97 Things Every Programmer Should Know

(Chris Devlin) #1

(^182) 97 Things Every Programmer Should Know

WET Dilutes Performance Bottlenecks .................

Kirk Pepperdine

THE iMPORTANCE OF THE DRY PRiNCiPLE (Don’t Repeat Yourself) is that
it codifies the idea that every piece of knowledge in a system should have a
singular representation. In other words, knowledge should be contained in
a single implementation. The antithesis of DRY is WET (Write Every Time).
Our code is WET when knowledge is codified in several different implemen-
tations. The performance implications of DRY versus WET become very clear
when you consider their numerous effects on a performance profile.

Let’s start by considering a feature of our system, say X, that is a CPU bottle-
neck. Let’s say feature X consumes 30% of the CPU. Now let’s say that feature
X has 10 different implementations. On average, each implementation will
consume 3% of the CPU. As this level of CPU utilization isn’t worth worrying
about if we are looking for a quick win, it is likely that we’d miss that this fea-
ture is our bottleneck. However, let’s say that we somehow recognized feature
X as a bottleneck. We are now left with the problem of finding and fixing every
single implementation. With WET, we have 10 different implementations that
we need to find and fix. With DRY, we would clearly see the 30% CPU utiliza-
tion and would have a tenth of the code to fix. And did I mention that we don’t
have to spend time hunting down each implementation?

There is one use case where we are often guilty of violating DRY: our use of
collections. A common technique to implement a query would be to iterate
over the collection and then apply the query in turn to each element:

public class UsageExample {
private ArrayList<Customer> allCustomers = new ArrayList<Customer>();
// ...
public ArrayList<Customer> findCustomersThatSpendAtLeast(Money amount) {
ArrayList<Customer> customersOfInterest = new ArrayList<Customer>();
for (Customer customer: allCustomers) {
if (customer.spendsAtLeast(amount))
Free download pdf