Elegant Online User Count with Redis Sorted Sets (ZSET)

Online user counting is a common requirement; this article shows a practical implementation using Redis sorted sets (ZSET).

1. Determine online status

If the site requires login, use token validity; for public sites, use a unique identifier such as browser fingerprint. Libraries like FingerprintJS or ClientJS can generate a visitorId.

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

The visitorId is stored in a cookie or header and sent to the backend.

2. Add online user with ZADD

Redis ZADD takes three arguments: key, score, member. The score is the expiration timestamp, ensuring each user has only one latest entry.

ZADD myzset 1 "one"

In Java the code adds a token with an expiration time as score:

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

3. Query online users with ZRANGEBYSCORE

ZRANGEBYSCORE returns members whose scores fall within a range. To get all currently online users, query from the current timestamp to +inf.

ZRANGEBYSCORE myzset 1 3

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

The size of userOnlineStringSet equals the online count.

4. Periodic cleanup with ZREMRANGEBYSCORE

Expired entries are removed by deleting members whose scores are less than the current time.

ZREMRANGEBYSCORE myzset 1 3

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

5. Remove user on logout with ZREM

When a user logs out, delete the corresponding member.

ZREM myzset "xxx"

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

Conclusion

The core idea is to maintain a Redis sorted set where each member represents an online user, the score stores the expiration time, and standard ZSET commands provide add, query, and cleanup operations, resulting in a simple and efficient online user count.