AWS identified two core use cases for the new Global Clusters feature: disaster recovery, for ensuring that the database stays live in the event of a regional outage, and low-latency reads across different local regions scattered across the globe. The Global Clusters feature expands on DocumentDB’s existing active-passive replication capability, where change events are replicated from the primary instance to read-only secondary instances. Until now, DocumentDB supported replication to a maximum of 15 replicas across three availability zones (AZs), within the same region. With Global Clusters, you can now spread the deployment across up to five secondary regions (the home region remains the primary), with up to 16 replicas in each secondary region. Like Aurora, DocumentDB uses storage-based replication to replicate data across regions. Document DB’s architecture has a unique advantage when it comes to replication processes. The task is handled by the storage volume, which departs from the traditional practice of running it from the compute node. As a result, there won’t be any resource contention between resources such as CPU and memory that otherwise add overhead to applications. In the MongoDB world, distributed capabilities have widely varied. MongoDB’s own Atlas cloud service started supporting read-only replication (like the new DocumentDB feature) a year after it was launched. MongoDB itself (on-premises and in the cloud) also has a limited distributed write capability that designates primaries at the shard level, meaning that different slices of the database control write capabilities for the portions of the data that they maintain. This capability is useful when data sovereignty policies require that specific records be stored and/or updated only within the country of origin. By contrast, Microsoft Azure Cosmos DB, a multimodel database that has a MongoDB-compatible API (like DocumentDB), supports fully distributed read/write capabilities. In a blog post that just went live, AWS claims that updates from the primary to read replicas are typically executed within a second. DocumentDB Global Clusters is available now.
