In many cases, we want to have a node that holds a copy of our data at an earlier point in time. This helps recover from a big subset of human errors like accidentally dropping a collection or an upgrade going horrendously wrong.
A delayed replica set member has to be priority 0 and hidden = true.
A delayed replica set member can vote for elections but will never be visible to clients (hidden = true) and will never become a primary (priority 0).
Following on from the preceding examples:
> cfg = rs.conf()
> cfg.members[0].priority = 0
> cfg.members[0].hidden = true
> cfg.members[0].slaveDelay = 7200
> rs.reconfig(cfg)
This will set the member[0] to a delay of 2 hours. Two important factors for deciding the delta time period between the primary and delayed secondary server are:
- Enough oplog size on the primary
- Enough time for maintenance to finish before the delayed member starts picking up data
| Maintenance window in hours | Delay | Oplog size on primary in hours |
| 0.5 | [0.5,5) | 5 |