/
replica-set-delayed-member.txt
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/
replica-set-delayed-member.txt
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.. index:: replica set members; delayed
.. _replica-set-delayed-configuration:
.. _replica-set-delayed-members:
.. _replica-set-delayed-replication:
===========================
Delayed Replica Set Members
===========================
.. default-domain:: mongodb
.. contents:: On this page
:local:
:backlinks: none
:depth: 1
:class: singlecol
Delayed members contain copies of a :term:`replica set's <replica
set>` data 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** :ref:`priority 0 <replica-set-secondary-only-members>`
members. Set the priority to 0 to prevent a delayed member from
becoming primary.
- **Should be** :ref:`hidden <replica-set-hidden-members>`
members. Always prevent applications from seeing and querying
delayed members.
- *do* vote in :term:`elections <election>` for primary.
Behavior
~~~~~~~~
Delayed members apply operations from the :term:`oplog` on a delay.
When choosing the amount of delay, consider that the amount of delay:
- must be is equal to or greater than your maintenance windows.
- must be *smaller* than the capacity of the oplog. For more
information on oplog size, see :ref:`replica-set-oplog-sizing`.
Sharding
~~~~~~~~
In sharded clusters, delayed members have limited utility when the
:term:`balancer` is 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, or an hour. This delayed member is also
*hidden* and is a *priority 0 member*.
.. include:: /images/replica-set-delayed-member.rst
Configuration
-------------
A delayed member has its
:data:`~replSetGetConfig.members[n].priority` equal to ``0``,
:data:`~replSetGetConfig.members[n].hidden` equal to ``true``, and
its :data:`~replSetGetConfig.members[n].slaveDelay` equal to the
number of seconds of delay:
.. code-block:: javascript
{
"_id" : <num>,
"host" : <hostname:port>,
"priority" : 0,
"slaveDelay" : <seconds>,
"hidden" : true
}
To configure a delayed member, see
:doc:`/tutorial/configure-a-delayed-replica-set-member`.