My goal is to maintain "real-time" (or as close as possible) information about the email messages sent by a group of users.
My ideal solution would be to periodically query the API for messages by all users in the group. This feature is not (yet?) implemented.
My second choice would be to create subscriptions (https://graph.microsoft.io/en-us/docs/api-reference/v1.0/api/subscription_post_subscriptions) for every member in the group and then request message information after I become aware of an event. The problem is, in practice, I am only allowed to create 20-30 simultaneous subscriptions (Issues to use Webhook for Microsoft Graph API), which might not be enough.
So I'm stuck with polling all the users in a cycle. The main problem with this approach is I can't find any information how many request are "too many", ie I get throttled. I want to maximize the number of requests to minimize the time it takes to get through one cycle.
A solution that comes to mind is to develop an adaptive program that slowly decreases the time between requests until throttled, then abruptly adds some time to it until a nice balance is found and maintained. This seems like a lot of work though. Right now I'm working on the assumption that 1/second is about the highest I can safely go (0.5 seconds on average per round trip, then a cool down of another 0.5 seconds).
What is the best way to deal with an unknown throttling limit in general, and Microsoft Graph in particular?
Edit:
While I think the accepted answer is a good response, it might not be suitable for all cases. For instance, if you don't want to use the 365 API, and you don't mind using beta features, perhaps you might check out this (delta tokens), which seem to be designed for real-time syncing with the data.
The only potential downside with the accepted answer is that you still need a subscription for each user you want to track (I think...), and there are limits on those. Very curious as to how other people tackled this problem.
While still in Preview, you may want to take a look at Outlook Streaming Notifications. These APIs provide a more robust notification model than simple web hooks. You would still need to establish multiple subscriptions but I expect you'll see far less latency.