A bank based in my region, one of the biggest in Europe, uses IBM mainframes and has hundreds of thousands of COBOL programmes. Their availability is disastrous. They need to regularly reboot LPARs (logical partitions of the mainframe) because they're so brittle, resulting in a segment of the clients unable to use online services. And on the other hand, services that need to stay available, like websites or instant settlement, obviously don't run on mainframes. That means that the website is up... but only displays a maintenance message.
It's the servers that go down! Not the 'frames. Those will go down maybe once a year for maintenance, if that. The mainframe engineers will brag your ears off about that and about the unreliability of the "distributed" systems (a.k.a. servers, a.k.a. what everyone else uses).
When you log on to your online banking you're interacting with servers, not with the mainframes directly. The servers are the interface, the mainframe is the, let's say far back end. That will be handling millions of transactions a second while the servers are down for maintenance- like handling payments and transfers etc, not just online banking.
Which is why it can't keep failing every few months or so.
I definitely remember my bank warning me about credit and debit card unavailability, so it's definitely not just the online systems.
If cards don't work and the online systems don't work, I don't know what else does. Those maintenance periods usually happen at night, so branches aren't open and wire transfer systems don't work.
> […] my bank warning me about credit and debit card unavailability […]
Your bank might have meant card balances being unavailable in the online banking which is more plausible than cards being generally unavailable for payments. It is not indicative of the mainframe actually being down (although not entirely improbable, either), and it is more likely that an intermediary (a service or a server) was undergoing maintenance.
Payment processing involves multiple tiers and multiple routing layers built into it – to ensure very high availability and that a payment is almost always guaranteed to process (successfully or otherwise – does not matter). Payment networks also impose stringent technical requirements onto the banks connecting to them. A Raspberry Pi running Slackware Linux from 1993 and powered by a dangling street pole wire would not be allowed to connect, for example.
Your bank (or mine, for the sake of the conversation) is the terminal point in this whole payment processing chain, with the payment network (Visa, Mastercard but not AmEx[0]) being the port of entry for a payment. Depending on the country, a country may have its own local payment Visa / MC processing centre and if that is the case, local payments will be routed to the local card payment processor. Otherwise, a global Visa/Mastercard will assume the payment. Then the payment network contacts your local bank to authorise the payment. Depending on the nature of the failure + other factors, the Visa/Mastercard can authorise certain payments on behalf of your bank if they fail to reach your bank and will forward the payment particulars onto your bank so that your bank could correctly process your card payment later. It is more common in overseas payment scenarios, i.e. you are travelling overseas and especially so when travelling outside the first world countries.
Payment networks and banks do not like such situations and actively loathe non-real time payment authorisations, yet they allow them for a narrow number of use cases at their own discretion.
[0] AmEx own their own global payment network and do not allow other financial institutions to gain access into it.
Maybe your bank goes down, but the Visa and Mastercard systems have been processing card payments 24/7 non stop and I dont even remember hearing a single incodent when system was down even partially
I don't know the particulars about your bank, obviously but the thing is, mainframes serve so many millions of clients, and that's clients of the largest money transfer networks (Amex, Visa, Master, everyone really) that if there's an outage it will make the news, internationally, and it will be front page news too. With live updates.
In fact, if I think about it, I don't think I remember any time when a serious outage that was eventually explained in the press was the fault of some mainframe going down. Usually it's something else, like someone misconfigured something or something didn't update correctly etc. stuff that sounds a lot like day-to-day web dev stuff.
So I think maybe it was something else that went on with your bank, that only affected your bank. Like inkyoto says below, maybe some DNS went down?