Client-driven request routing - AWS Prescriptive Guidance

Client-driven request routing

With client-driven request routing, the end user client (an application, a web page with JavaScript, or another client) keeps track of the valid application endpoints (for example, an Amazon API Gateway endpoint rather than a literal DynamoDB endpoint) and uses its own embedded logic to choose the Region to communicate with. It might choose based on random selection, lowest observed latencies, highest observed bandwidth measurements, or locally performed health checks.

        Client-driven request routing

As an advantage, client-driven request routing can adapt to things such as real-world public internet traffic conditions to switch Regions if it notices any degraded performance. The client must be aware of all potential endpoints, but launching a new Regional endpoint is not a frequent occurrence.

With write to any Region mode, a client can unilaterally select its preferred endpoint. If its access to one Region becomes impaired, the client can route to another endpoint.

With write to one Region mode, the client needs a mechanism to route its write requests to the currently active Region. This could be a basic mechanism, such as empirically testing which Region is presently accepting write requests (noting any write rejections and falling back to an alternate). Or it can be a complex mechanism, such as using a global coordinator to query for the current application state (perhaps built on the Amazon Route 53 Application Recovery Controller (ARC) routing control, which provides a five-Region, quorum-driven system to maintain global state for needs such as this). The client can decide if read requests can go to any Region for eventual consistency or must be routed to the active Region for strong consistency.

With the write to your Region mode, the client needs to determine the home Region for the dataset it’s working with. For example, if the client corresponds to a user account and each user account is homed to a Region, the client can request the appropriate endpoint assignment to use with its credentials from a global login system.

For example, a financial services company that helps users manage their business finances through the web uses global tables with a write to your Region mode. Each user must log in to a central service. That service returns credentials as well as the endpoint for the Region where those credentials will work. The Region that’s returned is based on where the user’s dataset is currently homed. The credentials are valid for a short time. After that, the webpage auto-negotiates a new login, which provides an opportunity to potentially redirect the user’s activity to a new Region.