Let's see how the write operation works in the Couchbase cluster. When a write command is issued using the set operation in the Couchbase cluster, the server immediately responds once the document is written to the memory of that particular node. How do clients know which nodes in the cluster will be responsible for storing the document? You might recall that every operation requires a document ID, using this document ID, the hash algorithm determines the vBucket in which it belongs. Then, this vBucket is used to determine the node that will store the document. All mapping information, vBucket to node, is stored in each of the Couchbase client SDKs, which form the cluster map. The Couchbase client library polls cluster to update the cluster map information all the time.

After the node has responded to the write operation, the Couchbase node puts a copy of the document in the disk queue to flush the document to the disk for durability...