How would you balance traffic between multiple clouds? What technologies would you use to build a global load balancer?

Balancing traffic between multiple clouds is a complex task, as it requires considering geography, fault tolerance, performance, and cost.

For a global load balancer, the following approaches and technologies can be used:

• DNS-based balancing with geo-targeting — for example, Amazon Route 53, Google Cloud DNS, Cloudflare DNS. They direct users to the nearest or least loaded region.

• Anycast IP — a technology where one IP address is announced from multiple data centers, and traffic automatically goes to the nearest node.

• Global load balancers — for example, Google Cloud Global Load Balancer, AWS Global Accelerator, which can balance traffic at L4/L7 level between regions and clouds.

• Using CDN — for static content and caching, to reduce load on backends.

• Custom solutions based on BGP and SD-WAN — for more granular control of routing between clouds.

Ultimately, I would build a global balancer based on DNS with geo-targeting and Anycast, supplementing it with cloud-based global load balancers and CDN. It is also important to monitor performance and automatically switch traffic in case of failures.