Database Reference
In-Depth Information
The preceding diagram shows you a replica set with four members: the primary member,
two
secondary
nodes with priority 1, and a
secondary
node that is hidden with
primary 0.
The last secondary member will be hidden from clients, and they can't route their read op-
erations to this node.
The delayed member
The delayed secondary member is a hidden and obviously a priority 0 member. Actually,
it is a subset of hidden members.
A delayed member's dataset is the same as the
primary
node but with a specified delay.
For instance, if the member syncs itself with a
primary
node now, if the current time is
10:00 A.M., and the member has a delay of one hour (or 3600 seconds), this member will
sync the dataset with the
primary
node again will be at 11:00 A.M. In other words, a
delayed member copies and applies
oplog
with a specific delay.
This behavior comes in handy when you want to keep an older version of the dataset to
prevent human errors, software upgrade issues, or invalid data entries.
However, a delayed member can vote in an election process, as shown in the following
diagram:
