Cascading replication, introduced with LightDB 21, enables a standby server to replicate from another standby server rather than directly from the primary, meaning replication changes "cascade" down through a hierarchy of servers. This can be used to reduce load on the primary and minimize bandwith usage between sites. For more details, see the LightDB cascading replication documentation.

ltcluster supports cascading replication. When cloning a standby, set the command-line parameter --upstream-node-id to the node_id of the server the standby should connect to, and ltcluster will create recovery.conf to point to it. Note that if --upstream-node-id is not explicitly provided, ltcluster will set the standby's recovery.conf to point to the primary node.

To demonstrate cascading replication, first ensure you have a primary and standby set up as shown in the Quick-start guide. Then create an additional standby server with ltcluster.conf looking like this:

    node_id=3
    node_name=node3
    conninfo='host=node3 user=ltcluster dbname=ltcluster'
    data_directory='/var/lib/lightdb/data'

Clone this standby (using the connection parameters for the existing standby), ensuring --upstream-node-id is provide with the node_id of the previously created standby (if following the example, this will be 2):

    $ ltcluster -h node2 -U ltcluster -d ltcluster -f /etc/ltcluster.conf standby clone --upstream-node-id=2
    NOTICE: using configuration file "/etc/ltcluster.conf"
    NOTICE: destination directory "/var/lib/lightdb/data" provided
    INFO: connecting to upstream node
    INFO: connected to source node, checking its state
    NOTICE: checking for available walsenders on upstream node (2 required)
    INFO: sufficient walsenders available on upstream node (2 required)
    INFO: successfully connected to source node
    DETAIL: current installation size is 29 MB
    INFO: creating directory "/var/lib/lightdb/data"...
    NOTICE: starting backup (using lt_basebackup)...
    HINT: this may take some time; consider using the -c/--fast-checkpoint option
    INFO: executing: 'pg_basebackup -l "ltcluster base backup" -D /var/lib/lightdb/data -h node2 -U ltcluster -X stream '
    NOTICE: standby clone (using pg_basebackup) complete
    NOTICE: you can now start your LightDB server
    HINT: for example: lt_ctl -D /var/lib/lightdb/data start

then register it (note that --upstream-node-id must be provided here too):

     $ ltcluster -f /etc/ltcluster.conf standby register --upstream-node-id=2
     NOTICE: standby node "node2" (ID: 2) successfully registered
    

After starting the standby, the cluster will look like this, showing that node3 is attached to node2, not the primary (node1).

    $ ltcluster -f /etc/ltcluster.conf cluster show
     ID | Name  | Role    | Status    | Upstream | Location | Connection string
    ----+-------+---------+-----------+----------+----------+--------------------------------------
     1  | node1 | primary | * running |          | default  | host=node1 dbname=ltcluster user=ltcluster
     2  | node2 | standby |   running | node1    | default  | host=node2 dbname=ltcluster user=ltcluster
     3  | node3 | standby |   running | node2    | default  | host=node3 dbname=ltcluster user=ltcluster