What to do if app misses notification - SequenceNumber jumps over by one or more?

Question

Where I am right now? I have subscribed to calendar notifications via office365's REST API. When any event in that calendar are created/updated/deleted office365 sends notification to my app's endpoint which I assigned when I created this subscription. Subscription code works like a charm and notifications are coming immediately.

I really want to make my code error tolerant and here comes the problem: what happens if notifications are coming in non-incremental order? For example if someone first creates event and then removes it. Can those notifications come in opposite order? 1. remove event, 2. create event. I failed to find this in documentation.

I am really interested about how you have been handling SequenceNumber in your code.

This is what documentation says:

SequenceNumber - A number in sequence for a notification, to help the client app identify if it's missing a notification.

Okay, it's nice to know if I missed a notification, but can you tell me what to do in that case? This seems to be really poorly explained in anywhere.

1. How SequenceNumber behaves e.g. can they come in non-incremental order?
2. What to do if you miss a SequenceNumber?

What I have figured out by testing this, I see that SequenceNumber is always in incremental order. But I'm not sure is this always the case?

And more importantly, what to do IF my app misses SequenceNumber? I mean if there comes 1, 2, 3 and next one is 5 or 6, what should I do then?

Also I'm not quite sure if ChangeType: Missed has something to do with this. I'm really confused about this so any help would be appreciated! :)

Answer 1

What I have tested it never jumps over a SequenceNumber, but good practice is to build a notification Queue per subscription and handle notifications one by one ordered by SequenceNumber.

ChangeType: Missed doesn't tell much. Pretty much it tells you only what subscription has missed notification. With calendar integration I ended up to check which calendar corresponds that subscriptionId and then just run full sync to validate if there has any changes in office we missed or not.