How to Track Online Users with Redis Sorted Sets (zset)

Online user counting can be efficiently implemented with Redis sorted sets (zset). The core Redis commands used are zadd, zrangeByScore, zremrangeByScore, and zrem.

1. Determine if a user is online

If the site requires login, user online status can be judged by the validity of the token. For public sites, a rule such as browser fingerprinting can be used, which combines User-Agent, HTTP headers, screen resolution, timezone, language, and plugins. Libraries like FingerprintJS or ClientJS simplify this process.

// 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);
  });
});

The obtained unique ID can be stored in a cookie or header and sent to the backend.

2. Add online user with zadd

zadd command introduction

The command takes three parameters:

key – the name of the sorted set.

score – a numeric value (integer or double) representing the expiration timestamp.

member – the identifier of the user (e.g., token or fingerprint).

Example: ZADD myzset 1 "one" In Java, adding a user token with an expiration score can be done as follows:

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

This ensures each user has only one latest entry.

3. Query online users with zrangeByScore

zrangeByScore command introduction

key – the sorted set name.

min and max – the score range (can use -inf and +inf).

Example to get members with scores between 1 and 3: ZRANGEBYSCORE myzset 1 3 To obtain the current online users:

String now = DateUtil.formatFullTime(LocalDateTime.now());
Set<String> userOnlineStringSet = redisService.zrangeByScore("user.active", now, "+inf");
// The size of userOnlineStringSet is the online count.

4. Periodically clean expired users with zremrangeByScore

zremrangeByScore command introduction

key – the sorted set name.

min and max – the score range to delete.

Example to delete members with scores between 1 and 3: ZREMRANGEBYSCORE myzset 1 3 Scheduled task example:

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

5. Remove user on logout with zrem

zrem command introduction

key – the sorted set name.

members – the members to delete.

Example:

ZREM myzset "xxx"

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

Conclusion

The core idea is to create a sorted set where the member is the user identifier and the score is the expiration timestamp. Adding, querying, and cleaning this set with the four Redis commands provides a simple yet effective way to track online users.