Google Calendar + Workspace 연동 예약 시스템 개발기

flowchart LR
    User[사용자] --> Roomly[Roomly 서버]
    Roomly <--> GCal[Google Calendar API]
    Roomly <--> Slack[Slack API]
    GCal -->|Webhook| Roomly

// Google OAuth 토큰 교환
public async Task<OAuthTokenResponse> ExchangeAuthorizationCodeForTokensAsync(
    string authorizationCode, 
    string? redirectUri)
{
    const string tokenEndpoint = "https://oauth2.googleapis.com/token";
    
    var content = new FormUrlEncodedContent(new Dictionary<string, string>
    {
        ["code"] = authorizationCode,
        ["client_id"] = _config.ClientId,
        ["client_secret"] = _config.ClientSecret,
        ["redirect_uri"] = redirectUri ?? _config.RedirectUri,
        ["grant_type"] = "authorization_code"
    });
 
    var response = await _httpClient.PostAsync(tokenEndpoint, content);
    // ...
}

// hd(Hosted Domain) 필드로 Workspace 계정 검증
if (string.IsNullOrEmpty(idToken.Hd))
{
    throw new RoomlyException(
        ResponseCode.WorkspacePersonalAccountNotAllowed,
        "Google Workspace account required."
    );
}

public async Task<Event> CreateEventAsync(
    string calendarId,
    Reservation reservation,
    IEnumerable<string> attendeeEmails)
{
    var calendarEvent = new Event
    {
        Summary = reservation.Title,
        Start = new EventDateTime 
        { 
            DateTimeDateTimeOffset = reservation.StartAt 
        },
        End = new EventDateTime 
        { 
            DateTimeDateTimeOffset = reservation.EndAt 
        },
        Attendees = attendeeEmails.Select(e => new EventAttendee { Email = e }).ToList()
    };
 
    return await _calendarService.Events.Insert(calendarEvent, calendarId).ExecuteAsync();
}

// RRULE 예시
"RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20260331T000000Z"
 
// This Event Only / This and Following Events / All Events
// 세 가지 수정 모드 모두 지원

public async Task RegisterWebhookAsync(long companyId, string calendarId)
{
    var channel = new Channel
    {
        Id = Guid.NewGuid().ToString(),
        Type = "web_hook",
        Address = $"{_serverDomain}/api/calendar/webhook",
        Expiration = DateTimeOffset.UtcNow.AddDays(7).ToUnixTimeMilliseconds()
    };
 
    await _calendarService.Events.Watch(channel, calendarId).ExecuteAsync();
}

// 매일 새벽에 만료 임박한 Webhook 갱신
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
    while (!stoppingToken.IsCancellationRequested)
    {
        await RefreshExpiringWebhooksAsync();
        await Task.Delay(TimeSpan.FromHours(24), stoppingToken);
    }
}

// Before: 단일 키에 모든 회사 데이터
"reminders:pending" → 100K items
 
// After: 회사별 분리
"reminders:pending:1" → 1K items (Company 1)
"reminders:pending:2" → 1K items (Company 2)