Simple Sketches for Diagramming your Software Architecture
Simon Brown, @simonbrown
If you're working in an agile software development team at the moment, take a look around at your environment. Whether it's physical or virtual, there's likely to be a story wall or Kanban board visualising the work yet to be started, in progress and done. Visualising your software development process is a fantastic way to introduce transparency because anybody can see, at a glance, a high-level snapshot of the current progress. As an industry, we've become pretty adept at visualising our software development process over the past few years although it seems we've forgotten how to visualise the actual software that we're building. I'm not just referring to post-project documentation, this also includes communication during the software development process. Agile approaches talk about moving fast, and this requires good communication, but it's surprising that many teams struggle to effectively communicate the design of their software.
Prescribed methods, process frameworks and formal notations
If you look back a few years, structured processes and formal notations provided a reference point for both the software design process and how to communicate the resulting designs. Examples include the Rational Unified Process (RUP), Structured Systems Analysis And Design Method (SSADM), the Unified Modelling Language (UML) and so on. Although the software development industry has progressed in many ways, we seem to have forgotten some of the good things that these older approaches gave us. In today's world of agile delivery and lean start-ups, some software teams have lost the ability to communicate what it is they are building and it's no surprise that these teams often seem to lack technical leadership, direction and consistency. If you want to ensure that everybody is contributing to the same end-goal, you need to be able to effectively communicate the vision of what it is you're building. And if you want agility and the ability to move fast, you need to be able to communicate that vision efficiently too.
As an industry, we do have the Unified Modelling Language (UML), which is a formal standardised notation for communicating the design of software systems. I do use UML myself, but I only tend to use it sparingly for sketching out any important low-level design aspects of a software system. I don't find that UML works well for describing the software architecture of a software system. While it's possible to debate this, it's often irrelevant because many teams have already thrown out UML or simply don't know it. Such teams typically favour informal "boxes and lines" style sketches instead but often these diagrams don't make much sense unless they are accompanied by a detailed narrative, which ultimately slows the team down. Next time somebody presents a software design to you focussed around one or more informal sketches, ask yourself whether they are presenting what's on the sketches or whether they are presenting what's still in their head.
Abandoning UML is all very well but, in the race for agility, many software development teams have lost the ability to communicate visually too. The example software architecture sketches (above) illustrate a number of typical approaches to communicating software architecture and they suffer from the following types of problems:
- Colour-coding is usually not explained or is often inconsistent.
- The purpose of diagram elements (i.e. different styles of boxes and lines) is often not explained.
- Key relationships between diagram elements are sometimes missing or ambiguous.
- Generic terms such as "business logic" are often used.
- Technology choices (or options) are usually omitted.
- Levels of abstraction are often mixed.
- Diagrams often try to show too much detail.
- Diagrams often lack context or a logical starting point.
Some simple abstractions
Informal boxes and lines sketches can work very well, but there are many pitfalls associated with communicating software designs in this way. My approach is to use a small collection of simple diagrams that each shows a different part of the same overall story. In order to do this though, you need to agree on a simple way to think about the software system that you're building. Assuming an object oriented programming language, the way that I like to think about a software system is as follows: a software system is made up of a number of containers, which themselves are made up of a number of components, which in turn are implemented by one or more classes. It's a simple hierarchy of logical technical building blocks that can be used to illustrate the static structure of most of the software systems Iíve ever encountered. Some diagrams will help to explain this further.
A context diagram can be a useful starting point for diagramming and documenting a software system, allowing you to step back and look at the big picture. Draw a simple block diagram showing your system as a box in the centre, surrounded by its users and the other systems that it interfaces with.
Letís look at an example. The techtribes.je website provides a way to find people, tribes (businesses, communities, interest groups, etc) and content related to the tech, IT and digital sector in Jersey and Guernsey, the two largest of the Channel Islands. At the most basic level, it's a content aggregator for local tweets, news, blog posts, events, talks, jobs and more. Here's a context diagram that provides a visual summary of this.
Detail isn't important here as this is your zoomed out view showing a big picture of the system landscape. The focus should be on people (actors, roles, personas, etc) and software systems rather than technologies, protocols and other low-level details. It's the sort of diagram that you could show to non-technical people.
Once you understand how your system fits in to the overall IT environment with a context diagram, a really useful next step can be to illustrate the high-level technology choices with a containers diagram. By "container" I mean something like a web server, application server, desktop application, mobile app, database, file system, etc. Essentially, what I call a container is anything that can host code or data. The following diagram shows the logical containers that make up the techtribes.je website.
Put simply, techtribes.je is made up of an Apache Tomcat web server that provides users with information, and that information is kept up to date by a standalone content updater process. All data is stored either in a MySQL database, a MongoDB database or the file system. It's worth pointing out that this diagram says nothing about the number of physical instances of each container. For example, there could be a farm of web servers running against a MongoDB cluster, but this diagram doesn't show that level of information. Instead, I show physical instances, failover, clustering, etc on a separate deployment diagram. The containers diagram shows the high-level shape of the software architecture and how responsibilities are distributed across it. It also shows the major technology choices and how the containers communicate with one another. It's a simple, high-level technology focussed diagram that is useful for software developers and support/operations staff alike.
Following on from a containers diagram showing the high-level technology decisions, I'll then start to zoom in and decompose each container further. However you decompose your system is up to you, but I tend to identify the major logical components and their interactions. This is about partitioning the functionality implemented by a software system into a number of distinct components, services, subsystems, layers, workflows, etc.
As illustrated by the containers diagram, techtribes.je includes a standalone process that pulls in content from Twitter, GitHub and blogs. The following diagram shows the high-level internal structure of the content updater in terms of components.
In addition to a number of core components, the content updater is made up of four components: a Scheduled Content Updater, a Twitter Connector, a GitHub Connector and a News Feed Connector. This diagram shows how the content updater is divided into components, what each of those components are, their responsibilities and the technology/implementation details.
This is an optional level of detail and I will typically draw a small number of high-level UML class diagrams if I want to explain how a particular pattern or component will be (or has been) implemented. The factors that prompt me to draw class diagrams for parts of the software system include the complexity of the software plus the size and experience of the team. Any UML diagrams that I do draw tend to be sketches rather than comprehensive models.
Think about the audience
There seems to be a common misconception that "architecture diagrams" must only present a high-level conceptual view of the world, so it's not surprising that software developers often regard them as pointless. In the same way that software architecture should be about coding, coaching and collaboration rather than ivory towers, software architecture diagrams should be grounded in reality too. Including technology choices (or options) is a usually a step in the right direction and will help prevent diagrams looking like an ivory tower architecture where a bunch of conceptual components magically collaborate to form an end-to-end software system.
A single diagram can quickly become cluttered and confused, but a collection of simple diagrams allows you to easily present the software from a number of different levels of abstraction. And this is an important point because it's not just software developers within the team that need information about the software. There are other stakeholders and consumers too; ranging from non-technical domain experts, testers and management through to technical staff in operations and support functions. For example, a diagram showing the containers is particularly useful for people like operations and support staff that want some technical information about your software system, but don't necessarily need to know anything about the inner workings.
Common abstractions over a common notation
This simple sketching approach works for me and many of the software teams that I work with, but itís about providing some organisational ideas and guidelines rather than creating a prescriptive standard. The goal here is to help teams communicate their software designs in an effective and efficient way rather than creating another comprehensive modelling notation.
UML provides both a common set of abstractions and a common notation to describe them, but I rarely find teams that are using either effectively. Iíd rather see teams able to discuss their software systems with a common set of abstractions in mind rather than struggling to understand what the various notational elements are trying to show. For me, a common set of abstractions is more important than a common notation. Most maps are a great example of this principle in action. They all tend to show roads, rivers, lakes, forests, towns, churches, etc but they often use different notation in terms of colour-coding, line styles, iconography, etc. The key to understanding them is exactly that - a key/legend tucked away in a corner somewhere. We can do the same with our software architecture diagrams.
Itís worth reiterating that informal boxes and lines sketches provide flexibility at the expense of diagram consistency because youíre creating your own notation rather than using a standard like UML. My advice here is to be conscious of colour-coding, line style, shapes, etc and let a set of consistent notations evolve naturally within your team. Including a simple key/legend on each diagram to explain the notation will help. Oh, and if naming really is the hardest thing in software development, try to avoid a diagram that is simply a collection of labelled boxes. Annotating those boxes with responsibilities helps to avoid ambiguity while providing a nice "at a glance" view.
"Just enough" up front design
As a final point, Grady Booch has a great explanation of the difference between architecture and design. He says that architecture represents the "significant decisions", where significance is measured by cost of change. The context, containers and components diagrams show what I consider to be the significant structural elements of a software system. Therefore, in addition to helping teams with effective and efficient communication, adopting this approach to diagramming can also help software teams that struggle with either doing too much or too little up front design. Starting with a blank sheet of paper, many software systems can be designed and illustrated down to high-level components in a number of hours or days rather than weeks or months.
Illustrating the design of your software can be a quick and easy task that, when done well, can really help to introduce technical leadership and instil a sense of a shared technical vision that the whole team can buy into. Sketching should be a skill in every software developer's toolbox. It's a great way to visualise a solution and communicate it quickly plus it paves the way for collaborative design and collective code ownership.
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