Accounting / Vote example

Suppose you have document library website. Each client have balance and you decrease it with 5 cents for each document the user requests.

Voting on websites have similar set-up, but instead of deducting money - increasing the object rating (user "karma", picture views etc).

If you do it with MySQL, you need to run query such "select balance from user where id = 5", then "update user set balance = balance - 0.05 where id = 5".

The update query blocks the table, so performance will be very poor. Second and most important, you do not know if another thread updated the table meanwhile - this will ruin whole accounting and billing will be dead wrong.

There is a way to avoid this issue with MySQL, but you will need to use two separate tables. However you can do hybrid way using both Redis and MySQL.

$r = new Redis();
$r->connect("127.0.0.1", "6379");

$userid = 5;

// Redis does not handle floating points, so we do conversion to cents.
$price  = 0.05 * 100;

$setkey = "all:users";

$userkey = "user:" . $userid;

// check if we had the data in Redis...
if (!$r->exists($userkey)){
	// No we don't.
	$balance = get_from_db("select balance from users where id = $userid");
	
	// We can be pretty sure there is no other thread who updated the balance, 
	// but anyway, we do it safe way.
	$r->setnx($userkey, $balance * 100);
	// Redis does not handle floating points, so we do conversion to cents.
	
	// Put the userid into a set, so later a crontab to be able to read the user balance 
	// and to put it back to the MySQL.
	$r->sadd($setkey, $userid);
}

$balance = $r->decrby($userkey, $price);

if ($balance < 0){
	// Opps, the user have no balance. Revert the "transaction"...
	$r->incrby($userkey, $price);
	
	// Exit
	exit;
}

// We show the requested document here...

echo "Document";

Redis connection note

The example uses generic connection to the Redis server. To make the example work with our service you will need to use code similar to this one:

// change these parameter according to the information in your instance list
$host = '50.30.35.9';
$port = 1234;
$password = 'somehashcode';
$db = 0;

$r = new Redis();
$r->connect($host, $port);
$r->auth($password);
$r->select($db);

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Note:
Because we want this page to be useful for memcached users, we tagged with memcached all examples that may be "recreated" for memcached server.


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