How to Accurately Track Online Users with Redis Sorted Sets

Online user counting can be implemented using Redis sorted sets (zset). The core methods are zadd, zrangeByScore, zremrangeByScore, and zrem.

1. How to identify whether a user is online

For sites that require login, online status is determined by the validity of the user's token. For public sites, you can use IP, deviceId, or browser fingerprint. Browser fingerprint libraries such as FingerprintJS or ClientJS can generate a unique identifier.

Example usage:

// Install: npm install @fingerprintjs/fingerprintjs

// Usage example:
import FingerprintJS from '@fingerprintjs/fingerprintjs';

// Initialize FingerprintJS library
FingerprintJS.load().then(fp => {
  // Get visitor ID
  fp.get().then(result => {
    const visitorId = result.visitorId;
    console.log(visitorId);
  });
});

The generated ID can be stored in a cookie or header and sent to the backend, where it is used as the member of the sorted set.

2. zadd command to add online users

The zadd command adds a member with a score (the expiration timestamp) to the sorted set.

// Set an expiration time for the user token
LocalDateTime expireTime = LocalDateTime.now().plusSeconds(expireTimeout);
String expireTimeStr = DateUtil.formatFullTime(expireTime);
// Add the user token to the sorted set
redisService.zadd("user.active", Double.parseDouble(expireTimeStr), userToken);

3. zrangeByScore command to query online users

Use zrangeByScore to retrieve members whose scores fall within a given range, which represents currently online users.

// Get the current timestamp
String now = DateUtil.formatFullTime(LocalDateTime.now());
// Query all users with scores from now to +inf
Set<String> userOnlineStringSet = redisService.zrangeByScore("user.active", now, "+inf");

The size of userOnlineStringSet equals the number of online users.

4. zremrangeByScore command to periodically clean offline users

Since a sorted set does not automatically remove expired members, a scheduled task can delete entries whose scores are older than the current time.

// Get the current timestamp
String now = DateUtil.formatFullTime(LocalDateTime.now());
// Remove all members with scores between -inf and now
redisService.zremrangeByScore("user.active", "-inf", now);

5. zrem command to delete a user on logout

When a user logs out, remove their token from the sorted set using zrem.

// Delete the member representing the user
redisService.zrem("user.active", "xxx");

Summary

The core idea is to create a Redis sorted set where each online user is a member and the expiration timestamp is the score. By adding, querying, and removing members with the appropriate commands, you can efficiently track online user counts.