ars-backend/app/Services/MessageSequence.php

<?php

namespace App\Services;

use App\Models\ChatSession;
use App\Models\Message;
use Illuminate\Support\Facades\Redis;

class MessageSequence
{
    public function nextForSession(ChatSession $session): int
    {
        $key = $this->redisKey($session->session_id);

        try {
            $current = Redis::get($key);

            if ($current === null) {
                $seed = $this->seedFromDatabase($session);
                Redis::setnx($key, $seed);
            } elseif ((int) $current < $session->last_seq) {
                Redis::set($key, (string) $session->last_seq);
            }

            return (int) Redis::incr($key);
        } catch (\Throwable) {
            return $session->last_seq + 1;
        }
    }

    public function syncToAtLeast(string $sessionId, int $seq): void
    {
        $key = $this->redisKey($sessionId);

        try {
            $current = Redis::get($key);
            if ($current === null || (int) $current < $seq) {
                Redis::set($key, (string) $seq);
            }
        } catch (\Throwable) {
            return;
        }
    }

    private function seedFromDatabase(ChatSession $session): int
    {
        $maxPersistedSeq = (int) (Message::query()
            ->where('session_id', $session->session_id)
            ->max('seq') ?? 0);

        return max($session->last_seq, $maxPersistedSeq);
    }

    private function redisKey(string $sessionId): string
    {
        return "chat_session:{$sessionId}:seq";
    }
}