Google Classroom API : Unable to receive push notifications for courseworks and student submissions (COURSE_WORK_CHANGES) - google-cloud-platform

Question

We are seriously blocked. We have followed the documentation below (among many others) to set up the pub/sub pipelines, create service accounts, assign permissions and use the right scopes and feed types for registrations.
https://developers.google.com/classroom/guides/push-notifications
So programmatically we are able to do the following in .net using the API :
We can Create Courses
We can create registrations for a given courseid
We create/update courseworks for the course that we have created a registration.
All good so far,
BUT, we don't receive notifications for that created/update course work.
some code for clarity :
ServiceAccountCredential credential = new ServiceAccountCredential(
new ServiceAccountCredential.Initializer("sa-something#precise-asset-259113.iam.gserviceaccount.com")
{
User = "impersonated user",
Scopes = new string[] { "https://www.googleapis.com/auth/classroom.coursework.students" ,
"https://www.googleapis.com/auth/classroom.courses",
"https://www.googleapis.com/auth/classroom.push-notifications" }}
.FromPrivateKey("My private key"));
//Authorize request
var result = credential.RequestAccessTokenAsync(CancellationToken.None).Result;
var service = new ClassroomService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
});
// We get courses
var courses = service.Courses.List().Execute();
// We get one course for registration
var course = courses.Courses.First();
// We create registration
var registration = service.Registrations.Create(new Google.Apis.Classroom.v1.Data.Registration()
{
Feed = new Google.Apis.Classroom.v1.Data.Feed()
{
FeedType = "COURSE_WORK_CHANGES",
CourseWorkChangesInfo = new Google.Apis.Classroom.v1.Data.CourseWorkChangesInfo()
{
CourseId = course.Id
},
},
CloudPubsubTopic = new Google.Apis.Classroom.v1.Data.CloudPubsubTopic()
{
TopicName = "projects/precise-asset-259113/topics/test"
},
});
//Successful response - We get a registrationID
var response = registration.Execute();
var courseWork = new CourseWork()
{
CourseId = course.Id,
Title = "Ver Test",
Description = "Calculus",
WorkType = "ASSIGNMENT",
MaxPoints = 20.0,
State = "PUBLISHED",
AlternateLink = "www.uni.com",
CreatorUserId = course.OwnerId,
CreationTime = DateTime.UtcNow,
DueTime = new TimeOfDay() { Hours = 5, Minutes = 10, Nanos = 10, Seconds = 10 },
DueDate = new Date() { Day = 3, Month = 12, Year = 2019 },
Assignment = new Assignment() { StudentWorkFolder = new DriveFolder() { AlternateLink = "Somewhere", Title = "My Calculus" } }
};
//Create course work for the course that we registered
var courseWorkResponse = service.Courses.CourseWork.Create(courseWork, course.Id).Execute();
SubscriberServiceApiClient subscriber = SubscriberServiceApiClient.Create();
SubscriptionName subscriptionName = new SubscriptionName("precise-asset-259113", "test");
PullResponse pullResponse = subscriber.Pull(
subscriptionName, returnImmediately: true, maxMessages: 20);
// Check for push notifications BUT ....NADA!!!
foreach (ReceivedMessage msg in pullResponse.ReceivedMessages)
{
string text = Encoding.UTF8.GetString(msg.Message.Data.ToArray());
Console.WriteLine($"Message {msg.Message.MessageId}: {text}");
}
Can you please assist?
Thanks

Answer 1

There are several things you need to change in order to ensure you get the notifications:
You need to create your subscription before you send the request that will generate the Pub/Sub message. Only messages published after the successful creation of a subscription are guaranteed to be received by subscribers for that subscription.
A single pull request with returnImmediately set to true is unlikely to return any messages, even if a message has been published. With this property set, if there are no messages immediately in memory of the server reached, the empty response is returned. You should always set returnImmediately to false. This still won't guarantee that messages are returned in a single request/response, even if there are messages available, but it will make it more likely.
Ideally, you would use the asynchronous client library, which opens a stream to the Cloud Pub/Sub service and receives messages as soon as they are available. If you are going to use the synchronous Pull method directly, then you need to keep many of them outstanding simultaneously in order to ensure delivery of messages with minimal latency. As soon as you receive a PullResponse for any of the outstanding requests, you should immediately open up another request to replace it. The goal of the asynchronous client library is to prevent one from having to take all of these step manually to ensure efficient delivery.