, we should be left with a common ground that enables a degree of interoperability without limiting individual applications from providing unique features.

### Opportunity

The domain of issue tracking is ripe for improvement. There are dozens of commercial and open source applications that address this domain, and yet they all use proprietary formats for interacting between server and client components. Issue data is locked in proprietary data stores that are tied to the application that created that data. We are limited to using client tools that were designed specifically for a particular issue data store. Why can’t I use my favorite issue management client to manage both the work items defined in my Bitbucket repositories and in my GitHub repositories? Why do I even need multiple stores? No one ever considered writing a web browser that is specific to Apache or IIS, so why do we insist on coupling client and server applications in other domains that are focused on distributed data?

Unfortunately, the type of sharing and reuse that the web architecture enables through the reuse of standard media types does not appear to be in the best interests of commercial organizations. Usually, it requires open source efforts to get the initial momentum going before commercial organizations take notice and realize that integration and interoperability can actually be major benefits to commercial software.

## Information Model

Before we can begin to develop web artifacts like media types, link relations, or semantic profiles, we must have a clearer understanding of the semantics that we need to communicate across the Web.

At its most basic, an issue could be described in just a short string of text. For example, "The application fails when clicking on button X in screen Y." Additionally, it is often desirable to include a more detailed description of the issue.

Consider the following extremely simple definition:

Issue
Title
Description (Optional)

This definition is potentially sufficient for someone to take this issue and resolve it. Although the issue contains no information about who created it or when, it is possible to capture that information from available ambient information when the request to create the issue is made. Having this extremely minimalist representation of an issue provides a very low barrier of entry that can later be evolved. It is good for getting a working application for demo purposes, and it is also useful for low-power clients like phones. There is nothing to stop someone from later using a more powerful client to fill in additional data.

### Subdomains

Before charging ahead and implementing this minimal representation, let’s get a better understanding of the larger set of data that can be associated with an issue. For organizational purposes I have broken down the information into four subdomains: descriptive, categorization, current state, and historical.

#### Descriptive

The descriptive subdomain includes the information we already discussed such as title and description, but also environmental information, such as software version, host operating system, hardware specifications, steps to reproduce, and screen captures. Any information used to provide precision about the situation that produced the issue will fall into this subdomain. One important characteristic of this information is that it is primarily just human readable. It does not tend to affect the issue workflow or impact any kind of algorithms.

#### Categorization

These pieces of information are used to help deal with sets of issues in meaningful groups. Attributes that belong to a certain predefined domain of values can be attached to an issue for the purpose of classifying issues for processing. Examples of this type of information include priority, severity, software module, application area, and issue type (defect, feature, etc). This information is used for searching, filtering, and grouping and is often used to dictate application workflow. Generally, this information is specified early on in the issue lifecycle and does not change unless information was specified erroneously.

#### Current state

An issue will generally have a set of attributes that define its current state. This includes information such as the current workflow state, the person actively assigned to the issue, hours remaining, and percent complete. This information will change numerous times over the life of an issue. It can also be used as classification attributes. The current state of an issue may also be annotated with textual comments.

#### Historical

Historical information is usually a record of an issue’s current state at a prior point in time. This information is generally not important for the processing of the issue, but may be useful for analytics of past issues, or investigating the history of an individual issue.

### Attribute Groups

Sometimes it is useful to group attributes together. This can help to make a representation easier to read. The groups can sometimes be used to simplify client code when a set of attributes can be processed as a whole. Perhaps a user agent does not want to deal with environmental attributes and therefore an entire group can be ignored. It is also possible to use attribute groups to introduce a conditional requirement for mandatory information. For example, if you include group X, then you must include attribute Y in the group. This allows us to support a very minimal representation but still ensure that key information is provided if a particular aspect of the issue is to be included. One specific example of this might be that when you include a history record that specifies the state of an issue at an earlier point in time, you must also provide the date and time attribute.

There is, however, a danger to having groups and using those groups within a media type. Deciding that an attribute has been put in a wrong group and moving it to a new group may end up being a breaking change, so it is important to be cautious with the use of groups.

Here is an example of an attribute group:

<resource>
<Title>App blows up</Title>
<Environment>
<OperatingSystem>Windows ME</OperatingSystem>
<AvailableRAM>284MB</AvailableRAM>
<AvailableDiskSpace>1.2GB</AvailableDiskSpace>
</Environment>
</resource>

### Collections of Attribute Groups

Attribute groups can be used when you want to represent a multiplicity in parts of a representation. Issues may have documents attached to them. Those documents would most likely be represented as links, but there may be additional attributes associated with the documents that can be grouped together. This allows multiple groups of these document attributes to be included in a single representation while maintaining the relationship between the document and its related attributes.

The following is an example of an attribute group collection:

<resource>
<Title>App blows up</Title>
<Documents>
<Document>
<Name>ScreenShot.jpg</Name>
<LastUpdated>2013-11-03 10:15AM</LastUpdated>
<Location>/documentrepository/123233</Location>
</Document>
<Document>
<Name>StepsToReproduce.txt</Name>
<LastUpdated>2013-11-03 10:22AM</LastUpdated>
<Location>/documentrepository/123234</Location>
</Document>
</Documents>
</resource>

### Information Model Versus Media Type

So far, we have talked about the information model that surrounds the domain of issue tracking. We have discussed, in abstract, how these pieces of information can be represented, grouped, and related (Figure 5-1). I have avoided the discussion of specific formats like XML and JSON because it is important to understand that the definition of the information model is independent of specific representation syntax. In the next chapter, when we talk about media types we will address the physical mapping of our conceptual model to the syntax of real media types and their particular formats.

There are several things to consider in relation to the reusability of this information model. Although it is quite extensive in its listed capabilities, the majority of this information is optional. This allows us to use the same model in both the simplest scenarios and the most sophisticated. However, in order to achieve any kind of interoperability we must draw a line in the sand and give specific names to attributes. Fortunately, what we are defining is purely an interface specification. There is no requirement for applications to use these same names when they persist data to their data stores, nor is it necessary for them to use those names in the client application’s user interface. As long as the semantic meaning of that data is accurately portrayed to the user, all is good.

When defining the media types we must consider what happens when an application wishes to include semantics that our information model currently does not support. Extensibility is an important goal; however, for this application, building interoperable extensibility is out of scope and so will not be part of the information model. That doesn’t mean we can’t allow media types to define their own extensibility options that will allow specific clients and servers to deal with extended data.

### Collections of Issues

In addition to the representation of an issue, our applications will probably need to be able to represent sets of issues. These are most likely to be representations returned from some kind of query request. In the next chapter, we will discuss the relative merits of building new media types to represent a set of issues versus reusing the list capabilities of existing media types and the variety of hybrid approaches that exist.

## Resource Models

Another major piece of application architecture to consider when building Web APIs are the resources exposed. It is not my intent to predefine a set of resources that must exist with an issue tracking API. One of the major differences between an evolvable API and and RPC/SOAP API is that the available resources are not part of the contract. It is expected that the client will discover resources, and the capabilities of the client will be limited to the resources that it can discover.

I do want to discuss the types of resources that an API might expose so that we have some ideas to work with when exploring the types of media types that our client needs to support. It is always good to start with a minimal set of resources. Resources should be quick and easy to create in a system, so as we gain real experience with consumers using the service we can easily add new resources to meet additional requirements.

### Root Resource

Every evolvable Web API needs a root resource. Without it, clients just cannot begin the discovery process. The URL of the root resource is the one URL that cannot change. This resource will mainly contain a set of links to other resources within the application. Some of those links may point to search resources.

### Search Resources

The canonical example of a search resource is an HTML form that has an input box and does a GET using the input value as a query string value. Search resources can be significantly more sophisticated than this, and sometimes can be completely replaced by the use of a URI template. Search resources will usually contain a link that returns some kind of collection resource.

### Collection Resources

A collection resource normally returns a representation that contains a list of attribute groups. Often each attribute group will contain a link to a resource that is being represented by the information in the attribute group.

An issue tracking application is likely to predefine a large number of collection resources, such as:

• Active issues
• Closed issues
• List of users
• List of projects

Often a very large number of collection resources can be defined for a Web API by using search parameters to produce filtered collections. It is important that when we use the term resource we understand the distinction between the concept and the underlying implementation that generates the representation. If I create an IssuesController that allows a client application to search on subsets of issues, the URLs /issues?foundBy='Bob' and /issues?foundBy='Bill' are two different resources even though it is likely that the exact same code generated the two representations. To my knowledge, there is no term in common usage that describes the shared characteristics of a set of resources that represent different instances of the same concept. From this point on, I will use the term resource class to identify this scenario.

### Item Resources

The bulk of the information retrieved via the API will be communicated via item resources. An item resource provides a representation that contains some or all of the information belonging to a single instance of some information model concept. It is likely that we will need to support multiple different levels of details. Considering the example of an issue, some clients may only want the descriptive attributes. Other clients may want all the details that could be edited.

There is a wide variety of subsets of information that a client may need for a particular use case. This is one reason why it is important that, for any issue-related media type that we might define, we remain very flexible regarding what information may or may not be included. Just because a representation of an issue resource does not contain the details of the issue history does not mean the information does not exist. This is where the idea of generating resource representations based on domain object serialization falls apart. An object has just one class definition; you cannot pick and choose which parts of the object you wish to serialize based on context with a generic object serializer.

When you are determining the appropriate subsets of attributes to include in a resource, it is important to consider several factors. Large representations have the advantage of requiring fewer roundtrips. However, when only a small amount of data is required, there will be some wasted bandwidth and processing time. Also, large representations have a higher chance of containing attributes with different levels of volatility. Including descriptive attributes in the same resource as current state attributes, we may find that the we cannot cache the resource representation for as long as we might like because the current state information changes frequently. Using data volatility as a guide for segregating data into distinct resources can be a very useful technique. The downside to breaking a single concept into multiple resources is that it can make doing atomic updates using a PUT more difficult, and it introduces more complexity during cache invalidation.

Having more, smaller resources means more links and more representations to manage, but it means there is more opportunity for reuse.

There is no formulaic approach for determining the best granularity for resources. It is essential to consider the specific use cases and the various factors we have just discussed and choose the most appropriate resource sizes for the situation.

Figure 5-2 shows one particular resource model that an issue tracking service might implement. Each of these resource or resource classes will be exposed at a particular URL by the service. We have chosen not to show what those URLs will be because they are not relevant to our design, nor should they be relevant to the client.

All too often, developers try to do "design by URL" when building an API. There are a number of problems with this approach. Designing by URL steers people to try to define their application as a hierarchial data structure rather than the application workflow/state machine that should be being modeled. Limitations of the chosen implementation framework’s ability to parse, process, and route URLs will tend to constrain the system design even further. Designing by URL also encourages developers to try to create a consistency in their URI structure that is completely unnecessary and potentially constraining to the design. Identifying resources and the relations between them can be completely independent of URI structure, and later on a URI space can be mapped to the resources. This is a unique benefit of systems where clients are hypermedia driven. When clients construct URIs based on knowledge of the server’s URI space, the need for a uniform URI space with a significant structure becomes pressing.

Any client who understands the media types and link relations that we will be discussing in the next chapter will be able to consume this service without any prior knowledge of any of these resources other than the root."

## Conclusion

In this chapter, we have considered the conceptual design of our application. We reviewed why we would want to build a system that can evolve and the costs of that choice. We identified the building blocks of our design and reviewed the application domain.

Fundamentally, this is a distributed application and will require communication between different systems. To implement this in a manner that will successfully evolve, we need to define the contracts that will be used to convey application semantics between the system components. This will be the focus of the next chapter.