cart issue in opencart - php

Question

I have problem in understanding one concept.
Suppose you have logged in and added some items to your cart but without checking out you have logged out. Next time you logged in you will find the cart with the items you have added previously.
I want to know how this concept works? After some study I have come to know it is using cache files. But how this cache files works? How this cache files are related to the backend database?

Answer 1

OpenCart uses a field in the customer table to store the cart items as a serialized array of data. The field is called cart. Similarly, the wishlist uses the same technique and is called wishlist, again in the customer table. To actually load the cached content, the values are loaded in the login() method in /system/library/customer.php, using this code
if ($customer_query->row['cart'] && is_string($customer_query->row['cart'])) {
$cart = unserialize($customer_query->row['cart']);
foreach ($cart as $key => $value) {
if (!array_key_exists($key, $this->session->data['cart'])) {
$this->session->data['cart'][$key] = $value;
} else {
$this->session->data['cart'][$key] += $value;
}
}
}
This code actually adds items to the current cart, so if you already have items in the basket, it merges them

Answer 2

As You asked more questions - one related to the cart and items storing and one or two cache related, and as Jay Gilford answered that cart related part, here is an answer for the cache:
Opencart caches the data You use most the time and at every page load: currencies, languages, categories, products, category product counts, weight classes, tax classes, etc. In models (lets talk about frontend/catalog for now) when fetching the data OpenCart looks for the cached data prior to querying the database. If there are cached data found they are fetched and unserialized (they are stored in plain text files using PHP's function serialize()) and returned to the controller without querying the database - thus the fetching should be much quicker1. If there are no cached data the database is queried and the database data fetched are then serialized (using PHP's function serilizae()) and saved to the corresponding cache file.
Here is one small problem that may occur - after You have edit the data directly in the database (using phpMyAdmin, may the reason be whatever for this) the change is not present at frontend. This is because there are older data cached that are fetched when asked for them. If You have to do such editing then do not forget to manualy delete corresponding cache file(s).
Which lead us to the administration (backend) part of models - here when editing or adding data (category, product, currencies, weight classes, etc) after the data is successfully stored to the database the corresponding cache file is immediately deleted so it could be re-created at frontend containing up-to-date data.
The cache files could be find at /system/cache/ folder.
EDIT: The cache is instanciated and registered within index.php and is then accessible in both controllers and models as $this->cache while methods set(), get() and delete() are callable. The cache class itself could be found at system/library/cache.php and is quite simple and straightforward.
Is such an answer sufficient?
1 I have seen a server with very fast PHP-MySQL communication while slow filesystem access...