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 this actually works is a slightly more advanced topic and is mainly relevant for those developers who want to create their own Couchbase SDK. This topic is more relevant for developers who want to create a Couchbase SDK for a language that is not yet supported. For more information, see, Section 7.2.2, “Handling vBucketMap Information”