Code Smell 49 - Caches

Caches are sexy. They are a one-night stand. We need to avoid them in a long-term relationship.

6 February, 2023

Contributors

Maxi Contieri

@mcsee

Caches are sexy. They are a one-night stand. We need to avoid them in a long-term relationship.

Problems

Coupling

Testability

Cache invalidation.

Maintainability

Premature Optimization

Erratic Behavior

Lack of transparency

Non-Deterministic behavior

Solutions

If you have a conclusive benchmark and are willing to pay for some coupling: Put an object in the middle.

Unit test all your invalidation scenarios. Experience shows we face them in an incremental way.

Look for a real-world cache metaphor and model it.

Sample Code

Wrong

[Gist Url]: # (https://gist.github.com/mcsee/c429e71f6b0239b83e7e307feb5927fe)

<?

final class Book {

    private $cachedBooks;

    public function getBooksFromDatabaseByTitle(string $title) {

        if (isset($cachedBooks[$title])) {
            return $cachedBooks[$title];
        } else {
            return $this->doGetBooksFromDatabaseByTitle($title);
        }
    }

    private function doGetBooksFromDatabaseByTitle(string $title) {
        globalDatabase()->selectFrom('Books', 'WHERE TITLE = ' . $title);
    }
}

Right

<?

final class Book {
    // Just Book related Stuff
}

interface BookRetriever {
    public function bookByTitle(string $title);
}

final class DatabaseLibrarian implements BookRetriever {
    public function bookByTitle(string $title) {
        // Go to the database (not global hopefully)
    }
}

final class HotSpotLibrarian implements BookRetriever {
    // We always look for real life metaphors
    private $inbox;
    private $realRetriever;

    public function bookByTitle(string $title) {
        if ($this->inbox->includesTitle($title)) {
            // We are lucky. Someone has just returned the book copy.
            return $this->inbox->retrieveAndRemove($title);
        } else {
            return $this->realRetriever->bookByTitle($title);
        }
    }
}