Compute-layer request routing

With compute-layer request routing, the code that runs in the compute layer determines whether to process the request locally or pass it to a copy of itself that’s running in another Region. When you use the write to one Region mode, the compute layer might detect that it’s not the active Region and allow local read operations while forwarding all write operations to another Region. This compute layer code must be aware of data topology and routing rules, and enforce them reliably, based on the latest settings that specify which Regions are active for which data. The outer software stack within the Region doesn’t have to be aware of how read and write requests are routed by the microservice. In a robust design, the receiving Region validates whether it is the current primary for the write operation. If it isn’t, it generates an error that indicates that the global state needs to be corrected. The receiving Region might also buffer the write operation for a while if the primary Region is in the process of changing. In all cases, the compute stack in a Region writes only to its local DynamoDB endpoint, but the compute stacks might communicate with one another.

The Vanguard Group uses a system called Global Orchestration and Status Tool (GOaST) and a library called Global Multi-Region library (GMRlib) for this routing process, as presented at re:Invent 2022. They use a follow-the-sun single primary model. GOaST maintains the global state, similar to the ARC routing control discussed in the previous section. It uses a global table to track which Region is the primary Region and when the next primary switch is scheduled. All read and write operations go through GMRlib, which coordinates with GOaST. GMRlib allows read operations to be performed locally, at low latency. For write operations, GMRlib checks if the local Region is the current primary Region. If so, the write operation completes directly. If not, GMRlib forwards the write task to the GMRlib in the primary Region. That receiving library confirms that it also considers itself the primary Region and raises an error if it isn’t, which indicates a propagation delay with the global state. This approach provides a validation benefit by not writing directly to a remote DynamoDB endpoint.