|Managing Your Data in GemFire XD / Replicating Tables|
GemFire XD replicates every single row of a replicated table synchronously to each table replica in the target server group(s). With synchronous replication, table reads can be evenly balanced to any replica in the cluster, with no single member becoming a contention point.
GemFire XD replicates data to all peers in the server groups where the table was created. Replication occurs in parallel to other peers over a TCP channel.
GemFire XD replicates table data both when a new table is initialized in a cluster and when replicated tables are updated.
When a non-persistent ("memory only") replicated table is created in a peer or server, it initializes itself using data from another member of the server group that hosts the table. A single data store member in the server group is chosen, and data for the table is streamed from that member to initialize the new replica. If the selected member fails during the replication process, the initializing process selects a different member in the server group to stream the data.
When an application updates a replicated table, GemFire XD distributes the update to all replicas in parallel, utilizing the network bandwidth between individual members of the cluster. The sending peer or server locks the updated row locally, and then distributes the update to the replicas. After each of the replicas processes the update and responds with an acknowledgment message, the originating GemFire XD peer returns control to the application. The update process is conservative in order to ensure that all copies of a replicated table contain consistent data. Each receiver processes the update entirely, applying constraint checks if necessary and updating the local table data, before responding with an acknowledgment. Each operation on a single row key is performed atomically per replica, regardless of how many columns are being read or updated.