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Singletons: Why Are They "Bad"?

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This was originally posted at Dev Leader.



Background

The very first thing I want to say is that I don’t think singletons are inherently bad–even if it means I am cast away from the rest of the programming world. There’s a time and a place for the singleton. It’s really common for people to get caught up with their perspective on something that they outright refuse to acknowledge the other side of it. I’d also like to point out that if you have a strong opinion on something and you find that other people also have a strongly opposing opinion on the same thing, there’s probably good take away points from either side. In this post though, I’m going to focus on why singletons are “bad”, because for me it means acknowledging one of the two main perspectives–that they are the best thing since cat videos met The Internet or they are the worst thing since Justin Bieber.

Let’s clarify what a singleton is here so we’re all on the same page. Maybe you’re under the impression it’s something slightly different than what I’m about to be talking about, so I’d rather make it clear from the beginning. And for what it’s worth, if this isn’t the exact meaning as set in stone by the singleton gods, then that’s sort of unfortunate… because this is going to be what I’m discussing. From Design Patterns: Elements of Reusable Object-Oriented Software by the Gang of Four, a singleton must:

Ensure a class has only one instance and provide a global point of access to it.


So, with that incredibly complex definition, let’s get into it.



Global Dependencies

There’s approximately 3.2 billion articles on The Internet that will tell you that global variables are the enemy. I mean, I could do some of the work for you, but you could check out this, or this, or this… There’s still billions more. Usually when we have ~99% of people agreeing on something, they probably have a pretty good point, right? You’ll notice after a bit of searching that one *big* problem with singletons is the fact that they are just a different way to dress a global variable. Thus, all of the arguments for why global variables are bad could be used against singletons.

There are a few (quite a few) major problems with having global dependencies:

  • You’ll be putting yourself at risk for dealing with deadlocks if you need to lock your resources
  • Your singleton can become the resource bottleneck in your application
  • It’s hard to know who you’ll affect by modifying the global variable
  • Testing becomes difficult because the tests may depend on the state of the singleton
  • Dependencies aren’t obvious from examining the API
  • Quite simply, global dependencies can be pretty scary. If you and you’re team are experienced enough, trying to weave in some new features or heavily modify code relying on singletons may not be that challenging for you. Maybe. But testing can get super messy.


When you write unit tests, you want to ensure that each test can be run independently of the others. You need complete control over your state. This can be a big problem if you run two tests in a row that depend on state provided by a singleton. Consider the following set of tests that depend on a singleton in a sample run:

  • Test1 increases the count property on a singleton by 1 as part of the side effect of the test.
  • Test2 also increases the count property of the same singleton instance by 1, just as Test1 did. However, Test2 is explicitly testing this property as part of it’s validation to ensure the value transitioned from 0 to 1.


When you run Test1 all by itself, it behaves as you expect. Great! Now you move onto Test2. Simple. You got it working. You’re all excited so you run them all together. Uh oh… The tests failed! But how? You don’t believe it, so you run them all again. Now they pass! This is a crappy spot to be in. If the test suite executes Test2 before Test1, your tests will work. If the test suite executes them in the opposite order, your tests will fail because the singleton instance will have had its count property increased twice. The global state of the singleton comes back to haunt you.



Tight Coupling

Most experienced developers know that you want to decrease coupling and increase cohesion because this, generally speaking, makes for a good API and extensible code base. Now this point is related to the global dependency points I was previously stating, but it deserves to be addressed on it’s own. The global dependency topic was more of a focus on the application at run time. That is, it becomes difficult to know and manage how your application behaves at run time when you share global state across the entire thing. It’s challenging to to start adding new functionality to your application or modify existing components of your application that depend on the global state because the dependencies just keep growing.

Coupling, in my opinion, is more of an issue with compile time. With singletons, you start to design classes that depend on singletons and thus you are relying on a specific implementation (Maybe you could check out dependency injected singletons?). Singletons, generally speaking, are a single instance of a concrete class. You start to code classes and object hierarchies that are then depending on this concrete class, and this can potentially (and I say potentially, because I would never claim it’s “always”) lead you to a dead end. Let’s consider a scenario:

We’re coding an application that uses a singleton for the data access layer of our application. We have a MySQL data model that we’ve coded as a singleton. We expose a few methods to read and write records to the database using some SQL, and things are great. Then, one day, we speak to our customers like we usually do to ensure we’re meeting their expectations. They inform us that we really need to be able to support a document database like MongoDB in addition to MySQL. Hold on. Wait. You mean our code that uses our data layer needs to be agnostic to the database under the hood?! But… But our singleton is only able to deal with MySQL… (I wrote about how to get yourself out of this situation here, although I would not claim it’s the ideal scenario). If we would have been passing around references to something that met a nice and clean model interface spec, we’d likely be able to hide the implementation details and completely avoid this problem.



Singletons Disrupt The API

Because I like to design complex systems in code, API and architecture is something I like to focus on. There’s an awesome posting over here by Miško Hevery about this very problem. He does a great job describing the problem with examples, so I’ll only try to summarize some of the main take-away points I got from it.

There’s nothing to stop someone from putting some heavy initialization logic in a singleton. I mean, you shouldn’t (because you can’t guarantee when this is going to happen!) but there’s nothing that prevents it. As such, simply calling a constructor on some class (which you might assume to happen pretty quickly) actually ends up taking seconds. Not a couple milliseconds… But seconds. Oh. I guess you didn’t realize your class was trying to call a singleton instance that was connecting over The Internet to some host on the other side of the planet during its initialization. Surprise. Singletons can mask this kind of stuff because it’s not explicit in the API. It kind of goes back to coupling but I wanted to point out that this kind of stuff can get scary.

This can be completely mitigated by incorporating the dependencies right into the API. There’s no hiding crazy database initialization or downloading data from the internet in a constructor (well… it just reduces the likelihood of you doing it so easily I guess). You use interfaces and construct classes in the order that you need them, and this doesn’t end up being some magical process that happens behind a curtain. If one class depends on another, so be it (it’d be nice if it just depended on the interface…), but pass in the reference that you require. Singletons often end up being the shortcut, but what’s easier for you to code now may not be easier for someone to extend upon and understand in the future.



Summary

Singletons. You’ve likely seen them. You’ve likely heard bad things about them. You may have even used one yourself. Shame on you. Like all debatable things though, there’s always going to be another side to it. If you take away anything from reading this, I hope it’s that you question what the other perspective is. Fully understanding something requires you to look at all sides.



References

This article was based on information I obtained from the following sources (as well as my own experiences, of course!):





This was originally posted at Dev Leader.

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