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(SP: 1) [Backend] Move session heartbeat from DB writes to Redis sorted set #411

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Merged
ViktorSvertoka merged 1 commit into from
Mar 22, 2026
Merged

(SP: 1) [Backend] Move session heartbeat from DB writes to Redis sorted set #411

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99 changes: 58 additions & 41 deletions frontend/app/api/sessions/activity/route.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -5,8 +5,11 @@ import { NextResponse } from 'next/server';

import { db } from '@/db';
import { activeSessions } from '@/db/schema/sessions';
import { getRedisClient } from '@/lib/redis';

const SESSION_TIMEOUT_MINUTES = 15;
const SESSION_TIMEOUT_MS = SESSION_TIMEOUT_MINUTES * 60 * 1000;
const REDIS_KEY = 'online_sessions';

function getHeartbeatThrottleMs(): number {
const raw = process.env.HEARTBEAT_THROTTLE_MS;
Expand All @@ -17,56 +20,70 @@ function getHeartbeatThrottleMs(): number {
return Math.max(floor, parsed);
}

export async function POST() {
async function heartbeatViaRedis(sessionId: string): Promise<number | null> {
const redis = getRedisClient();
if (!redis) return null;

try {
const now = Date.now();
const pipeline = redis.pipeline();
pipeline.zadd(REDIS_KEY, { score: now, member: sessionId });
pipeline.zremrangebyscore(REDIS_KEY, '-inf', now - SESSION_TIMEOUT_MS);
pipeline.zcard(REDIS_KEY);

const results = await pipeline.exec();
return results[2] as number;
} catch (err) {
console.warn('Redis heartbeat failed, falling back to DB:', err);
return null;
}
}

async function heartbeatViaDb(sessionId: string): Promise<number> {
const now = new Date();
const heartbeatThreshold = new Date(
now.getTime() - getHeartbeatThrottleMs()
);

await db
.insert(activeSessions)
.values({ sessionId, lastActivity: now })
.onConflictDoUpdate({
target: activeSessions.sessionId,
set: { lastActivity: now },
setWhere: lt(activeSessions.lastActivity, heartbeatThreshold),
});

if (Math.random() < 0.05) {
const cleanupThreshold = new Date(Date.now() - SESSION_TIMEOUT_MS);
db.delete(activeSessions)
.where(lt(activeSessions.lastActivity, cleanupThreshold))
.catch(err => console.error('Cleanup error:', err));
}

const countThreshold = new Date(Date.now() - SESSION_TIMEOUT_MS);
const result = await db
.select({ total: sql<number>`count(*)` })
.from(activeSessions)
.where(gte(activeSessions.lastActivity, countThreshold));

return Number(result[0]?.total || 0);
}


export async function POST() {
try {
const cookieStore = await cookies();
let sessionId = cookieStore.get('user_session_id')?.value;

if (!sessionId) {
sessionId = randomUUID();
}

const now = new Date();
const heartbeatThreshold = new Date(
now.getTime() - getHeartbeatThrottleMs()
);

await db
.insert(activeSessions)
.values({
sessionId,
lastActivity: now,
})
.onConflictDoUpdate({
target: activeSessions.sessionId,
set: { lastActivity: now },
setWhere: lt(activeSessions.lastActivity, heartbeatThreshold),
});

if (Math.random() < 0.05) {
const cleanupThreshold = new Date(
Date.now() - SESSION_TIMEOUT_MINUTES * 60 * 1000
);

db.delete(activeSessions)
.where(lt(activeSessions.lastActivity, cleanupThreshold))
.catch(err => console.error('Cleanup error:', err));
}

const countThreshold = new Date(
Date.now() - SESSION_TIMEOUT_MINUTES * 60 * 1000
);
const redisCount = await heartbeatViaRedis(sessionId);
const online = redisCount ?? (await heartbeatViaDb(sessionId));
Comment on lines +83 to +84
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@coderabbitai coderabbitai bot Mar 22, 2026

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⚠️ Potential issue | 🟠 Major

Avoid switching back to Redis count immediately after an outage.

Sessions written via heartbeatViaDb() exist only in active_sessions, so redisCount ?? ... undercounts as soon as Redis starts responding again. After a transient outage, the online number can collapse until every still-active user heartbeats back into Redis. Consider keeping DB as the source of truth for one timeout window after a Redis failure, or seeding Redis from recent active_sessions rows before switching back.

🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@frontend/app/api/sessions/activity/route.ts` around lines 83 - 84, The
current logic flips back to Redis immediately when heartbeatViaRedis(sessionId)
returns a value, causing undercount after transient Redis outages; change the
logic so that when heartbeatViaRedis(sessionId) returns null/undefined
(indicating an outage) you record the outage time and use
heartbeatViaDb(sessionId) as the source of truth for at least one timeout window
(e.g., SESSION_TIMEOUT_MS) after that outage, or alternatively seed Redis from
recent active_sessions before switching; update the code around redisCount,
online, and the functions heartbeatViaRedis/heartbeatViaDb to check a
lastRedisOutage timestamp (or a boolean flag) and continue using DB results
until the grace period expires, then resume using Redis normally.


const result = await db
.select({
total: sql<number>`count(*)`,
})
.from(activeSessions)
.where(gte(activeSessions.lastActivity, countThreshold));

const response = NextResponse.json({
online: Number(result[0]?.total || 0),
});
const response = NextResponse.json({ online });

response.cookies.set('user_session_id', sessionId, {
httpOnly: true,
Expand Down
2 changes: 1 addition & 1 deletion frontend/lib/about/stats.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ export const getPlatformStats = unstable_cache(

const linkedinCount = process.env.LINKEDIN_FOLLOWER_COUNT
? parseInt(process.env.LINKEDIN_FOLLOWER_COUNT)
: 1700;
: 1800;

let totalUsers = 243;
let solvedTests = 1890;
Expand Down
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