When the DDS is activated, the default structure includes 1 primary, 1 secondary and 1 hidden node. Users can configure the read policy by specifying readPreference after connecting to the DDS instance. Currently, readPreference supports the following policies:
primary: Default rule, all read requests are sent to the primary node.
primaryPreferred: Read the primary node first. If the primary node is unreachable, request the secondary node.
secondary: All read requests are sent to the secondary node.
secondaryPreferred: Read the secondary node first. If the secondary node is unreachable, request the primary node.
nearest: The client probes the network records of each node and sends read requests evenly to the nearest reachable node. By default, if the network distance between two nodes and the client is no more than 15ms, they are considered "equally close".