A shard contains a subset of sharded data for a sharded cluster. Together, the cluster's shards hold the entire data set for the cluster.分片包含分片集群的分片数据子集。集群的分片共同保存了集群的整个数据集。
Shards must be deployed as a replica set to provide redundancy and high availability.分片必须作为副本集部署，以提供冗余和高可用性。
Important
Sharded clusters use the write concern 分片集群在许多内部操作中使用写关注"majority" for a lot of internal operations. Using an arbiter in a sharded cluster is discouraged due to Performance Issues with PSA replica sets."majority"。由于PSA副本集的性能问题，不建议在分片集群中使用仲裁器。
Warning
Typically, do not perform operations directly on a shard because they might cause data corruption or data loss.通常，不要直接在分片上执行操作，因为它们可能会导致数据损坏或数据丢失。 Users, clients, or applications should only directly connect to a shard to perform local administrative or maintenance operations.用户、客户端或应用程序应仅直接连接到分片以执行本地管理或维护操作。
Performing queries on a single shard only returns a subset of data. 对单个分片执行查询只返回数据的子集。Connect to the 连接到mongos to perform cluster level operations, including read or write operations.mongos以执行集群级操作，包括读取或写入操作。
Important
Primary Shard主分片
Each database in a sharded cluster has a primary shard. It is the default shard for all unsharded collections in the database. All unsharded collections for a database are created on the database primary shard by default. 分片集群中的每个数据库都有一个主分片。它是数据库中所有未分片集合的默认分片。默认情况下，数据库的所有未分片集合都是在数据库主分片上创建的。Starting in MongoDB 8.0, you can move unsharded collections to another shard using 从MongoDB 8.0开始，您可以使用moveCollection. The primary shard has no relation to the primary in a replica set.moveCollection将未分片的集合移动到另一个分片。主分片与副本集中的主分片没有关系。
The mongos selects the primary shard when creating a new database by picking the shard in the cluster that has the least amount of data. mongos在创建新数据库时，通过选择集群中数据量最少的分片来选择主分片。mongos uses the totalSize field returned by the listDatabases command as a part of the selection criteria.mongos使用listDatabases命令返回的totalSize字段作为选择条件的一部分。
To change the primary shard for a database, use the 要更改数据库的主分片，请使用movePrimary command. movePrimary命令。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. 迁移主分片的过程可能需要很长时间才能完成，在完成之前，您不应该访问与数据库关联的集合。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.根据迁移的数据量，迁移可能会影响整个集群操作。在尝试更改主分片之前，请考虑对集群操作和网络负载的影响。
When you deploy a new sharded cluster with shards that were previously used as replica sets, all existing databases continue to reside on their original replica sets. 当您使用以前用作副本集的分片部署新的分片集群时，所有现有数据库继续驻留在其原始副本集上。Databases created subsequently may reside on any shard in the cluster.随后创建的数据库可以驻留在集群中的任何分片上。
Shard Status分片状态
Use the 使用sh.status() method in mongosh to see an overview of the cluster. mongosh中的sh.status()方法查看集群的概述。This reports includes which shard is primary for the database and the chunk distribution across the shards. 此报告包括哪个分片是数据库的主分片以及分片之间的块分布。See 有关更多详细信息，请参阅sh.status() method for more details.sh.status()方法。
Sharded Cluster Security分片群集安全
Use Self-Managed Internal/Membership Authentication to enforce intra-cluster security and prevent unauthorized cluster components from accessing the cluster. 使用自我管理的内部/成员身份验证来强制实施集群内安全，并防止未经授权的集群组件访问集群。You must start each 您必须使用适当的安全设置启动集群中的每个mongod in the cluster with the appropriate security settings in order to enforce internal authentication.mongod，以强制执行内部身份验证。
Starting in MongoDB 5.3, SCRAM-SHA-1 cannot be used for intra-cluster authentication. 从MongoDB 5.3开始，SCRAM-SHA-1不能用于集群内身份验证。Only SCRAM-SHA-256 is supported.仅支持SCRAM-SHA-256。
In previous MongoDB versions, SCRAM-SHA-1 and SCRAM-SHA-256 can both be used for intra-cluster authentication, even if SCRAM is not explicitly enabled.在以前的MongoDB版本中，即使没有明确启用SCRAM，SCRAM-SHA-1和SCRAM-SHA-256都可以用于集群内身份验证。
See Deploy Self-Managed Sharded Cluster with Keyfile Authentication for a tutorial on deploying a secured sharded cluster.有关部署安全分片集群的教程，请参阅部署具有键文件身份验证的自我管理分片集群。
Shard Local Users分片化本地用户
Each shard supports Role-Based Access Control in Self-Managed Deployments (RBAC) for restricting unauthorized access to shard data and operations. 每个分片都支持自我管理部署（RBAC）中的基于角色的访问控制，以限制对分片数据和操作的未经授权的访问。Start each 使用mongod in the replica set with the --auth option to enforce RBAC. --auth选项启动副本集中的每个mongod以强制RBAC。Alternatively, enforcing Self-Managed Internal/Membership Authentication for intra-cluster security also enables user access controls via RBAC.或者，为集群内安全实施自我管理的内部/成员身份验证也可以通过RBAC实现用户访问控制。
Starting in MongoDB 5.3, SCRAM-SHA-1 cannot be used for intra-cluster authentication. 从MongoDB 5.3开始，SCRAM-SHA-1不能用于集群内身份验证。Only SCRAM-SHA-256 is supported.仅支持SCRAM-SHA-256。
In previous MongoDB versions, SCRAM-SHA-1 and SCRAM-SHA-256 can both be used for intra-cluster authentication, even if SCRAM is not explicitly enabled.在以前的MongoDB版本中，即使没有明确启用SCRAM，SCRAM-SHA-1和SCRAM-SHA-256都可以用于集群内身份验证。
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 每个分片都有自己的分片本地用户。这些用户不能在其他分片上使用，也不能通过mongos.mongos连接到集群。
See Enable Access Control on Self-Managed Deployments for a tutorial on enabling adding users to an RBAC-enabled MongoDB deployment.有关启用向启用RBAC的MongoDB部署添加用户的教程，请参阅在自我管理部署上启用访问控制。