The Singleton Anti-Pattern

In programming, the whole idea is to avoid re-inventing the wheel, and re-use as much as possible. Some clever coders discovered that there were some mechanism that were used over and over again. For example, the “producer/consumer” mechanism, whereby one or more threads are “producers” and one or more threads are “consumers”. Instead of coders figuring out how to do this properly over and over again, a group of people decided to write a book that described how to solve some of these problems. “Design Patterns: Elements of Reusable Object-Oriented Software” they called it. In the business, the authors became known as the “Gang of Four”.

One of the patterns they described is a “Singleton“: A singleton is essentially a global object, that is instantiated when needed. The idea being that the user doesn’t need to know when, or how, the underlying object is created/destroyed, they can just use it, and all parts of the code then shares the same object. Isn’t that cool. It’s like global variables were suddenly being endorsed in a book, and by some clever people too!!

There are cases (rare, constrained) where a global variable makes sense; it makes sense when the physical properties that the software is trying to model, matches with a single object. E.g. a singular file on a disk or a specific camera in a network. It’s perfectly appropriate to model these objects as global, because there truly is only one of them.

Let’s consider a log mechanism. There may be several things that are logging data, but if all that data goes into just one file, then it’s OK to use a singleton for the file, but certainly not for the log abstractions. If there are three or four different modules that are all logging to the same file, then those modules must have their own logger instance, and the various instances that are made, can then write to the same file using the singleton.

A primitive class diagram could look like this:

             Module A -> Log A 
Parent  ->                        -> Singleton File
             Module B -> Log B

When you are acutely aware of this composition, you should eventually realize that each logger instance must add some identifier when it writes to the disk. Otherwise you get a log file that looks like this

File Open
File Open
File Write Failed
File Write Succeeded
File Close
File Close

What you want, in the file, is this

Module A: File Open
Module B: File Open
Module B: File Write Failed
Module A: File Write Succeeded
Module B: File Close
Module A: File Close

This appears to solve the problem; except there’s a caveat. Say someone writes an app that creates two instances of the parent module. Since the log file is a singleton, all log data is written to the same file. This, in turn, means that two instances of the parent will also write to the same file.

Consider this diagram

                              Module A -> Log A
                 Parent ->               
                              Module B -> Log B
Aggregator  ->                                       -> Singleton File
                              Module A -> Log A
                 Parent ->
                              Module B -> Log B

We are now in hell.

Module A: File Open
Module B: File Open
Module B: File Write Failed
Module A: File Open
Module B: File Write Failed
Module A: File Write Succeeded
Module B: File Close
Module A: File Write Succeeded
Module A: File Close

This issue is relatively easy to fix, and it’s still valid to have a requirement that there is just one log file (might be better to create one per parent, but that’s a matter of taste).

But what about issues where things like username, password, preferences etc. are stored in a singleton that contains “user info”. In that case, when the aggregator sets the username, the username change applies to ALL modules, regardless of where they reside in the aggregator tree. It’s therefore impossible for the aggregator to set a different username for Parent 1 and Parent 2. The aggregator, therefore, breaks.

Essentially, the coder might as well have said “let’s make the username a global variable”. 99% of all coders will object when they hear that (or “goto”). But 50% of all coders remain silent when the same pattern is described using the “singleton” moniker.

The morale of the story: don’t use singletons. Not even if you think you know what you are doing. Because if you think you know what you are doing, then you almost certainly do not.

 

Do Managers in Software Companies Need to Code?

I think so.

The horrible truth is that there are good and bad coders, there are good and bad managers and there are easy and hard projects.

A project, taken on by good coders and good managers can fail simply because the project was too complex and was too intertwined with system that the team had no control over. You could argue that the team never should have taken on the task, but that’s why you warn the customer of the risk of non-completion and bill by the hour.

When doing research on the skills needed to be a good software project manager, there seems to be an implied truth that the coders simply do what they are told, and that coding/design errors are always the managers fault. At the same time, you’ll find that people complain about micromanagement, and not letting the coders find their own solution. I find these two statements at odds with one another.

Coders will sometimes do things that are just wrong, yet it still “works”. How do you handle these situations? Do you, as a manager insists that the work is done “correctly”, which the coder may think is just a matter of taste, and not correct vs incorrect? Or do you leave the smelly code in there, and keep the peace?

If you don’t know how to code, and you’re the manager, you won’t even notice that the code is bad. You’ll be happy that it “works”. Over time, though, the cost of bad code will weigh down on productivity, the errors start piling up, good coders leave as there is no reward for good quality and they’re fed up with refactoring shitty code. If you have great coders, you might not run into that situation, but how do you know if you have great coders if you can’t code?

Maybe you’re the best coder in the world, and you’re in a managerial position facing some smelly code, you might consider two approaches: scold the coder(s), and demand that they do it the “correct” way (which is then interpreted as micromanagement), or alternatively, if you’re exhausted from the discussions, you just do a refactor yourself on a Sunday, while the kids are in the park?

In the real world, though, the best solution is for the manager to have decent coding skills, and posses that rare ability to argue convincingly. The latter is very hard to do if you do not understand the art of coding. Furthermore I don’t think coders are uniquely handicapped in being persuasive and certainly not when dealing with other coders (n00b managers wearing a tie are universally despised in the coding world).

Every coder is different, and act differently depending on the time of day, week or year. Some coders have not fully matured, some are a little too ripe, and some just like to do things the way they always did (or “at my old job we…”), different approaches are needed to persuade different people.

I must confess that this is what I have observed, the few times I have been wearing anything with any resemblance to a managerial hat, I have walked away being universally despised and feared as some sort of “Eye of Sauron” who picks up on the smallest error with no mercy when dishing out insults, but in theory at least, I think I know how thing ought to be.

So,if you are managing software projects and interacting with coders, you need to know how to code.