I can't help but think this isn't the best idea, it raises so many questions. How does fail over work (it failed for me!), can you safely fail over, how does product think about the stack, is it funded well enough, how do you scale a system from zero to all traffic instantly, how do you effectively keep this environment warm, how do you determine which services are critical enough to be in it, how do you manage finding the bugs in two separate stacks, and so on and on and on.
It seems like it would be better to invest in a platform that is distributed across multiple failure domains. Some companies call this a cell architecture. For Monzo this might mean true multi cloud, but they already have bare metal services for bank integrations so they are already hybrid in some respects.
It's not the decision I would have made, and I'm not sure I agree with their reasoning, but I'll also admit that I have not run a bank, I don't know all their constraints, and there are some good tech folks at Monzo. That said, I am an SRE at Google so I'd like to think I'm not completely naïve here.
Edit: after reading in more detail I'm puzzled. The justification for Stand-In and not traditional scale-out is the difficulty of data consistency (agreed!), but then Stand-In is just eventually consistent anyway, and introduces sync problems in both directions. Admittedly I'm spoilt by Spanner, but I do wonder if Monzo is being bitten by past architecture choices here.
It seems like it would be better to invest in a platform that is distributed across multiple failure domains. Some companies call this a cell architecture. For Monzo this might mean true multi cloud, but they already have bare metal services for bank integrations so they are already hybrid in some respects.
It's not the decision I would have made, and I'm not sure I agree with their reasoning, but I'll also admit that I have not run a bank, I don't know all their constraints, and there are some good tech folks at Monzo. That said, I am an SRE at Google so I'd like to think I'm not completely naïve here.
Edit: after reading in more detail I'm puzzled. The justification for Stand-In and not traditional scale-out is the difficulty of data consistency (agreed!), but then Stand-In is just eventually consistent anyway, and introduces sync problems in both directions. Admittedly I'm spoilt by Spanner, but I do wonder if Monzo is being bitten by past architecture choices here.