案例：用Redis来存储关注关系

Redis提供了丰富的数据类型，比起关系型数据库或者简单的Key-Value存储（比如Memcached）来，Redis的数据模型与实际应用的数据模型更相近。比如下面说到的好友关系的存储，原作者使用了Redis的 Sets（集合）数据结构。

具体存储方式如下：对于每一个用户，其关注关系存储两份列表，一份为此用户关注的人的UID列表，另一份为此用户粉丝的UID列表，这两个列表都使用Sets（集合）。比如对于用户ID为123的用户，graph:user:123:following 保存的是其关注人的列表，graph:user:1:followed_by 保存的是关注他的人的列表。

下面是一个PHP代码的关注关系类，包括了常规的关注关系操作查询等方法，具体可看注释：

<?

/*
 * This example would probably work best if you're using
 * an MVC framework, but it can be used standalone as well.
 *
 * This example also assumes you are using Predis, the excellent
 * PHP Redis library available here:
 * https://github.com/nrk/predis
 */
class UserNode {
    // The user's ID, probably loaded from MySQL
    private $id;

    // The redis server configuration
    private $redis_config = array(
        array('host' => 'localhost', 'port' => 6379 )
    );

    // Stores the redis connection resource so that
    // we only need to connect to Redis once
    private $redis;

    public function __construct($userID) {
        $this->id = $userID;
    }

    private function redis() {
        if (!$this->redis) {
            $this->redis = new Predis\Client($redis_config);
        }

        return $this->redis;
    }

    /*
     * Makes this user follow the user with the given ID.
     * In order to stay efficient, we need to make a two-way
     * directed graph. This means when we follow a user, we also
     * say that that user is followed by this user, making a forward
     * and backword directed graph.
     */
    public function follow($user) {
        $this->redis()->sadd("graph:user:{$this->id}:following", $user);
        $this->redis()->sadd("graph:user:$user:followed_by", $this->id);
    }

    /*
     * Makes this user unfollow the user with the given ID.
     * First we check to make sure that we are actually following
     * the user we want to unfollow, then we remove both the forward
     * and backward references.
     */
    public function unfollow($user) {
        if ($this->is_following()) {
            $this->redis()->srem("graph:user:{$this->id}:following", $user);
            $this->redis()->srem("graph:user:$user:followed_by", $this->id);
        }
    }

    /*
     * Returns an array of user ID's that this user follows.
     */
    public function following() {
        return $this->redis()->smembers("graph:user:{$this->id}:following");
    }

    /*
     * Returns an array of user ID's that this user is followed by.
     */
     public function followed_by() {
         return $this->redis()->smembers("graph:user:{$this->id}:followed_by");
     }

    /*
     * Test to see if this user is following the given user or not.
     * Returns a boolean.
     */
    public function is_following($user) {
        return $this->redis()->sismember("graph:user:{$this->id}:following", $user);
    }

    /*
     * Test to see if this user is followed by the given user.
     * Returns a boolean.
     */
    public function is_followed_by($user) {
        return $this->redis()->sismember("graph:user:{$this->id}:followed_by", $user);
    }

    /*
     * Tests to see if the relationship between this user and the given user is mutual.
     */
    public function is_mutual($user) {
        return ($this->is_following($user) && $this->is_followed_by($user));
    }

    /*
     * Returns the number of users that this user is following.
     */
    public function follow_count() {
        return $this->redis()->scard("graph:user:{$this->id}:following");
    }

    /*
     * Retuns the number of users that follow this user.
     */
    public function follower_count() {
        return $this->redis()->scard("graph:user:{$this->id}:followed_by");
    }

    /*
     * Finds all users that the given users follow in common.
     * Returns an array of user IDs
     */
    public function common_following($users) {
        $redis = $this->redis();
        $users[] = $this->id;

        $keys = array();
        foreach ($users as $user) {
            $keys[] = "graph:user:{$user}:following";
        }

        return call_user_func_array(array($redis, "sinter"), $keys);
    }

    /*
     * Finds all users that all of the given users are followed by in common.
     * Returns an array of user IDs
     */
    public function common_followed_by($users) {
        $redis = $this->redis();
        $users[] = $this->id;

        $keys = array();
        foreach ($users as $user) {
            $keys[] = "graph:user:{$user}:followed_by";
        }

        return call_user_func_array(array($redis, "sinter"), $keys);
    }

}

下面是使用这个类来操作关注关系的例子：

<?
// create two user nodes, assume for this example
// they're users with no social graph entries.
$user1 = UserNode(1);
$user2 = UserNode(2);

$user1->follows(); // array()

// add some followers
$user1->follow(2);
$user1->follow(3);

// now check the follow list
$user1->follows(); // array(2, 3)

// now we can also do:
$user2->followed_by(); // array(1)

// if we do this...
$user2->follow(3);

// then we can do this to see which people users #1 and #2 follow in common
$user1->common_following(2); // array(3)