======
Shards
======

.. default-domain:: mongodb

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A :term:`shard` contains a subset of sharded data for a :term:`sharded
cluster`. Together, the cluster's shards hold the entire data set for the
cluster.

As of MongoDB 3.6, shards must be deployed as a :term:`replica set` to
provide redundancy and high availability.

Users, clients, or applications should only directly connect to a shard to
perform local administrative and maintenance operations.

Performing queries on a single shard only returns a subset of data. Connect to
the :binary:`~bin.mongos` to perform cluster level operations, including read or
write operations.

.. important::

   MongoDB does not guarantee that any two contiguous :term:`chunks<chunk>`
   reside on a single shard.

.. _primary-shard:

Primary Shard
-------------

Each database in a sharded cluster has a :term:`primary shard` that holds all
the un-sharded collections for that database. Each database has its own
primary shard. The primary shard has no relation to the :term:`primary` in a
replica set.

The :binary:`~bin.mongos` selects the primary shard when creating a new database
by picking the shard in the cluster that has the least amount of data.
:binary:`~bin.mongos` uses the ``totalSize`` field returned by the
:dbcommand:`listDatabase` command as a part of the selection criteria.

.. include:: /images/sharded-cluster-primary-shard.rst

To change the primary shard for a database, use the :dbcommand:`movePrimary`
command. The process of migrating the primary shard may take significant time
to complete, and you should not access the collections associated to the
database until it completes. Depending on the amount of data being migrated,
the migration may affect overall cluster operations. Consider the impact to
cluster operations and network load before attempting to change the primary
shard.

When you deploy a new :term:`sharded cluster` with shards that were
previously used as replica sets, all existing databases
continue to reside on their original replica sets. Databases created
subsequently may reside on any shard in the cluster.

Shard Status
------------

Use the :method:`sh.status()` method in the :binary:`~bin.mongo` shell to
see an overview of the cluster. This reports includes which shard is
primary for the database and the :term:`chunk` distribution across the
shards. See :method:`sh.status()` method for more details.

Sharded Cluster Security
------------------------

Use :doc:`/core/security-internal-authentication` to enforce intra-cluster
security and prevent unauthorized cluster components from accessing the
cluster. You must start each :binary:`~bin.mongod` in the cluster with the
appropriate security settings in order to enforce internal authentication.

See :doc:`/tutorial/deploy-sharded-cluster-with-keyfile-access-control` for a
tutorial on deploying a secured sharded cluster.

Shard Local Users
~~~~~~~~~~~~~~~~~

Each shard supports :doc:`/core/authorization` *(RBAC)* for restricting
unauthorized access to shard data and operations. Start each :binary:`~bin.mongod`
in the replica set with the :option:`--auth <mongod --auth>` option to enforce RBAC.
Alternatively, enforcing :doc:`/core/security-internal-authentication` for
intra-cluster security also enables user access controls via RBAC.

Each shard has its own shard-local users. These users cannot be used
on other shards, nor can they be used for connecting to the cluster
via a :binary:`~bin.mongos`.

See :doc:`/tutorial/enable-authentication` for a tutorial on enabling
adding users to an RBAC-enabled MongoDB deployment.
