Monzo's Stand-in: A Backup Banking Platform That Survives Full Cloud Outages

Monzo Stand-In

Monzo has built a completely separate backup banking infrastructure, Monzo Stand-in, to keep customers spending, transferring, and checking balances even during a full AWS outage. Running on GCP with just 18 microservices versus the primary platform's 3,000, it costs around 1% of the primary's cost. The system syncs data asynchronously, accepts eventual consistency, and uses 'Monzo Advices' to reconcile effects when the primary returns. Crucially, it runs different software to avoid the same failure affecting both platforms, and can be enabled even if the primary is completely down.