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.. _replica-set-secondary-only-members:
==============================
Priority 0 Replica Set Members
==============================
.. default-domain:: mongodb
.. contents:: On this page
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:class: singlecol
A :rsconf:`priority 0 <members[n].priority>` member is a member that
**cannot** become :term:`primary` and **cannot** trigger
:term:`elections <election>`. Priority 0 members can acknowledge write
operations issued with :ref:`write concern <write-concern>` of
``w : <number>``. For ``"majority"`` write concern, the priority 0
member must also be a voting member (i.e. :rsconf:`members[n].votes` is
greater than ``0``) to acknowledge the write. Non-voting replica set
members (i.e. :rsconf:`members[n].votes` is ``0``) cannot contribute to
acknowledging write operations with ``"majority"`` write concern.
Other than the aforementioned restrictions, secondaries that have
:rsconf:`priority 0 <members[n].priority>` function as normal
secondaries: they maintain a copy of the data set, accept read
operations, and vote in elections.
Configure a secondary to have *priority 0* to prevent it from
becoming primary, which is particularly useful in multi-data center
deployments.
For example, in the following diagram, one data center hosts the
primary and a secondary. A second data center hosts a secondary with
*priority 0*. Only the members in data center 1 can become primary.
.. include:: /images/replica-set-three-members-geographically-distributed.rst
Priority 0 Members as Standbys
------------------------------
A secondary with :rsconf:`priority 0 <members[n].priority>` can
function as a standby. In some replica sets, it might not be possible
to add a new member in a reasonable amount of time. A standby member
keeps a current copy of the data to be able to replace an unavailable
member.
In many cases, you need not set standby to *priority 0*. However, in replica
sets with varied hardware or :ref:`geographic distribution
<replica-set-geographical-distribution>`, a *priority 0* standby
ensures that only certain members become primary.
A *priority 0* standby may also be valuable for some members of a set
with different hardware or workload profiles. In these cases, deploy a
member with *priority 0* so it can't become primary. Also consider
using an :ref:`hidden member <replica-set-hidden-members>` for this
purpose.
If your set already has seven voting members, also configure the
member as :ref:`non-voting <replica-set-non-voting-members>`.
Failover Considerations
-----------------------
When configuring a secondary to have :rsconf:`priority 0
<members[n].priority>`, consider potential failover patterns, including
all possible network partitions. Always ensure that your main data
center contains both a quorum of voting members and members that are
eligible to be primary.
Example
-------
To configure a secondary to have :rsconf:`priority 0
<members[n].priority>`, see
:doc:`/tutorial/configure-secondary-only-replica-set-member`.
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