replica-set-hidden-member.txt

.. index:: replica set members; hidden
.. _replica-set-hidden-configuration:
.. _replica-set-hidden-members:

==========================
Hidden Replica Set Members
==========================

.. default-domain:: mongodb

.. contents:: On this page
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A hidden member maintains a copy of the :term:`primary's <primary>`
data set but is **invisible** to client applications. Hidden members
are good for workloads with different usage patterns from the other
members in the :term:`replica set`. Hidden members must always be
:ref:`priority 0 members <replica-set-secondary-only-members>` and
so **cannot become primary**. The :method:`db.hello()` method does not
display hidden members. Hidden members, however, **may vote** in
:ref:`elections <replica-set-elections>`.

In the following five-member replica set, all four secondary members
have copies of the primary's data set, but one of the secondary members
is hidden.

.. include:: /images/replica-set-hidden-member.rst

Behavior
--------

Read Operations
~~~~~~~~~~~~~~~

Clients will not distribute reads with the appropriate :doc:`read
preference </core/read-preference>` to hidden members. As a result, these
members receive no traffic other than basic replication. Use hidden
members for dedicated tasks such as reporting and
backups.

.. include:: /includes/important-delayed-replica-set-members.rst


In a sharded cluster, :binary:`~bin.mongos` do not interact with hidden
members.

Voting
~~~~~~

Hidden members *may* vote in replica set elections. If you stop a
voting hidden member, ensure that the set has an active majority or the
:term:`primary` will step down. 

For the purposes of backups,

- .. include:: /includes/extracts/wt-fsync-lock-compatibility.rst

Write Concern
~~~~~~~~~~~~~

Hidden replica set members can acknowledge write operations issued
with :writeconcern:`w: \<number\> <\<number\>>`. For write operations
isued with :writeconcern:`w : "majority" <"majority">`, however,
hidden members must also be voting members (i.e. :rsconf:`members[n].votes`
greater than ``0``) to acknowledge the ``"majority"`` write operation.
Non-voting replica set members (i.e. :rsconf:`members[n].votes` 
is ``0``) cannot contribute to acknowledging write operations with 
``majority`` write concern.

Further Reading
---------------

For more information about backing up MongoDB databases,
see :doc:`/core/backups`. To configure a hidden member, see
:doc:`/tutorial/configure-a-hidden-replica-set-member`.