# Zones, Regions and Disaster Recovery — Scalability, Availability & Reliability

Source: https://www.skillbyai.com/en/scalability/a-regions

> Choose a deployment footprint and DR strategy from RTO and RPO targets.

## How far apart should copies be?

Cloud **availability zones** are separate datacentres within a region with independent power and networking; spreading across zones protects against a datacentre-level failure with low latency between copies, and is the standard baseline for production. **Multi-region** deployments survive the loss of a whole region but are much harder: cross-region latency, data replication, consistency and cost. Two numbers drive the design. **RTO (Recovery Time Objective)**: how long the service may be down. **RPO (Recovery Point Objective)**: how much recent data may be lost. Common **disaster recovery** strategies, from cheapest to most expensive: **backup and restore** (hours of RTO, RPO equal to backup interval), **pilot light** (core data replicated, minimal infrastructure ready to scale up), **warm standby** (a scaled-down full copy running), and **multi-site active-active** (near-zero RTO and RPO, highest cost and complexity).

## Matching DR strategy to targets

Pick the cheapest option that meets the business's RTO and RPO.

```text
strategy               typical RTO      typical RPO        relative cost
---------------------  ---------------  -----------------  -------------
backup and restore     hours to a day   hours (last backup)   $
pilot light            tens of minutes  minutes              $$
warm standby           minutes          seconds to minutes   $$$
active-active          near zero        near zero            $$$$

ask the business: "If the region vanished now, how long can we be down,
                  and how much data can we lose?"  -> that sets RTO / RPO
```

## Spare keys

A spare key in your drawer (zone redundancy) helps when you lose one key at home. A spare key with a relative in another city (multi-region) helps when the whole house is inaccessible, but it takes longer to fetch and you must remember to update it when you change the lock.

**Quiz:** The business can tolerate 4 hours of downtime and losing up to 24 hours of data for an internal tool. Which DR strategy is most appropriate?

- [ ] Multi-site active-active
- [ ] Warm standby in three regions
- [ ] No backups at all
- [x] Backup and restore

*Answer:* Backup and restore. Generous RTO and RPO targets are met cheaply by backup and restore.