We got our first email (from Academic Affairs) notifying us that it was down at 5:17pm EDT this afternoon, with little info; followup emails were sent at 6:24 and 6:57 with more info, but mostly about how we would be compensating for it and not about what actually was going on (other than, "nationwide shutdown" and "cybersecurity attacks", no further detail). I don't get a sense that they know much more than that, not that I would expect them to.
A perhaps telling detail: they're instructing us to have students email us directly with any work that had been submitted via Canvas. That suggests that they have no particular confidence that it will come back up soon.
I personally am only slightly affected; as a CS professor a lot of my students' work is done on department machines, and submitted that way, and I do the actual exams on paper. More importantly, I've never liked or trusted Canvas's gradebook, and so although I do upload grades to Canvas so students can see them, my primary gradebook is always a spreadsheet I maintain locally.
But I have a lot of colleagues for whom this is catastrophic at a level of "the whole building burnt down with all my exams and gradebooks in it"---even many of those that teach 100% in person have shifted much or all of their assessment into Canvas (using the Canvas "quiz" feature for everything up to and including final exams), and use the Canvas gradebook as their source-of-truth record. We've been encouraged to do so by our administration ("it makes submitting grades easier"). For faculty in that situation, they have few or zero artifacts that the students have produced, the students themselves don't have the artifacts to resubmit via email because they were done in Canvas in the first place, and they have no record of student grades or even attendance (because they managed that all inside Canvas). I guess they have access to the advisory midterm grades from March, if they submitted them (most do, some don't), but that might be it.
My gut feeling on this is that this is either resolved in hours (they have airgapped backups and can be working as soon as they can spin up new servers), or weeks (they don't). Very little in-between. And if that's true and we wake up tomorrow with this unresolved, I really have no idea what a lot of professors at my university and across the country are going to do to submit grades that are fair and reasonable. In the extreme case, they may have to revert to something we did in the pandemic semester (and before that, at my school, in the semester that two major academic buildings actually did burn to the ground a week before finals): let classes that normally count for a grade just submit grades as pass-fail. Because what else can you do?
(Well, one thing you can do is not put your eggs all in one basket, and not trust "the cloud" quite so much, but that ship's already sailed. I do wonder if in the longer term, anybody learns any lessons from this....)
UPDATE: As of 11:45pm EDT, my university's canvas instance is up and running! Here's hoping it stays (but I'll be downloading some stuff just in case...)
