Recently I needed to assure that a process could not be started for a second time on the same machine. I had done this before with a mutex but rather than rummaging through old code on my own personal machine, I did the expedient thing and found this question on Stack Overflow.

I liked the answer as many others did and after a few minutes of tweaking came up with this useful adaptation. I share it here in part to help others but mostly to create a permanent “note to self” as this will surely come up again in the future.

private static void Main(string[] args)
{
  // assure only one instance running
  RunExclusively(() =>
  {
    // do your exclusive stuff
  });
}

static void RunExclusively(Action action)
{
  // global mutex to prevent multiple instances from being started
  // get application GUID as defined in AssemblyInfo.cs
  string appGuid = ((GuidAttribute)
           Assembly.GetExecutingAssembly()
           .GetCustomAttributes(typeof(GuidAttribute), false)
           .GetValue(0))
           .Value;

  // unique id for global mutex - Global prefix means it is global to the machine
  string mutexId = string.Format("Global\\{{{0}}}", appGuid);

  using (var mutex = new Mutex(false, mutexId))
  {
    // set security settings for mutex - allow everyone
    var allowEveryoneRule = new MutexAccessRule(
      new SecurityIdentifier(WellKnownSidType.WorldSid, null),
      MutexRights.FullControl, AccessControlType.Allow);
    var securitySettings = new MutexSecurity();
    securitySettings.AddAccessRule(allowEveryoneRule);
    mutex.SetAccessControl(securitySettings);

    var hasHandle = false;
    try
    {
      try
      {
        // wait to acquire for up to five seconds
        if (!mutex.WaitOne(5000, exitContext: false))
        {
          throw new TimeoutException("Timeout waiting for exclusive access");
        }
      }
      catch (AbandonedMutexException)
      {
        // mutex abandoned in another process
        // it will still get acquired
        hasHandle = true;
      }
      action(); //execute work
    }
    finally
    {
      if (hasHandle) mutex.ReleaseMutex();
    }
  }
}

If you know of a better way or see any flaws in this one, please do share.

You can find the well written brief on Phillippe Kruchten’s 4+1 architectural view model on Wikipedia along with a link to the original white paper published nearly 20 years ago. Kruchten worked for Rational, as I understand it, and so this is often referred to as the RUP 4+1 model.

Courtesy of Wikipedia

Although I have used this model or some variant of it at times, I had quite forgotten its name and origin until someone reminded me of it the other day. Of course, we often forget that UML also came out of Rational’s famous Grady Booch, Ivar Jacobson and James Rumbaugh..

And yet today I find it difficult to find software developers who know much of anything about the 4+1 model and only a few regularly use UML and even then only for simple class diagrams. So why the disconnect?

Perhaps a Version 2 of these approaches is required. What would you get if you mixed the 4+1 model with more clearly defined non-functional requirement scenarios as I’ve previously discussed and the C4 model presented by Simon Brown along with just a bit more UML that most developers are not currently accustomed to using? I’m thinking this through and will post more thoughts on it soon.

Two days ago I posted Non-Functional Requirements for the Software Architect in which I suggested that quality attributes or non-functional requirements could be categorized into a two by two grid, operational and non-operational by internal and external.

Thinking further along those lines led me to picture the illustration below and hop out of bed to put it to virtual paper.

Most quality attributes will fit into one or two of the big four: security, performance, usability and modifiability. And each of these largely corresponds to its depicted quadrant.

Security is largely an internal concern. Often external stakeholders will profess their concern with this attribute, and hence the Venn-like spill over into the external quadrant. Generally the concern for security and assuring security falls within the domain of internal stakeholders and the implementation team.

External stakeholders are mostly concerned with performance and usability. There are a number of other quality attributes such as availability that fall within or at least are closely related to one or both of those.

Modifiability is almost exclusively a concern of internal stakeholders and implementation teams. Interoperability and testability are largely related to modifiability while interoperability nearly always shares at least some security concerns.

You may mix these differently to match your circumstances and priorities, but the illustration covers the majority of software development efforts but of course does not spell out the details. For guidance on that, I recommend my previous post. If you are concerned with software architecture, perhaps this will help you to visualize how these quality attributes or non-functional requirements compete and compliment one another.

To paraphrase Bass in Software Architecture in Practice, an architecture will either inhibit or enable the achievement of a system’s desired quality attributes. Understanding your desired quality attributes well will drive the critical decisions in your architecture. And your architecture will then provide the requisite containers into which you will place your functionality. To quote, “Functionality is not so much a driver for the architecture as it is a consequence of it.”

While I am not completely certain that is always the case, it is a principle that is well worth considering when creating and refactoring the architectural elements of your system. And it is my hope that by seeing this illustrated, it will get you thinking about how your own desired quality attributes interact with internal and external teams, which are operational and non-operational, and how they may compete with or compliment on another.

I grew up on a small farm in eastern Utah a few miles west of Roosevelt. My dad is still farming but on a smaller scale these days. He and my mom dropped by for a visit the other day. They asked about work and as I described to them what I do, it struck me that there are many similarities between agile software development and farming.

Here are a few that came to mind:

There is one very big difference between software and farming. The former pays better and is easier.

Countless failed software development projects have been kicked off with non-functional requirements delivered to the implementation team with little to no detail. Here are a few of the worst but most common examples:

• Security – The software must be secure.
• Performance – The software must be fast.
• Usability – The software must be easy to use.

Most software professionals have been taught that non-functional requirements are important, but many projects skip over them in order to get to functional use cases and writing code. The result can be profound, leaving the implementation team without sufficient input to make critical design decisions that will be very costly to change when the non-functional requirement is later clarified.

What Every Non-Functional Requirement Needs

For every non-functional requirement, the software architect should assure that the following questions have been adequately answered.

• To whom is this quality important?
• Users and integrators
• Management team
• Implementation team
• Operations team
• Who will assure this quality is met?
• Implementation team
• Operations team
• Management team
• How will this quality be met?
• Cross cutting constraints in software
• System and network constraints
• Log analysis and oversight
• How will we know this quality is met?
• Scenarios with measures
• Monitoring and review
• Acceptable tolerance percentiles

The items below each question are not meant to be an exhaustive list but simply to give you an idea of what may be involved in answering those questions.

Classification of Non-Functional Software Quality Requirements

Clarifying and prioritizing non-functional software quality requirements may be easier when you classify them into one of four groups by answering two questions: operational or non-operation, and internal or external. The following table is anything but exhaustive but it will give you the general idea.

 
Business stakeholders are generally more interested in and will support efforts to meet external qualities. Implementation and IT teams sometimes have to work a little more to garner support for time and effort and expense for internal qualities.

It is often easier to build into an implementation the cross cutting concerns to measure operational qualities. Collecting performance, reliability and security metrics from executing code is always possible with well planned constraints early on in the development effort. If these qualities are defined later, the refactoring process can be challenging.

For non-operational qualities, other systems such as those used to manage support issues and ongoing development efforts are often helpful in measuring the cost of change to the system or whether usability goals are being met. Sometimes time series log analysis can be utilized to extract measures for non-operational qualities, especially those most important to external parties.

Use an Agile Approach to Non-Functional Requirements

However you choose to collect and document non-functional software quality requirements, you should continue to improve and tweak them throughout the development process just as you would with functional requirements, grooming your backlog and prioritizing based on ongoing feedback from stakeholders, users and developers.

I have been enjoying an e-book called Software Architecture for Developers by Simon Brown that was very well worth the price. I also just finished watching the author’s presentation at the 2014 GOTO Conference. A very thought provoking presentation.

Here are a few things that I like very much from the book and presentation.

• “The code is the single point of truth. It is the embodiment of the architecture.”
• TDD does not replace architecture. Do TDD inside a set of boundaries and frameworks provided by the architecture. (see Why Most Unit Tests Are a Waste by James O Coplien.)
• If the diagrams don't reflect the code, the diagrams are basically pointless. We're just deceiving ourselves.
• Component testing is preferable over unit testing but unit tests and mocks for testing against async systems are still useful.
• Layered architecture can lead to a big ball of mud because too much functionality is exposed for public use by other layers.
• Component organization over layered is preferred. Components have limited public interfaces (aka ports) and may have layers within the component that are not improperly publicly accessible.
• If a system has a very large number of unit tests, we may have an out of control layered architecture.
• “If your software system is hard to work with, change it. This is entirely within your hands.”

C4: Context, Containers, Components, Classes

I especially like Brown’s use of what he calls C4. In essence, every software system can be broken down into a simple hierarchy:

• Contexts (systems) made up of
• Containers (web server, app server, database, browser, file system, etc.) which host
• Components which expose one or more interfaces (sometimes referred to by others as ports) and contain
• Classes which implement those interfaces and the layers behind them.

By creating architecture diagrams that follow this hierarchy, it is possible to create code that matches it. It is an architecture that developers can use directly. (Side note: I have often used a Visual Studio solution to lay out a project in similar terms directly in code. I have also gone down the layered road only to regret it later and end up pulling those layers apart and encapsulating them into a component-like approach to ensure that the responsibilities and behaviors exposed to the outside world are used properly and that the system remains well ordered.)

Here is my simplification of what the author already makes rather simple with respect to these constructs. I recommend you buy the book and get full details and example diagrams, but the ideas are what is most important.

• Context Diagram - A context diagram answers what are we building, who is using it and how does it fit into our IT environment and business. It is important to note at a high level how users will interact with the system.
• Container Diagram - In the container diagram, you define the shape of the software, high-level technology decisions, how responsibilities are grouped and separated, how each container communicates with other containers, and where the code lives for each container.
• Component Diagram - The component diagrams answers what course grained building blocks of functionality are required, what are their responsibilities, and how will they interact with other components (interface and communication mechanisms such as sync, async, batched, etc.).
• Class Diagram – Brown does not explicitly discuss this level, as far as I’ve read. I assume this is because it is rather intuitive to most architects and in part because the design at the class level is often better left to the implementation team. However, from my own perspective, the architect should consider defining the public interfaces and primary classes to name and separate key responsibilities into distinct code structures that will be intuitive for the implementation team to complete—naming conventions that separate what may be thought of as traditional layers and that the implementation team will understand is crucial here.

Some of what Brown proposes breaks with what others may consider traditional software architecture. I am excited about Brown’s challenging of the status quo, questioning our assumptions. I believe his ideas will help us narrow the model-code gap (see Just Enough Software Architecture by George Fairbanks—one I’ve just added to my Kindle collection for some fun future reading).

Measurable Non-Functional Quality Scenarios

Brown covers the importance of quality scenarios, but on this topic I prefer the more pedantic but measurable approach of Len Bass in Software Architecture in Practice. I believe his emphasis on defining metrics driven quality scenarios is well worth pursuing, especially to the point of implementing logging and monitoring systems that allow you to constantly measure non-functional quality and improve against those measures. Here’s the structure that Bass recommends:

• Source – where does the input come from?
• Stimulus – what is the input?
• Artifact – what container and component are affected?
• Environment – in what context does this occur?
• Response – how and with what did the software respond?
• Measure – how much time, how many clicks, how many errors, were proper notifications were sent?

Creating quality scenarios that developers understand and can incorporate into the software they are building is critical to long term success of the software. To paraphrase the quality gurus, if you can measure it, you can understand and improve it.

Most important, if your implementation team and your stake holders understand your architecture diagrams and documents, you are more likely to succeed. And if your code does not mirror your diagrams, no amount of code reviewing will tell you whether your architecture has been implemented. And that is Brown’s greatest point in my opinion.

About 18 months ago, I started some fun open source projects shared on GitHub and published on NuGet where I just crept over the 4,000 downloads mark. Wahoo! Of course, this is not really a very high number in comparison with the most popular packages on NuGet.org but I’m glad there are some people enjoying some benefit from using my work, and I don’t mind patting myself on the back just a little..

Here are my favorites:

DuoVia.FuzzyStrings--A collection of fuzzy string algorithms:

• Dice coefficient based on bigrams
• Levenshtein distance with transposition
• LCS (longest common subsequence)
• Double metaphone.

ServiceWire--A fast and light weight service host and dynamic client library that simplifies the development and use of high performance remote procedure call (RPC) communication between .NET processes over named pipes or TCP/IP. Supports the following:

• Out and ref parameters
• Dynamic client proxy generation from service interface
• Fast serialization of most native types and arrays of those types
• Multiple service interface hosting on the same endpoint
• Aspect oriented interception with easy pre-, post- and exception handling cross cutting

ServiceRunner--Easiest way to turn a console app into a Windows Service but still debug as a console app.

DuoVia.Net.Distributed--A Distributed Task Parallel Processing Library for .NET.

DuoVia.MpiVisor--Distributed Parallel Computing for .NET.

I hope you’ll give one or more of them a try.

Platform or technology stack specific modifiers on the title of Software Architect are common. Most software architects know that the platform, like a framework, is an interchangeable implementation choice and not really part of the software architecture (see my post entitled Practical Agile Software Architecture).

Why the distinction then? Two reasons:

• Architect as Implementer  - Many software architects are involved in guiding and contributing to platform specific implementation which requires specialization that is not specific to the architecture itself. I prefer this combination.
• Platform Specific Language - The platform specific software architect may be most comfortable or even required to produce artifacts that adopt the vernacular of that platform in order to make those artifacts more easily consumed by the implementation team.

The most common platform and implementation specific language elements in architecture artifacts replace the more generic terms of module, component, and software. Here’s an overly simple conversion table that I have found helpful for Java and .NET.

.
Connectors are generally common across these stacks, with a few exceptions such as WCF and JAX. More often connectors are literally specified as a SOAP or REST or even a custom TCP based protocol. Message based connectors are very often technology specific, indicating the specific message queue but be careful to avoid limiting your architecture by specifying implementation choices. The implementation choices should be, as much as possible, left to the design and implementation team effort.

The majority of my implementation experience has been in the .NET stack. But software architecture should be the same across implementation stacks. Of course, there may be some things easier said than done in a specific platform and technology stack, and that may have an impact on your software architecture choices, but minimally so.