Data modelling

This is a draft paper about a lightweight data modelling framework that I am developing as a home project, for use in other home projects of mine.  It is incomplete; I will be amending it as I find time to write more and as my understanding evolves of what this framework is supposed to do.

Introduction

Every single software project in existence deals in one way or another with data. Some projects have small amounts of data, some have large amounts of data, some even have "big" data. The data almost always exhibit a certain well defined structure, known as the Schema, and the loosely defined term Data Model is used to refer either to the data, or to the schema, or non-specifically to both.

In virtually all cases, the data model is highly application-specific, but many characteristics and operations are common or even ubiquitous across data models.

The MVVM architectural design pattern

Here is a brief technical explanation of MVVM, which contains enough detail (borrowed from its WPF implementation) and examples to allow the reader to grasp how it actually works.

Object Lifetime Awareness

The Thinker (French: Le Penseur) by Auguste Rodin (From Wikipedia)

Abstract

Garbage collectors have given us a false sense of security with respect to what happens to an object once we stop thinking about it. The assumption is that it will be magically taken care of, but this does not always go as hoped, resulting in memory leaks and bugs due to failure to perform necessary cleanup. Tools for troubleshooting such problems are scarce, and not particularly helpful, so finding and fixing such problems is notoriously difficult. 

A methodology is presented, which differs from current widespread practices, for maintaining awareness of, and exercising full deterministic control over, the lifetime of certain objects in a garbage-collected environment. We issue hard errors in the event of misuse, and accurate diagnostic messages in the event of omissions, thus improving the robustness of software and lessening the troubleshooting burden.

Coherence: The Assertable Lock

Abstract

A Software Design Pattern for concurrent systems which makes race conditions something that can be asserted against and thus deterministically eliminated rather than stochastically reduced or minimized. (Subject, of course, to the thoroughness of the assertions.)

Image by reginasphotos from pixabay.com

A description of the problem

Every Software Engineer who has dealt with concurrency knows that it is hard. The bane of concurrency is race conditions: when a thread accesses data without taking into account the fact that the data is shared with other concurrently running threads which may alter that data at any unforeseeable moment in time.

The famous "Could not load file or assembly or one of its dependencies" error message

If you have ever done any software development under Microsoft Windows you have probably come across this famous error message: "System.IO.FileNotFoundException : Could not load file or assembly 'Acme.dll' or one of its dependencies. The specified module could not be found." 

Modern software makes heavy use of dynamic link libraries, and the problem with this kind of libraries is that for various reasons they might not be there when you need them, resulting in runtime errors. This is the runtime error you get under Windows when this happens.

Naturally, when you see this message, the first thing to do is to check whether Acme.dll is there, and what you usually discover is that the file is indeed there. When dealing with computers, most error messages that you come across tend to leave some room for troubleshooting, but when the system is reporting that a certain file does not exist on your very own filesystem, while the file is most certainly there, the situation seems really hopeless. You are stymied. 

Ferolli HR OptiFor OT-V Ventilation Unit User's Manual in English

In a previous post I published the Ferolli BlueSense Boiler User's Manual in English.  Here is the manual of the ventilation unit.

Click on the pictures below for the documents.

 

  

The Wild, Wild Web

This is a little history of the early World Wide Web (WWW) for the benefit of the younger generation which may have not experienced the Internet in its infancy and therefore might not be aware of the horrors that it involved, and why certain things have come to be the way they are today.

As you are reading this, and thinking to yourself that it could not possibly have been as bad as I am describing it, remember, the general public was experiencing it using 2400 baud modems.

Domain Oriented Programming

A Software Design Pattern which brings the principles of Inheritance, Encapsulation and Polymorphism one level up from the Class level to the Subsystem level, and offers a way of realizing relationships between classes so as to achieve dependency inversion by means of propagation instead of injection.

Part 1: Dependency Inversion

The software that we write often invokes other software to get parts of the job done. These are known as Services or Dependencies. If Class A is making use of some Class B, then Class A depends on Class B, so Class B is a dependency of Class A. 

The principle of Dependency Inversion (⬀) says that a class should not contain any direct calls to specific instances of any of its dependencies. Instead, it should receive these instances as parameters during initialization.

That's all very nice, but passing dependencies around can become quite a complicated business, and in large systems it can become a nightmare.

On Validation vs. Error Checking

Let me start with a couple of pedantic definitions; stay with me, the beef follows right afterwards.

Conventional wisdom says that validation is different from error checking.

Validation is performed at the boundaries of a system, to check the validity of incoming data, which is at all times presumed to be potentially invalid. When invalid data is detected, validation is supposed to reject it by returning an appropriate result, not throwing an exception. Validation is supposed to be always on, you cannot switch it off on release builds and only have it enabled on debug builds.

Error checking, on the other hand, is performed inside a system, checking against conditions that should never occur, to keep making sure that everything is working as intended. In the event that an error is encountered, the intent is to signal a catastrophic failure (throw an exception) instead of causing some result to be returned. Essentially, the term Error Checking is shorthand for Internal Error Checking.  It can be implemented using assertions, thus being active on the debug build only, and having a net cost of zero on the release build.

So far so good, right?

Stackoverflow is not suitable for poignant questions

As the screenshot proves.