Ask software professionals what software architecture is and you will get many different answers.
In the past I have worked as a software architect and development lead on a variety of projects and teams. Two years ago I joined a larger software development organization as an individual contributor, and for the last eight months I’ve been the sole engineer assigned to create a very large in-memory graph data service that provides sub-second traversals across tens of thousands of edges amongst several billion. It has been a fun and successful project, but lately I’ve been thinking more and more about software architecture.
I’m sure that millions of words have been written on the subject of software architecture. I have read of them. Most recently I’ve been perusing two tomes on the subject: Software Architecture Foundations, Theory and Practice (Taylor, et al) and Software Architecture in Practice - 3rd Edition (Bass, et al). The latter is quickly becoming my favorite, but both of these are exhaustive academic textbooks, so most software professionals would find it impractical to put them to good use. Even so, I recommend them to you.
So what do I mean by “practical agile software architecture” and how can it be used by an agile software development organization? That is the question I will attempt to answer here. Distilling some of my reading through the filter of my own experience, I offer the following thoughts on the matter.
Principles of Practical Agile Software Architecture
- Architecture is part of the product or a product dependency – Architecture should come first. Any large and complicated software project should have a architectural artifacts (see below) sufficiently defined to guide product backlog story definition to seed and sustain ongoing implementation with minimal risk of disruptive architectural changes.
- Architecture stories go on the backlog – Architecture stories are generated from the stakeholder/view matrix (see below). Stories may repeat by adding detail to or modifying existing artifacts. Modifications should therefore be taken carefully considering the implementation changes that may be required.
- Architecture story implementation produces artifacts (scenarios and views) – The architectural scenarios and views (Bass) include the definition of non-functional quality attributes such as security, reliability and performance. These artifacts document the architecture sufficient for the implementation team as well as stakeholders.
- Architecture quality attributes should be measurable – Simply declaring that the application must be fast is not enough. Quality attribute definitions, as one type of artifact (see scenarios below), must produce measurable, observable, testable indicators.
- Architecture artifacts evolve iteratively – Each iteration should produce a useable architecture, sufficient to inform stakeholder decisions and guide implementation. This does not mean that the architecture is never revisited. Architecture should be a regular part of every iteration, even if the story is simply to review some part or the whole of the architecture that is relevant to other stories being worked on during that iteration.
- Architecture change can affect product backlog – Architecture stories that result in a modification to an artifact will generally produce one or more changes to the product backlog in order to accommodate the modification in product implementation. Care should be taken to communicate each new version of the architecture to relevant stakeholders and the implementation team.
- Architecture complexity should be decomposed – In multi-team multi-service multi-product development organizations, it may be useful to create a centralized architecture team that defines and develops higher level architectural views upon which implementation teams take a dependency. Changes to the central architecture may produce changes to a service or product’s architecture related stories on their own backlog. As scrum of scrums can help to decompose more complex projects, so can the same approach be used to break down the complexity of the overall architecture to ensure agility.
Architecture Artifacts and Views
Documenting an architecture can be challenging. Some documents are produced as an overview. Some with varying levels of detail. Much of this depends upon the extent to which the architecture is complete and the intended audience for the specific view. Whether you call these documents a “view” or a treatment or a physical or logical or quality requirements document, the result should be the same. These documents should convey as concisely as possible the information needed to by stakeholders and implementation teams to make decisions.
These are summarized from Bass with my own particular spin.
- Qualities Scenarios – There are seven primary non-functional software qualities: security, availability, interoperability, modifiability, performance, testability and usability. These should be well defined with concrete scenarios specifying stimulus source, stimulus, environment, artifact, response and acceptable response measures. Here’s a summary example for each of the seven qualities.
- Security – A user (source) attempts to view private information (stimulus) in the patient records systems (artifact) on the beta testing system (environment) and an audit trail is maintained (response) and the attempt is prevented and reported (measure).
- Availability – A heartbeat monitor (source) receives server unresponsive status (stimulus) on a web service (artifact) in production operations (environment) and informs the failover system to take traffic (response) and no downtime is recorded (measure).
- Interoperability – Our patient billing system (source) sends account to collections (stimulus) to the collections system (artifact) in the testing system (environment) and a test letter is sent (response) and the test letter queue increases by one (measure).
- Modifiability – Developer (source) is required to change a data source (stimulus) on the records repository (artifact) in the source code (environment) and makes the change with a unit test (response) and completes the task in three hours (measure).
- Performance – A user (source) save changes to his profile (stimulus) in the company intranet (artifact) on the intranet (environment) and the change is displayed (response) and the change takes less than two seconds (measure).
- Testability – An automated unit test runner (source) executes a unit test (stimulus) on the code (artifact) in the test server (environment) and the test passes (response) with 85% code coverage (measure).
- Usability – A user (source) adds a widget to his blog (stimulus) on the admin page (artifact) in the production system (environment) and is taken to a page with the widget activated (response) with fewer than 3 modifications (measure).
- Module View – In the module view, design principles such as separation of concerns and loose coupling help to define boundaries around responsibility sets. Often a module is a separately compiled unit (e.g. a .NET assembly DLL).For example, the data repository module depends on and implements the data repository interfaces module which provides data models and the interface definitions for persisting those models. The data repository module can be replaced when a decision is taken to move from one database server to another.
- Elements – module = implementation units (sets of responsibilities)
- Relations – is a part of X, depends on Y, and is a Z
- Constraints – visibility and availability of module
- Usage – implementation blueprint, impact analysis, requirements tracing, information design, users stories and use cases
- Component & Connector View – Components and connections define logical processing units (processes or sub-processes) or data stores and their ports (sometimes called API) for connections to connect to. This could be a web service and the port (not TCP port) that defines the external boundary of that processing unit. A connector may me a message queue pushing and pulling messages or a browser connecting to a RESTful web service.
- Elements – component = processing unit or data store, connector = paths of interaction (external API and use of same)
- Relations – attachment = connector connected to a component port, interface delegation = a connector between compatible ports
- Constraints – components attach via port only to components and vice-versa, never to each other
- Usage – illustrate how the system works, define communication boundaries, guide implementation and design structure, and guide establishment of quality attributes for each component
- Physical Allocation View – Define the software that will run on each part of the physical deployment in each environment.
- Elements – software = what runs on each part of the environment, environment = where software is hosted and on what
- Usage – assists reasoning about quality attributes, distribution of work and teams, concurrent access to versions of the software, and the form and mechanics of software deployment
- Qualities View – A quality view document helps to correlate specific qualities and their scenarios with respect to the following:
- Security – define components and behaviors that have an impact on security and how security risks will be mitigated and prioritized.
- Communications – document how components will communicate, what networks channels and internal and external systems will be involved in data transfer, and how quality of service (QoS) will be maintained and how communication interruptions will be managed
- Exception / error handling – define how errors will be detected, reported, monitored and resolved
- Reliability – define how timing, integrity and other quality measures will be recorded and monitored
- Performance – establish traffic load expectation and handling patterns, latency and system and network metrics
For more complete details on how these views may be composed, I recommend picking up a copy of Software Architecture in Practice as previously mentioned.
Stakeholder View Matrix
Because the number of possible views and scenarios with varying levels of required detail can overwhelm the architecture backlog, I endorse the Bass recommendation of the stakeholder/view matrix. I’ll try to summarize it here.
Create a matrix of stakeholders, including implementation and testing roles, with common architecture views and qualities. Let’s assume you have executive, product manager, implementation and testing stakeholders. And you have seven qualities and a minimum of four views. This gives you a four by eleven matrix.
For each cell in the matrix enter a level of detail word (none, overview, moderate, high) and a priority number between 1 and 10. Use this matrix to identify artifacts that need to be produced first and what level of detail is required for the first, second and third iterations. This exercise will help you identify the most important architectural stories for your backlog and prioritize your work on the architecture.
Thoughts on Architecture and the Role of the Architect
Much of what a software architect is often required to do is not, strictly speaking, architecturally related. But if an architect wants a team to be successful at implementing the software, she or he will take special care to incorporate these things into their regular work.
- Patterns and Frameworks – architecture is NOT a framework. In fact, a framework should be able to be swapped out without really having to change the architecture. Frameworks are tools and as such should be carefully chosen for their implementation multiplier capabilities as well as their simplicity and adaptability. The same is true for implementation patterns. Developers who are guided to select appropriate implementation patterns by being provided prototypes and samples can be far more productive and will produce much more maintainable code. The effort invested by the hands-on architect in these will yield handsome returns.
- BDD and TDD – architects often wear the design hat as well and when doing so, they should adopt Behavior Driven and Test Driven Development. Read the linked content. It will really help simplify and clarify your user stories, use cases, and test designs. Of course, most design is driven by functional requirements which can affect the architecture in order to support the behaviors defined by the functional requirements. The software architect is appropriately involved in gathering and understanding these requirements. And breaking them down by behavior driven forms is an ideal way to fully understand and document these requirements.
- UML and Other Modeling Languages – there are many tools available to create diagrams and other artifacts that may help you communicate your architectures and designs. A word of caution. Unless your audience is familiar with the notation and the meaning of the symbols and shapes you use, you need to provide a key with sufficient detail that a less familiar reader can understand what it is your are trying to communicate. If the diagram uses too many abstractions, the reader, even a seasoned software engineer, may have trouble understanding your meaning. And when stakeholders and implementation team members do not understand it, they will not always ask. Most of the time they will either ignore or or make erroneous assumptions that will bite you in the tender flesh later.
- DevOps and Infrastructure – one of the things that can kill a great architecture and implementation is the lack of sufficient means to build, test and deploy, and roll back when necessary, in an automated fashion. A great DevOps team can make or break your large software development organization. And if you have the necessary operations monitoring and SLA failover triggers enabled with redundancy, you can maintain a very high level of service and reliability. A software architect’s input into the creation and improvement of these is critical to success.
Software architecture is not necessarily easy in part because we often make it more complicated than it need be. It is also true that we rarely define it well within an organization. Many organizations hire and promote to the position of software architect with the expectation that they will be an experienced software engineer who has picked up on what works and what does not and will help lead a team to success. Many experienced software engineer’s make this transition and learn the craft of architecture well. Some do not.
Some software architects spend very little time defining and refining the actual architecture of a the software. Instead they spend far more time chasing down design flaws, resolving thorny implementation issues and untangling behaviors that are at odds with business requirements. And yet, had the architect spent sufficient time and energy producing concise and precise architecture artifacts discussed in this post, continually improving them, reviewing them and coordinating their impact on implementation backlogs, many of the problems they spend so much time on would have been avoided.
I know you have differing opinions. Please share them here.