I’m working on a C# application that syncs data with Google Calendar API and I keep running into rate limit problems.
The Problem:
My app processes multiple users who have authorized calendar synchronization. Even though I stay within the daily quota (1M requests per day, 25K per 100 seconds per user), I still get this error for certain users:
Google.Apis.Requests.RequestError Rate Limit Exceeded [403] Errors [ Message[Rate Limit Exceeded] Location[ - ] Reason[rateLimitExceeded] Domain[usageLimits] ]
My Code Structure:
// Process each authorized user
foreach (var user in databaseContext.UserProfiles)
{
var dataStore = CreateFileDataStore();
// Retrieve stored token
var userToken = dataStore.GetAsync<TokenResponse>(user.ID).Result;
var userCredential = new UserCredential(GetAuthFlow(dataStore), user.ID, userToken);
// Refresh expired tokens
if (userCredential.Token.IsExpired(userCredential.Flow.Clock))
{
if (!userCredential.RefreshTokenAsync(CancellationToken.None).Result)
{
// Remove invalid token
userCredential.Flow.DeleteTokenAsync(user.ID, CancellationToken.None).Wait();
}
}
// Initialize Calendar service
var calendarService = new CalendarService(new BaseClientService.Initializer()
{
HttpClientInitializer = userCredential,
ApplicationName = APP_NAME,
});
// Fetch events from primary calendar
EventsResource.ListRequest eventsRequest = calendarService.Events.List("primary");
// ERROR OCCURS HERE
var calendarEvents = eventsRequest.Execute().Items.ToList();
// Additional operations
calendarService.Events.Delete("primary", event.GoogleCalendarID).Execute();
calendarService.Events.Update(updatedEvent, "primary", updatedEvent.Id).Execute();
}
What could be causing this rate limiting issue and how can I fix it? The error seems to happen randomly for specific users even when I’m not hitting the documented limits.
Google’s rate limiting is weird - they have undocumented burst limits that’ll catch you off guard. I’ve worked with Calendar API for 3 years and those 403 errors happen when you hit these hidden thresholds.
Your problem is the synchronous processing. You’re looping through users and firing requests immediately, so Google sees burst traffic even when you’re under quota. Their system flags this as suspicious and starts blocking you.
I fixed this with a token bucket approach. Don’t delay between users - instead, limit concurrent Calendar service instances to 3 max. Process users in batches with a semaphore:
using var semaphore = new SemaphoreSlim(3, 3);
var tasks = databaseContext.UserProfiles.Select(async user => {
await semaphore.WaitAsync();
try {
// Your calendar processing logic here
}
finally {
semaphore.Release();
}
});
This keeps decent throughput while flying under Google’s radar. The randomness happens because their rate limiter looks at request velocity patterns, not just raw counts.
I encountered a similar issue with the Google Calendar API while developing a multi-user app. The root cause often stems from exceeding the allowed request threshold not per user, but collectively as an application. Introducing exponential backoff and a slight random delay between requests can significantly mitigate these rate limiting errors. I found that implementing a strategy to delay retries exponentially—starting from 1 second and increasing accordingly—helped immensely. Additionally, staggering requests among users can distribute load and avoid hitting the limits set by Google’s system, which considers usage patterns beyond just endpoint limits.
This isn’t about per-user quotas - it’s Google’s undocumented concurrent request limits kicking in. Even when you’re under the official quotas, too many requests at once will trigger rate limiting.
I hit this exact issue last year with a calendar sync service. The problem was processing users in a tight loop with zero throttling.
Here’s what fixed it:
Throttle requests between users:
foreach (var user in databaseContext.UserProfiles)
{
// Your existing code...
try
{
var calendarEvents = eventsRequest.Execute().Items.ToList();
}
catch (GoogleApiException ex) when (ex.HttpStatusCode == HttpStatusCode.Forbidden)
{
// Wait and retry once
await Task.Delay(2000);
var calendarEvents = eventsRequest.Execute().Items.ToList();
}
// Add delay between users
await Task.Delay(500);
}
Batch your operations:
Don’t make individual delete/update calls - batch them together. Cuts your total request count way down.
The random behavior happens because Google looks at your app’s overall request pattern, not just individual quotas. Some users get blocked while others work fine depending on timing.
Also, ditch .Result and .Wait() - use proper async/await. Those blocking calls mess with timing and make rate limiting worse.
Been there, done that - this is a common nightmare. Your quotas aren’t the problem. Google’s burst detection kicks in way before you hit any documented limits.
That foreach loop creates request spikes that trigger their anti-abuse system. Sure, you can add delays, but then you’re stuck managing token refreshes, retry logic, batching, and monitoring across multiple users. It’s a mess.
I stopped fighting rate limits in code and automated the whole flow instead. Built a workflow that processes each user through a queue with built-in rate limiting and automatic retries. It handles API calls, manages tokens, and spreads requests naturally.
No more burst patterns since requests get processed at controlled intervals. Random 403 errors? Gone. Semaphores? Don’t need them. You can scale to way more users without touching code.
For calendar syncing, workflows can trigger on schedules, handle webhook updates, and batch operations automatically. Much cleaner than orchestrating everything in your C# app.
Check out https://latenode.com for properly automating Google Calendar integrations.