How to Track Online Users with Redis ZSET: A Step‑by‑Step Guide

1. Introduction

Online user counting is a common requirement; this article shows how to implement it using Redis sorted sets (zset) with four core commands: zadd, zrangeByScore, zremrangeByScore, and zrem.

2. Implementation Steps

2.1 How to determine if a user is online?

For authenticated sites, online status can be inferred from the validity of the user's token. For public sites, a rule such as browser fingerprinting (User‑Agent, headers, screen resolution, etc.) can be used, with libraries like 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);
  });
});

Store the visitorId in a cookie or header and pass it to the backend.

2.2 zadd – add online user

zadd takes three arguments: key, score, member. Example:

ZADD myzset 1 "one"

In the Java example, the score is the expiration timestamp, ensuring each user has only one entry.

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

2.3 zrangeByScore – query online users

zrangeByScore retrieves members whose scores fall within a range. Example:

ZRANGEBYSCORE myzset 1 3

To get current online users:

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

The size of the returned set equals the number of online users.

2.4 zremrangeByScore – periodic cleanup

zremrangeByScore removes members whose scores fall within a range. Example:

ZREMRANGEBYSCORE myzset 1 3

Scheduled task to delete expired entries:

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

2.5 zrem – delete on logout

zrem removes a specific member:

ZREM myzset "xxx"

When a user logs out:

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

3. Summary

The approach creates a Redis sorted set where each online user is a member, the expiration timestamp is the score, and standard ZSET commands provide add, query, and cleanup operations, offering a simple yet effective online‑user counting solution.