Delayed Replica Set Members
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Delayed members contain copies of areplica set’sdata set. However, a delayed member’s data set reflects an earlier, or delayed, state of the set. For example, if the current time is 09:52 and a member has a delay of an hour, the delayed member has no operation more recent than 08:52.
Because delayed members are a “rolling backup” or a running “historical” snapshot of the data set, they may help you recover from various kinds of human error. For example, a delayed member can make it possible to recover from unsuccessful application upgrades and operator errors including dropped databases and collections.
Considerations
Requirements
Delayed members:
- Must be priority 0 members. Set the priority to 0 to prevent a delayed member from becoming primary.
- Should be hidden members. Always prevent applications from seeing and querying delayed members.
- do
vote in
elections
for primary, if
members[n].votesis set to 1.
Behavior
Delayed members copy and apply operations from the sourceoplogon a delay. When choosing the amount of delay, consider that the amount of delay:
- must be equal to or greater than your expected maintenance window durations.
- must be smaller than the capacity of the oplog. For more information on oplog size, see Oplog Size .
Sharding
In sharded clusters, delayed members have limited utility when thebalanceris enabled. Because delayed members replicate chunk migrations with a delay, the state of delayed members in a sharded cluster are not useful for recovering to a previous state of the sharded cluster if any migrations occur during the delay window.
Example
In the following 5-member replica set, the primary and all secondaries have copies of the data set. One member applies operations with a delay of 3600 seconds (one hour). This delayed member is alsohidden_and is a_priority 0 member.
Configuration
A delayed member has itsmembers[n].priorityequal to0,members[n].hiddenequal totrue, and itsmembers[n].slaveDelayequal to the number of seconds of delay:
{
"_id" : <num>,
"host" : <hostname:port>,
"priority" : 0,
"slaveDelay" : <seconds>,
"hidden" : true
}
To configure a delayed member, seeConfigure a Delayed Replica Set Member.