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

Keep it in balance

You should keep several criteria in balance. I'd consider a few topics and ask a few questions.

Developing time VS maintenance time

Can I develop what I need in a few hours? If yes, why do I need a library? If I get a lib am I sure that it will not cause hours spent to debug and documentation reading? The answer - if I need something obvious and straightforward I don't need an extra-flexible lib.

Simplicity VS flexibility

If I need just an error wrapper do I need a lib with flexible types and stack tracing and color prints and.... Nope! Using even beautifully designed but flexible and multipurpose libs could slow your code. If you plan to use 2% of functionality you don't need it.

Dig task VS small task

Did I faced a huge task and I need external code to solve it? Definitely AMQP or SQL operations is too big tasks to develop from scratch but tiny logging could be solved in place. Don't use external libs to solve small tasks.

My own libs VS external libs

Sometimes is better to grow your own library because it is for 100% used, for 100% appropriate your goals, you know it best, it is always up to date with your applications. Don't build your own lib just to be cool and keep in mind that a lot of libs in your vendor directory developed "just to be cool".