Create Rolling Index Builds on Sharded Clusters在分片集群上创建滚动索引构建

Unique Indexes唯一索引

To create unique indexes using the following procedure, you must stop all writes to the collection during this procedure.要使用以下过程创建唯一索引，您必须在此过程中停止对集合的所有写入。

If you cannot stop all writes to the collection during this procedure, do not use the procedure on this page. Instead, build your unique index on the collection by issuing db.collection.createIndex() on the mongos for a sharded cluster.如果在此过程中无法停止对集合的所有写入，请不要使用此页面上的过程。相反，通过在分片集群的mongos上发出db.collection.createIndex()，在集合上构建唯一索引。

Oplog Size操作日志大小

Ensure that your oplog is large enough to permit the indexing or re-indexing operation to complete without falling too far behind to catch up. See the oplog sizing documentation for additional information.确保oplog足够大，以允许索引或重新索引操作完成，而不会落后太多而无法赶上。有关更多信息，请参阅oplog大小文档。

Rolling index builds lower the resiliency of your cluster and increase build duration.滚动索引构建会降低集群的弹性，并增加构建持续时间。

A. Stop Migrations停止迁移

Connect mongosh to a mongos instance in the sharded cluster and disable migrations for the collection where you want to perform the rolling build index:将mongosh连接到分片集群中的mongos实例，并禁用要执行滚动构建索引的集合的迁移：

db.adminCommand(
  {
    setAllowMigrations: "<db>.<collection>",
    allowMigrations: false
  }
)

The preceding command ensures the correct set of shards is targeted for rolling index builds because no migration for the collection will be allowed to commit.前面的命令确保滚动索引构建的目标是正确的分片集，因为不允许提交集合的迁移。

If the command returns the following error, it means the collection is unsharded. You can safely ignore the error and continue with the next step.如果该命令返回以下错误，则表示该集合未被记录。您可以放心地忽略错误并继续下一步。

MongoServerError[NamespaceNotSharded]: Collection must be sharded so migrations can be blocked

B. Determine the Distribution of the Collection确定藏品的分布

To determine which shards must be involved in the rolling index build, run the following aggregation on the collection that you want to build the index on:要确定滚动索引构建中必须涉及哪些分片，请在要构建索引的集合上运行以下聚合：

db.getSiblingDB(<db>).getCollection(<collection>).aggregate([{$collStats:{}},{$group: {_id: "$ns", shard_list: {$addToSet: "$shard"}}}])

For example, if you want to create an index on the records collection in the test database:例如，如果要在test数据库中的records集合上创建索引：

db.getSiblingDB("test").getCollection("records").aggregate([{$collStats:{}},{$group: {_id: "$ns", shard_list: {$addToSet: "$shard"}}}])

[ { _id: 'test.records', shard_list: [ 'shardA', 'shardC' ] } ]

C. Build Indexes on the Shards That Contain Collection Chunks在包含集合块的分片上构建索引

For each shard that contains chunks for the collection, follow the procedure to build the index on the shard.对于包含集合块的每个分片，按照以下过程在分片上构建索引。

net:
   bindIp: localhost,<hostname(s)|ip address(es)>
   port: 27218
#   port: 27018
#replication:
#   replSetName: shardA
#sharding:
#   clusterRole: shardsvr
setParameter:
   skipShardingConfigurationChecks: true
   disableLogicalSessionCacheRefresh: true

mongod --config <path/To/ConfigFile>

mongod --port 27218 --setParameter skipShardingConfigurationChecks=true --setParameter disableLogicalSessionCacheRefresh=true

db.records.createIndex( { username: 1 } )

net:
   bindIp: localhost,<hostname(s)|ip address(es)>
   port: 27018
replication:
   replSetName: shardA
sharding:
   clusterRole: shardsvr

mongod --config <path/To/ConfigFile>

mongod --port 27018 --replSet shardA --shardsvr

D. Repeat for the Other Affected Shards对其他受影响的分片重复此操作

Once you finish building the index for a shard, repeat C. Build Indexes on the Shards That Contain Collection Chunks for the other affected shards.一旦你完成了一个分片的索引构建，重复C.为其他受影响的分片在包含集合块的分片上构建索引。

E. Enable Migrations启用迁移

Connect mongosh to a mongos instance in the sharded cluster and re-enable the migration with setAllowMigrations:将mongosh连接到分片集群中的mongos实例，并使用setAllowMigrations重新启用迁移：

db.adminCommand(
  {
    setAllowMigrations: "<db>.<collection>",
    allowMigrations: true
  }
)

If the command returns the following error, it means the collection is unsharded. You can safely ignore the error.如果该命令返回以下错误，则表示该集合未被记录。您可以放心地忽略该错误。

MongoServerError[NamespaceNotSharded]: Collection must be sharded so migrations can be blocked