Scenario

A social media platform requires statistics of user likes and online users in real time. A Redis-based real-time counter is used for real-time statistics and high concurrency requirements.

Business Requirements

It is necessary to collect the number of user likes and that of online users in real time to display real-time updates of likes on the user interface and

to help the administrator monitor the number of online users in the background.

Analysis

Counters can be updated in real time and deal with a large number of concurrent requests.

Counter data needs to be flushed to disks to prevent data loss.

Implementation Solution

Use the counter function of Redis to store and update the number of likes and online users.
Use the key expiration feature of Redis to automatically clear expired online users.
Use Redis DataBase (RDB) and Append Only File (AOF) to ensure the persistence of data.
Use the transaction function of Redis to realize the atomic update of counters.

Sample Code

The following shows a simplified code example explaining the key parts of a real-time counter based on Redis:

// The user gives a like.
public void likePost(String postId, String userId) {
    Jedis jedis = new Jedis("localhost");
    jedis.incr("post_likes:" + postId); // Increase the number of likes.
    jedis.sadd("post_liked_users:" + postId, userId); // Add a like user.
    jedis.close();
}

// Get the number of likes for a post.
public long getPostLikes(String postId) {
    Jedis jedis = new Jedis("localhost");
    long likes = Long.parseLong(jedis.get("post_likes:" + postId));
    jedis.close();
    return likes;
}

// The user goes online.
public void userOnline(String userId) {
    Jedis jedis = new Jedis("localhost");
    jedis.setex("user_online:" + userId, 300, "true"); // Set the user online flag and the expiration time to 300s.
    jedis.close();
}

// Get the number of online users.
public long getOnlineUserCount() {
    Jedis jedis = new Jedis("localhost");
    long count = jedis.keys("user_online:*").size();
    jedis.close();
    return count;
}

In this example, the counter function of Redis is used to store the number of likes, and the Redis sets are used to store the users who give a like. The key expiration function of Redis is used to set the expiration time of online flags, and the query keys of Redis are used to obtain the number of online users.

DCS Redis Help Documentation

Real-Time Counter