Your web application does not need to directly handle any information about where a document resides; Couchbase SDKs automatically retrieve updates from Couchbase Server about the location of items in a cluster. Multiple web application instances can access the same item at the same time using Couchbase SDKs.
How an SDK gets updates on cluster topology is a slightly more advanced topic and is mainly relevant for those developers who want to create their own Couchbase SDK. For instance developers who want to create a Couchbase SDK for a language not yet supported would be interested in this topic. For more information, see, Section 8.2, “Getting Cluster Topology”.