What does the expression "Fail Early" mean, and when would you want to do so?

Question

What does the expression "Fail Early" mean, and under what circumstances is this approach most useful, and when would you avoid the approach?

Answer 1

Essentially, fail fast (a.k.a. fail early) is to code your software such that, when there is a problem, the software fails as soon as and as visibly as possible, rather than trying to proceed in a possibly unstable state.

Fail Fast
by Jim Shore
edited by Martin Fowler
http://www.martinfowler.com/ieeeSoftware/failFast.pdf

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Fortunately, there’s a simple technique that will dramatically reduce the number of these bugs in your software. It won’t reduce the overall number of bugs, at least not at first, but it’ll make most defects much easier to find.

The technique is to build your software to “fail fast.”

Immediate and visible failure

Some people recommend making your software robust by working around problems automatically. This results in the software “failing slowly.” The program continues working right after an error but fails in strange ways later on.

A system that fails fast does exactly the opposite: when a problem occurs, it fails immediately and visibly. Failing fast is a nonintuitive technique: “failing immediately and visibly” sounds like it would make your software more fragile, but it actually makes it more robust. Bugs are easier to find and fix, so fewer go into production.
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Also note the related concept of a fail-fast iterator - an iterator that, after certain modifications to the collection outside of the iterator, throws as soon as possible rather than proceed in a potentially unstable, or non-deterministic state.

Answer 2

It means: "Catch bugs as early as possible". If possible, you want to know they are there as soon as they are there =).

The earlier you catch a bug, the cheaper it is to remove it. If you could know the bug right at the time you wrote the buggy line, it would be awesome. You would know exactly what you were intending to do, and have the most power to remove that bug quickly.

On the other hand, if you just catch the bug one month later, or after it's released, the damage is a LOT greater. Users already have to deal with it, you won't remember what you were thinking so well (or maybe you won't be even working for the company anymore, so someone will need to find out what your thoughts were).

Answer 3

Failing early embodies the idea that when building software the earlier you fail or a test fails or you find a bug the easier it is to correct (and cheaper as well). It also applies to your business model. Better to find out early (in beta for example) than after you have launched.

Answer 4

"Fail Early" means that the program should raise an exception and stop working if something goes wrong. (It is described in the Pragmatic Programmer's list of tips as Crash Early)

In my bioinformatics work, I tend to use a "Fail Early" approach because my highest concern is ensuring correctness. By contrast, Rails allows you to hide failures. For example, Rails' try allows you to call something on an object, and it won't raise an exception if that object is nil. I guess this is because with web sites, ensuring that the program keeps running is more important than correctness.

Answer 5

I once had a junior oracle programmer working for me who put an "ignore everything" exception block around all his code so errors were never seen. This initially made his code look impressive but: A) Bugs took far longer to find; and B) I lost all (well, 80% of my) faith in his abilities after that.

I have since taught people that this is a very bad thing as it hides errors.

Do not confuse this with a code module being able to cope with poor quality inputs (such as HTML which is not also valid XHTML in a browser) - these do not need to result in a failure at all. More likely, they did cause a failure many years ago but the way to resolve it was to make the system take reasonable assumptions about how to recover.