SFX, like most link resolvers, depends on CrossRef for DOI-resolving functionality. This is pretty core functionality for the link resolver, required to take an incoming link that has only a DOI (as many do these days, most notably from Google Scholar), and actually DO anything useful with it.
Over the past year or so, every two or three months, CrossRef goes down. For 6 hours, or 24 hours, or 48 hours. During this period, depending on how the link resolver software is written/configured, you either get a timeout from the link resolver itself (as it waits forever for CrossRef), or you get a response from the link resolver which in many cases is pretty useless as it lacks DOI resolution.
This is a danger with relying on third party network services for core functionality in general. It’s especially a danger when you’re relying on a free third party service, for which you have no contract and no service-level agreement.
I’m actually not sure if we libraries pay anything for Crossref resolution services. If we do, it’s a nominal amount. But if it’s even a nominal amount, that might encourage me to complain to them that we need better service, including failover provisions on their end. If it’s entirely free, I wouldn’t even bother, they don’t really owe us anything. But either way, it’s a problem for our software.
To make matters more inconvenient, actually getting from “the service isn’t working right” to a diagnosis of “it’s because CrossRef is down” is increasingly time-consuming in our ever more complex infrastructural stacks. Good logs and hidden-but-on-page debugging info are increasingly crucial to save the time of the programmer.
( Incidentally, the case of JStor, which sends significant numbers of ‘bad’ DOIs to Google Scholar, you get the same kind of ‘broken-ness’ as a matter of course, even when CrossRef is performing fine. I had my first actual report from a patron of a mal-functioning service that ended up being caused by JStor’s bad metadata sent to Google. I assume that for every report from a user, there are 10x or 100x more users that are encountering the same problem, but don’t bother to report it.)