Tag Archives: software

Low-Level Software Design

[Please note, this article isn’t about formal design methods (LLD), UML, Design Patterns, nor about object-oriented design, etc. It’s written with a view towards the kind of software project I typically get to work on – embedded / Linux OS related, with the primary programming language being ‘C’ and/or scripting (typically with bash).]

When one looks back, all said and done, it isn’t that hard to get a decent software design and architecture. Obviously, the larger your project, the more the thought and analysis that goes into building a robust system. (Certainly, the more the years of experience, the easier it seems).

However, I am of the view that certain fundamentals never change: get them right and many of the pieces auto-slot into place. Work on a project enough and one always comes away with a  “feel” for the architecture and codebase – it’s robust, will work, or it’s just not.

So what are these “fundamentals”? Well, here’s the interesting thing: you already know them! But in the heat and dust of release pressures (“I don’t care that you need another half-day, check it in now!!!”), deadlines, production, we tend to forget the basics. Sounds familiar? 🙂

The points below are definitely nothing new, but always worth reiterating:

Low-level Design and Software Architecture

  • Jot down the requirements: why are we doing this? what do we hope to achieve?
  • Draw an overall diagram of the project, the data structures, the code flow, as you visualise it. You don’t really need fancy software tools- pencil and paper will do, especially at first.
    pencil20on20notebook20-20writing20concept
    Arrive, gently, at the software architecture.
  • Layering helps (but one can overdo it)
    • To paraphrase- “adding a layer can be used to solve any problem in computer science” 🙂 Of course, one can quite easily add new problems too; careful!
  • It evolves – don’t be afraid to iterate, to use trial and error
    • “Be ready to throw the first one away – you’re going to anyway” – paraphrased from that classic book “The Mythical Man Month”
    • “There is no silver bullet” – again from the same book of wisdom. There is no one solution to all your problems – you’ll have to weigh options, make trade-offs. It’s like life y’know 😉
  • Design the code to be modular, structured
  • A function encapsulates an intention
    • Requirement-driven code: why is the function there?
  • Each function does exactly one thing
    • This is really important. If you can do this well, you will greatly reduce bugs, and thus, the need to debug.
  • Use configuration files (Edit: preferably in plain ASCII text format).

Coding

  • Insert function stubs – code it in detail later, get the overall low-level design and function interfacing correct first. What parameters, return value(s)? 
  • Avoid globals
    • use parameters, return values
    • in multithreaded / multiprocess environments, using any kind of global implies using a synchronization primitive of some sort (mutex, semaphore, spinlock, etc) to take care of concurrency concerns, races. Be aware – beware! – this is often a huge source of performance bottlenecks!
      Edit: When writing MT software, use powerful techniques TLS and TSD to further avoid globals.
  • Keep it minimal, and clean: Careful! don’t end up using too many (nested functions) layers – leads to “lasagna / spaghetti code” that’s hard to follow and thus understand
  • If a function’s code exceeds a ‘page’, re-look, redesign.

Of course, a project is not a dead static thing – at least it shouldn’t be. It evolves over time. Expect requirements, and thus your low-level design and code, to change. The better thought out the overall architecture though, the more resilient it will be to constant flux.

For example: you’re writing a device driver and a “read” method is attempting to read data from the ‘device’ (whatever the heck it is), but there is no data available right now, what should we do? Abort, returning an error code? Wait for data? Retry the operation thrice and see?

The “correct” answer: follow the standard. Assuming we’re working on a POSIX-compliant OS (Unix/Linux), the standard says that blocking calls must do precisely that: block, wait for data until it becomes available. So just wait for data. “But I don’t want to wait forever!” cries the application! Okay, implement a non-blocking open in that case (there’s a reason for that O_NONBLOCK flag folks!). Or a timeout feature, if it makes sense.

Shouldn’t the driver method “retry” the operation if it does not succeed at first? Short answer, No. Follow the Unix design philosophy: “provide mechanism, not policy”. Let the application define the policy (should we retry, if yes, how often; should we timeout, if yes, what’s the timeout, etc etc). The mechanism part of it – your driver’s read method implementation must work independent of, in fact ignore, such concerns. (But hey, it must be written to be concurrent and reentrant -safe. A post on that another day perhaps?).

Configuration

Using the same example: lets say we do want the “read” method of our device driver to timeout after, say, 1 second. Where do we specify this value? Recollect, “provide mechanism, not policy”. So we’ll do so in the application, not the driver. But where in the app? Ah. I’d suggest we don’t hardcode the value; instead, keep it in a simple ASCII-text configuration file:

app_config
   read_timeout=1

Of course, with ‘C’ one would usually put stuff like this into a header file. Fair enough, but please keep a separate header – say, app_config.h .

Try and do some crystal-ball-gazing: at some remote (or not-so-remote) point in the future, what if the project requires a GUI front-end? Probably, as an example, we will want to let the end-user view and set configuration – change the timeout, etc – easily via the GUI. Then, you will see the sense of using a simple ASCII-text configuration file to hold config values – reading and updating the values now becomes simple and clean.
Finally, nothing said above is sacred – we learn to take things on a case-by-case basis, use judgement.


A few Resources

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