Database / Google Spanner Database Interview questions
Which is better and why: multi-region or regional Spanner configuration for a global app?
Neither is universally better; the right choice depends on which failure mode and latency profile the application can least tolerate.
| Regional | Multi-region |
| Lower write latency - all replicas are close together. | Higher write latency due to cross-region quorum. |
| 99.99% availability SLA. | 99.999% availability SLA, survives a full region outage. |
| Reads are fast but only from one geography. | Reads can be served near users in multiple geographies. |
A regional configuration is the better fit when the user base and write-heavy traffic are concentrated in one geography and the priority is minimizing write latency - most single-market SaaS backends fall here. A multi-region configuration is worth its added write latency and cost when the application genuinely serves users across continents and needs to survive a full regional outage without downtime, such as global payment or ad-serving systems where an extra region-level nine of availability matters more than shaving milliseconds off each write. Teams sometimes get this wrong by defaulting to multi-region "for safety" on an app that's actually single-region in practice, paying a real latency tax for availability they don't need yet.
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