rHXN - Monzo Stand-In

danpalmer - 1 hour ago

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.

ricardobayes - 35 minutes ago

No, it's a great idea. Any systems engineer knows redundancy is good, even better if it's across providers. I see no issues with this from a system architecture point of view. As to how the implementation is done that's up to them to do right.

danpalmer - 31 minutes ago

Redundancy is great, absolutely, but there are multiple ways to do it and this is not necessarily the right one.

Which is better: 3 DCs for 2 DCs worth of traffic, running the same stack but isolated failure domains... or 4 DCs for 2 DCs worth of traffic, that work in completely different ways, where 2 are left cold until needed. I can see some advantages of the latter, but not a lot.

This also isn't about fault tolerance at the code level like in, say, avionics. In those scenarios having multiple distinct implementations makes sense, but that's very different, you run all paths at the same time and check they agree etc.

sdcfgy - 2 hours ago

Due to the recent outage where it didn’t work properly, this should be renamed Monzo Limp-In.

qmarchi - 2 hours ago

Something like this has been top-of-mind for us since we have a heavy reliance on AWS, which was made evident with the VPC Origin failure earlier this year.

Multi-cloud is best cloud.

IshKebab - 36 minutes ago

Monzo is great. Rare example of a consumer banking company that makes things easy. I just signed up for their phone service in literally 3 minutes. Didn't even need to talk to anyone to port my old number. I'm going to move all my ISAs and SIPP to them next I think.

danpalmer - 21 minutes ago

> Didn't even need to talk to anyone to port my old number

That seems like pretty low stakes – I wouldn't expect to talk to anyone to sign up and transfer to a consumer service in the UK. I don't think I've spoken to anyone for something like this in the last 10 years or so.

broadben12 - 17 minutes ago

This is either a bot comment, or you have a very poor understanding of the UK financial market. They are quite far down the leaderboard for ISA rates and SIPPs.

StevenWaterman - 10 minutes ago

The parent comment didn't mention anything about ISA rates and SIPPs in terms of performance, they said that it was easy. You might disagree with their priorities but that doesn't change whether it's the right decision given those priorities

guiriduro - 1 hour ago

By using another US hyperscaler for your backup platform you're still unhedged against US governance failure/breakdown.

lmz - 1 hour ago

Surely by being a bank, you are already exposed to that.

guiriduro - 51 minutes ago

Exposed yes, but not fatally if you're working with alternative de-dollarised banking systems in the eurozone, for example.

sdcfgy - 1 hour ago

An increasing risk for sure.

ascorbic - 1 hour ago

[2025]