Friday, October 26, 2007

The Fail Fast Rule

Every time you're writing a piece of code and you have to deal with unknown corner cases ...
Every time you're implementing some kind of logic and you don't know what to do when things go bad ...
Every time you think: this can never happen, I don't have to check for it ...
Every time you don't know what to do or what would happen ...
Please ...

Fail fast.

This is the Fail Fast Rule, as I call it, a term borrowed from the Java Collections iterators behavior:

Every time your code might fail, and you don't know what to do, just make it fail abruptly.

That is: do not ignore it, do not (just) log it, but let your code throw an exception.

An example was how we computed a shopping cart total in one of my previous posts:

public class ShoppingCart {

private static final int MAX_TOTAL = 10000;

private List orders = new ArrayList();

public void addOrder(Order o) {

public double computeTotal() {
double total = 0;
Iterator it = this.orders.iterator();
while (it.hasNext()) {
Order current = (Order);
total += current.computeSubTotal();
if (total > MAX_TOTAL) {
throw new SomeException();

return total;

As soon as the total gets too high, an exception is thrown. Probably this will change later, because we may want to handle it more gracefully, but in the meantime: make it fail.


Because if an exception will pop up in front of you, you'll have found a bug, a corner case, something that you have not still properly implemented or that you have not thought of.

So remember: make it good, or fail fast.

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