Sent by ns_server to the producer specifying the additional metadata required to set the vBucket state. The command can optionally accept a value, which encodes a JSON object to configure the vBucket.
The following keys can be include in the JSOn object.
topology- for setting a replication topology required for Synchronous Replication
topology should be set by ns_server to tell the producer memcached about a
given replication topology. This topology is used to determine which nodes
must ack DCP_PREPARES as part of Synchronous Replication to satisfy durable
writes. This field should only be sent to active vBuckets.
A topology consists of an array of replication chains. A replication chain
consists of a series of string values that correspond to the names of memcached
nodes. The first name in each replication chain should correspond to the active
node. A replication chain could have a null value instead of a string to
indicate that there is an undefined replica (i.e. we may have 1 replica
configured but a failover will cause that replica to become undefined or null).
A topology may contain up to two replication chains. The first chain is the
current replication topology. The second chain is used during the later stages
of rebalance to tell memcached about the post-rebalance topology so that
memcached can ensure durable writes are satisfied with the new topology too.
For example:
A replication topology with a single chain with active and one replica
{
"topology" : [[<active_name>, <replica_name>]]
}
A replication topology with a single chain with active and one undefined replica
{
"topology" : [[<active_name>, null]]
}
A replication topology with two chains
{
"topology" : [[<active_name>, <replica_name>],[<new_active_name>, <new_replica_name>]]
}