In a nutshell, Active-Active solution gets all the services on the user call path deployed across multiple AWS Regions — in this case US-East-1 in Virginia and US-West-2 in Oregon. In order to do so, several requirements must be satisfied

Services must be stateless — all data / state replication needs to handled in data tier.

They must access any resource locally in-Region. This includes resources like S3, SQS, etc. This means several applications that are publishing data into an S3 bucket, now have to publish the same data into multiple regional S3 buckets.

there should not be any cross-regional calls on user’s call path. Data replication should be asynchronous.

In a normal state of operation, users would be geo-DNS routed to the closest AWS Region, with a rough split of 50/50%. In the event of any significant region-wide outage, we have tools to override geo-DNS and direct all of users traffic to a healthy Region.