Monzo Stand-In | Hacker News Reader

Monzo Stand-In

monzo.com

130 pointscoffeefuel68 comments

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.

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.

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.

I've never run a bank either, but their current setup is very cost effective. A true multi cloud architecture would be "much" (a couple of 9s) more available, but I believe it would introduce other issues. The goal was to ensure the use of a subset of features for short periods of time, which is probably already above the market standard.

The cost effectiveness doesn't really add up for me either though (and again, I lack a lot of context). 1% of cost to run everything essential doesn't pass the sniff test, I assume it's much more during an actual failover. But I'm also not suggesting running any more hardware than the number of failure domains you want to be resilient to losing, and on cloud you could choose to run less and scale (assuming you aren't concerned about stockouts).

My guess is a little of both. I would bet that the 1% is the unused cost, but that for periods that they are running the stand-in system it's much closer to normal. But from what I've heard of their infrastructure I'd also bet that stand-in is significantly cheaper in part because it had the benefit of hindsight and could resolve issues in the first system.

None of this really changes my original feeling though. A cell based architecture spreads the risk, failover is essentially built-in without being a special mode you have to put the system into and then recover from again, on cloud you can scale those up and down with traffic and they also take peaks from each other, and so on. There's a reason why there are very few cloud outages that cross all regions for a provider, and if you're paranoid you can run multi-cloud (as they already do).

"Just do multi-cloud" is burying the lede a bit. Monzo is a bank, so it's not a given that they can easily partition all their operations into fully independent cells. I'm reasonably confident that teams of average competence could implement a second system for failover. I'm much less confident in teams of average competence doing complicated distributed systems work correctly, and doing it incorrectly is potentially existential risk for a bank.

In your Google scope this all works out however if you're a mature but not yet prevalent, neobank whose reputation rides almost solely on availability, I'd still say theirs is a great approach. Even in the unlikely event that AWS throws in the towel and has a hard failure, users can still enjoy a subset of functions without elevated latency. On top of it, it might well be that user data can never leave UK, so geographical redundancy might not even be a thing here.

And yes you are completely right a completely distinct implementation is probably a no-go, but I'd think you could deploy a similar codebase with feature flags, with some frontend differences. I'm speculating but pretty sure they didn't reimplement backend functionality for this.

Agree, I know nothing about their system but in terms of clear auditability and accountability of systems I agree it's great. Every subsystem failure or inconsistency can be clearly attributed to one or the other site/stack, and the stand-in could for example apply other rules (e.g. approve only smaller transaction amounts that are accepted by an incident-only set of risk rules).

I'm a bit curious about Spanner. At $EMPLOYER we were going to do multi cloud with multi-region CockroachDB clusters but gave up on it because write latencies and costs were too high. Do you not get the same thing with Spanner, where technically you can have multi region but it's very unattractive to actually do?

I'm afraid I'd have to defer to the public docs on this. I've only used it internally and a bunch of that is not public.

What I would say is that it's considered to be much better than CockroachDB, but that there's no getting around physics. Spanner is subject to the same speed of light... but in every other way it does what it can do to improve things at every level. Google's use of GPS hardware for better timing is public.

I suspect Monzo was bitten by complexity. The network should not be assumed to be reliable and if you make a thousand things (microservices) with network between them regardless of what you think may happen, it’s entirely impossible to test it with any level of certainty. And that’s worse if you have a complete failover scenario.

Add the instability of the average cloud control plane during an outage and things get rather nasty rather quickly in reality. We’ve been through that with AWS’ numerous outages.

My company is victim of this mindset. We sacrificed tested and assured reliability on physical hardware in multiple redundant data centres for cloud and microservices. Mostly driven by Monzo’s claims, semi religious thinking and me too.

Reliability is measurably down, cost is higher, latency is higher and we cannot fail over to another provider because someone went to an AWS conference and started using all the toys. Even if we did get rid of those dependencies, during an outage the kubernetes control plane is so tied into the vendor that we’d be fucked anyway. Last time the ingress controller couldn’t change ALBs for example.

Only winner here is Bezos and the decision makers CVs. The losers are the customers. We live in the stupidest of times.

For truly critical systems and independently developed redundant or backup system that can perform at least the most important functions is clearly superior. Regardless of how much effort goes into making one implementation reliable, there's always the possibility of an issue that takes it out, or forces it to be taken offline.

I don't know how common this is in retail banking but I'm happy to see Monzo do it.

I think the biggest problem is the data consistency. Since they are planning for eventual consistency, should make it easier, but they most probably had to re-write code to support atomic operations and proper transaction isolations.

But the scaling problem is still there, when they switched over I assume they would start both horizontal and vertical scale of k8 pods and maybe even bump up DB instances as well which could result in a small congestion at the start of the switch over

Foundationdb has a good solution to this. Uses three regions, sync writes to regions 1 and 2 which should be near each other, sync writes to region 3. If region 1 dies then region 3 pulls last commits from region 2 before serving traffic. Small cluster manager runs in all three regions

monzo r the same guys who went in on micro services. at a time they were bragging 3 micro services per engineer. so yeah a lot of things don't make sense.

So I've been spending a lot of time in the financial services/payment space and might be able to answer some of your questions.

The EU activated the Digital Online Resiliency Act (DORA) last year (passed in 2023) which, amongst other things, mandates that financial services providers who run their core business functions in the cloud (i.e. Monzo) must be able to demonstrate that they are not entirely reliant on a single cloud provider. Otherwise, big, big fines.

Like GDPR, there are auditors that are responsible for signing off on EU DORA compliance. Also like GDPR, there are ways to check those boxes without _really_ checking them. This is probably why Stand-In only has 18 services in its GCP failover "region" instead of the 1K+ production services in AWS; it's just enough to keep critical services (payments/card networks, balances, ledger, etc.) operational and, more importantly, keep DORA auditors at bay.

This article IMO is more for other FSIs who _want_ to run _entirely_ in the cloud (there are very, very few that do). Financial services is a very risk-averse industry. Most providers have biz-critical systems running on mainframes still. (Mainframes are practically indestructible and are much more technically-interesting than people give them credit for, but that's another post.) Being able to tell AWS "yeahhhhh we can run this business on GCP and/or Azure in literally 10 minutes" is a money line for procurement to use during price negotiations. Or at least it was until RAM prices mooned hard enough to make their existing datacenter inventory suddenly very attractive.

Cloud-aside, pretty much every bank in the US has some architecture like this for their critical banking systems, except they are usually an old-school hot-cold (sometimes hot-hot) datacenter failover strategy. Historically failover has been done manually with a runbook (with some automation) and tested during DR/BCP days (sometimes more frequently). They, of course, have zonal/regional replication in place.

It kind of makes sense that critical financial infrastructure would be one of the things that doesn’t just run on AWS and accepts it goes down with everything else when us-east-1 goes down

So much stuff in AWS depends on us-east-1 being up; it's crazy.

Fwiw to acquire a banking license and get the relevant approvals I believe you need to prove a redundancy plan that goes beyond just scaling.

I used to work in fintech where we’d face the same requirements and know a bunch of folks from Monzo. You can fight the status quo only for so long until it becomes much easier to just do what you’re being asked and produce a totally separate service that solidly answers any audit you may face.

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

Monzo I've Fallen And I Can't Get Up

Simple Bank, when they were a thing, published this "look at how awesome we are" post about moving to Riak from Cassandra. (I think that's what it was; it's been a long time.)

"We're going to be so reliable after this; big banks don't stand a chance" was what I took away from it.

The migration went horribly. Tier 0 clusterfuck. Debit cards were completely unusable and they lost ledger data. That and the occasional duplicate transaction (like the duplicated student loan payment that put me in a really bad spot at the time, which I was only given $50 for the inconvenience) was enough for me to _run_ bank to Chase.

So, yeah, I get why the big banks are totally fine with keeping their mostly-rock-solid core banking functions on the mainframe and stick with datacenters everywhere.

100% why I bank with an old bank that runs mainframes.

Minor thing but I do like they use the word "Stand In" here in the payments / financial sense of Stand-in processing (STIP), which is usually used when a bank is offline to keep essential services such as public transport working.

There are 3 things I have in mind when reading this:

- it should be so frustrating for a dev to work on a full core banking that is mostly useless and userless except maybe 1h every 3 years.

- if you build a totally different implementation to handle high traffic transaction from third parties that don't operate for days or months, i would expect it to be fragile. Like for example suddenly you receive unicode special chars that you didn't expect in labels and boom.

- last, I think that in most incident cases, the situation is not clear enough to be able to switch to the full different system. Rarely it will be your whole infra but part of it, so there is always the question to what to do in such a case, or if your main infra goes crazy with loops for example, or if you lose control because of a network, aws, infrastructure issue thr problem might be that you have no visibility on what is working and what is not, not necessarily the control to stop the faulty services, etc...

"it should be so frustrating for a dev to work on a full core banking that is mostly useless and userless except maybe 1h every 3 years."

When other banks go down and Monzo stays up, even for 1 hour, that is a massive PR win and will bring in new customers. It's also been used multiple times as of the article (2025), and they are always exposing some users to it for testing.

I don't say that it is not useful, and that it is not a good thing for the company. More than I would be very frustrated to spend my life on that if it was my job.

Offtopic but I remembered one HN comment about where someone was working for an internal app for a company for the fast food/ice-cream or food app in general (or maybe it was for the contractor)

I think some people found that job really peaceful when others might've found it more frustrating.

To me, I think though the issue isn't the scale or even the internal part of it but rather in this case, there's a difference because its being used infrequently (1 hour) as compared to that other HN comment was still being used quite frequently (it was used every day)

Though, I must admit, I don't find it too frustrating. I can be wrong I usually am but, Sysadmin/DevOps/Security engineer does the work as well where sometimes 1 hour can matter much more than months and they do work for months so that they don't get to face that 1 hour issue just because of how devastating it can get, Though Sysadmins do a lot of regular work as well.

There are many jobs where they exist for reasons where the employee exists to handle the bad things. You would wish for the bad thing to not happen and the employee might feel like they might not be doing much if the bad thing doesn't happen, BUT when that bad thing happens, You would be happy that they would then be there for you.

Maybe not a direct 1:1 comparison but hopefully I can express my point. I feel like there are many jobs in tech, or maybe in the world in general which fall into a similar fashion?

> Monzo Stand-in is an independent set of systems that run on Google Cloud Platform (GCP)

Given the US Cloud Act, how Monzo is protecting customers' financial data from being accessed by foreign government without Monzo's knowledge?

Everyone does it, does not excuse it.

Also shows lack of faith in British business.

Which British businesses provide this sort of infrastructure?

I ask this as a Brit who ran stuff on a few UK providers, and ultimately moved to a cloud platform because of the combination of terrible service and lack of features. The landscape may have changed, but in my experience it was bleak circa 2020.

There's a now dead comment giving a few options. One of those options was the main driver for my previous company moving fully to GCP, they were atrocious and caused a major security incident for us. They also regularly took days to action urgent fixes.

I'm not saying the UK doesn't have data centres, but it's easy to do a crap job of running one. In my experience there's a level of competence that is missing from UK operations. Hetzner and OVH are noticeably better, but still not quite at the level of the main cloud providers.

Just pointing out this is from February 2025, so has been a thing for a while

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.

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.

> 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.

I've never talked to someone for it, and I would've first done it in something like 2008. (It's 'always' been done by SMS, well before 'apps' that might let you (and I don't recall it on websites).)

Well I haven't moved my number for a couple of decades at least, but the last time I did you had to phone them and go through the whole "are you sure? Let me see if I can get you a better deal" nonsense.

Cancellation is different to porting a number. You'll still need to cancel your previous plan, but legislation fixed carriers holding numbers hostage a while ago.

No, when you port the number it automatically cancels the service.

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.

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

I don't think they actually are. Platform fees are capped at £250 and their default managed fund fees is 0.14% which is pretty normal. Also I deliberately want a platform that doesn't allow trading individual shares because otherwise I have to report those to my employer every quarter and I can't be bothered with that admin.

While Monzo are expensive their approach of rewarding loyalty (5% off per year for Monzo Mobile) is a good idea compared to the endless switching you have to do to get the best rates.

they don't offer a phone contract..? this makes no sense

Yes they do - https://monzo.com/mobile

Compared to Lebara or other cheap MVNOs their offering is not great. While they eventually give you 30% off (after 6 years) their starting price is 30% higher.

Same service with HSBC and Giffgaff here. Meh.

Yeah I seriously doubt that. I used to be an HSBC customer. Awful tech.

How long ago? Everything is instant self service now. I haven’t spoken to a human in ten years.

Almost seems like they can do anything at monzo.com

The only limit is themselves.

[2025]

I think this is what distributed systems has a phrase for: "graceful degradation".

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

An increasing risk for sure.

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

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

Won't help if they sanction you and cut you off from SWIFT (or the card networks) so it just depends on the degree of the governance issues here.

Most european banking entities already have and use SEPA.

Any alternative non US, Europe wide card network? I suppose you could use Mir, or UnionPay.

This is an issue, but one Europe is acutely aware of and working on. WERO is one strand of that.

https://www.reneweuropegroup.eu/news/2026-06-23/independence...

https://europeanbusinessmagazine.com/eu-payments-sovereignty...

https://stripe.com/en-fr/resources/more/wero-how-europes-uni...

Ah.. SWIFT is an international cooperative, originated in Europe and based in Brussels..

Monzo are a UK bank that briefly but afaik no longer operate in the US

They are partially Chinese owned, so you can imagine Trump having a fit could be ordering AWS and GCP to stop servicing this bank.

You have no choice really if you’re a UK business, there is no UK (or French, or German, or Japanese for that matter) cloud that can do what AWS or GCP does

Really? None other than Airbus chose Scaleway (a personal favourite) for their migration of core services off AWS. There's also Hetzner, OVH, NetCUPs etc. I'd agree they're not AWS/GCP scale (yet) but as more big names move for reasons of EU sovereignty that multiplies available capital and soon they become hyperscale themselves. And sovereign.

My understanding is OVH and Hetzner were traditionally more bare metal/VPS orientated (though maybe that's changed)?

Fair though, I hadn't even heard of Scaleway, and that does look a lot more like a direct AWS/GCP alternative. Maybe because I'm UK based that's why I never hear of the EU clouds

Monzo is nothing more than a science project

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