Implementing Online User Counting with Redis Sorted Sets (ZSET)

Online user counting is a common requirement, and a practical way to implement it is by using Redis sorted sets (ZSET) to store user identifiers with expiration timestamps as scores.

1. Introduction

The core Redis commands used are zadd , zrangeByScore , zremrangeByScore , and zrem .

2. Implementation Steps

2.1 Determining Whether a User Is Online

If the site requires login, the validity of the user's token can be used to judge online status. For public sites, a custom rule such as browser fingerprinting is recommended. Fingerprint data may include User-Agent, headers, screen resolution, timezone, language, plugins, etc.

Example using FingerprintJS:

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

2.2 Adding Online Users with zadd

The zadd command requires a key, a score, and a member. Example:

ZADD myzset 1 "one"

In Java, adding a user token with an expiration timestamp as the score:

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

This ensures each user has only one latest entry because the score (timestamp) is unique.

2.3 Querying Online Users with zrangeByScore

To get all users whose scores are within a range (e.g., from now to +inf):

ZRANGEBYSCORE myzset 1 3

Java example:

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

The size of userOnlineStringSet represents the number of online users.

2.4 Periodically Removing Expired Users with zremrangeByScore

Example to delete members with scores between 1 and 3:

ZREMRANGEBYSCORE myzset 1 3

Java scheduled task:

// Get current time
String now = DateUtil.formatFullTime(LocalDateTime.now());
// Remove entries from -inf to now
redisService.zremrangeByScore("user.active", "-inf", now);

2.5 Removing a User on Logout with zrem

Example to delete a specific member:

ZREM myzset "xxx"

Java code:

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

3. Summary

The solution creates a Redis sorted set where each online user is a member, the expiration timestamp is the score, and the set is manipulated using the four ZSET commands to add, query, and clean up user sessions efficiently.

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