Implementing Online User Counting with Redis Sorted Sets (ZSET)

The article introduces an online user counting feature and suggests using Redis sorted sets (ZSET) for its implementation.

Core Redis commands used are zadd , zrangeByScore , zremrangeByScore and zrem .

Step 1: Determine whether a user is online. For authenticated sites, check token validity; for public sites, use browser fingerprinting with libraries such as FingerprintJS or ClientJS.

// Install: npm install @fingerprintjs/fingerprintjs
// Usage example:
import FingerprintJS from '@fingerprintjs/fingerprintjs';
FingerprintJS.load().then(fp => {
  fp.get().then(result => {
    const visitorId = result.visitorId;
    console.log(visitorId);
  });
});

Step 2: Add online users to a ZSET using zadd . Example command:

ZADD myzset 1 "one"

LocalDateTime expireTime = LocalDateTime.now().plusSeconds(expireTimeout);
String expireTimeStr = DateUtil.formatFullTime(expireTime);
redisService.zadd("user.active", Double.parseDouble(expireTimeStr), userToken);

Step 3: Query online users with zrangeByScore . Example command:

ZRANGEBYSCORE myzset 1 3

String now = DateUtil.formatFullTime(LocalDateTime.now());
Set
userOnlineStringSet = redisService.zrangeByScore("user.active", now, "+inf");

Step 4: Periodically clean up expired users using zremrangeByScore :

String now = DateUtil.formatFullTime(LocalDateTime.now());
redisService.zremrangeByScore("user.active", "-inf", now);

Step 5: Remove a user when they log out with zrem :

redisService.zrem("user.active", "xxx");

The core logic is to maintain a ZSET where the key is the online‑user set, members are user identifiers, and scores are expiration timestamps, enabling efficient add, query, and cleanup operations.

In addition to the technical tutorial, the article contains promotional material for ChatGPT services, a paid community, and related offers, which are not part of the technical content.