从多个线程（并行）使用GraphServiceClient-发送邮件

Question

I have a need to send multiple emails using Microsoft Graph from Windows Service.
I'm using Microsoft.Graph NuGet package.
I'm creating GraphServiceClient and sending mail like so:

IGraphServiceClient graphClient = new GraphServiceClient("https://graph.microsoft.com/v1.0", authenticationProvider);
var email = new Message
{
    Body = new ItemBody
    {
        Content = "Works fine!",
        ContentType = BodyType.Html,
    },
    Subject = "Test",
    ToRecipients = recipientList
};

await graphClient.Users["test@example.onmicrosoft.com"].SendMail(email, true).Request().WithMaxRetry(5).PostAsync();

When I send emails one-by-one:

for (var j = 0; j < 20; j++)
{
    await graphClient.Users["test@example.onmicrosoft.com"].SendMail(email, true).Request().WithMaxRetry(5).PostAsync();
    progressBar1.PerformStep();
}

everything works fine, but when I use Parallel.For :

var res = Parallel.For(0, 20, async (i, state) =>
{
    var email = new Message
    {
        Body = new ItemBody
        {
            Content = "Works fine!",
            ContentType = BodyType.Html,
        },
        Subject = "Test",
        ToRecipients = recipientList
    };

    await graphClient.Users["test@example.onmicrosoft.com"].SendMail(email, true).Request().WithMaxRetry(5).PostAsync();
});

i get errors, because I get Too Many Requests (429) and then Unsupported Media Type (415).

This is the error code:

Code: RequestBodyRead Message: A missing or empty content type header was found when trying to read a message. The content type header is required.

This is how it looks in Fiddler:

My question is: can I use and how I should use Graph with Parallel.For to avoid this kind of errors. I'm already setting WithMaxRetry(5) for each request.

I'm aware of usage limits, but I thought WithMaxRetry(5) will help.

Answer 1

It doesn't have to do with the threads. It has to do with throtteling aka you can only do x amount of requests in a certain time period.

The dotnet graph api client doesn't support batching (sadly enough). But batching those requests yourself is easily implemented. Then you can send 15 mails with one request.

Answer 2

The reason why you saw this is because of the missing content type header. It wasn't being cloned when we cloned the httprequestmessage. This has been fixed and will be in the next release of the client. With regards to parallel threads, we plan on implementing a resource based shared retry queue so that we use a single retry scheme across multiple requests that target the same resource (and the same throttling policy).