How do you decide whether to use a library or write your own implementation

Question

Inspired by this question which started out innocently but is turning into a major flame war.

Let\'s say you need to a utility method - reasonably straightforward but no

Answer 1

If the functionality is only a small part of the app, or if your needs are the same as everyone else's, then a library is probably the way to go. If you need to consume and output JSON, for example, you can probably knock something together in five minutes to handle your immediate needs. But then you start adding to it, bit by bit. Eventually, you have all the functionality that you would find in any library, but 1) you had to write it yourself and 2) it isn't a robust and well document as what you would find in a library.

If the functionality is a big part of the app, and if your needs aren't exactly the same as everyone else's, then think much more carefully. For example, if you are doing machine learning, you might consider using a package like Weka or Mahout, but these are two very different beasts, and this component is likely to be a significant part of your application. A library in this case could be a hindrance, because your needs might not fit the design parameters of the original authors, and if you attempt to modify it, you will need to worry about a much larger and more complex system than the minimum that you would build yourself.

There's a good article out there talking about sanitizing HTML, and how it was a big part of the app, and something that would need to be heavily tuned, so using an outside library wasn't the best solution, in spite of the fact that there were many libraries out that did exactly what seemed to be called for.