convertToCapped

convertToCapped

Warning警告

Do Not Run This Command In Sharded Clusters不要在分片集群中运行此命令

MongoDB does not support the convertToCapped command in a sharded cluster.MongoDB在分片集群中不支持convertToCapped命令。

The convertToCapped command converts an existing, non-capped collection to a capped collection within the same database.convertToCapped命令将同一数据库中的现有非封顶集合转换为封顶集合。

The command has the following syntax:该命令具有以下语法：

{ convertToCapped: <collection>,
  size: <capped size>,
  writeConcern: <document>,
  comment: <any>
}

The command takes the following fields:该命令接受以下字段：

Field字段	Description描述
convertToCapped	The name of the existing collection to convert.要转换的现有集合的名称。
size	The maximum size, in bytes, for the capped collection.上限集合的最大大小（字节）。
writeConcern	Optional. 可选。A document expressing the write concern of the drop command. 表示drop命令的写入关注的文档。Omit to use the default write concern.忽略使用默认的写入关注。
comment	Optional. 可选。A user-provided comment to attach to this command. 用户提供了附加到此命令的注释。Once set, this comment appears alongside records of this command in the following locations:设置后，此注释将与此命令的记录一起显示在以下位置： mongod log messages, in the attr.command.cursor.comment field.mongod日志消息，在attr.command.cursor.comment字段中。 Database profiler output, in the command.comment field.数据库探查器输出，在command.comment字段中。 currentOp output, in the command.comment field.输出，在command.comment字段中。 A comment can be any valid BSON type(string, integer, object, array, etc).注释可以是任何有效的BSON类型（字符串、整数、对象、数组等）。 New in version 4.4.在版本4.4中新增。

convertToCapped takes an existing collection (<collection>) and transforms it into a capped collection with a maximum size in bytes, specified by the size argument (<capped size>).convertToCapped获取一个现有集合（<collection>），并将其转换为一个有上限的集合，该集合的最大大小由size参数（<caped size>）指定，以字节为单位。

During the conversion process, the convertToCapped command exhibits the following behavior:在转换过程中，convertToCapped命令显示以下行为：

• MongoDB traverses the documents in the original collection in natural order and loads the documents into a new capped collection.MongoDB按自然顺序遍历原始集合中的文档，并将文档加载到一个新的封顶集合中。
• If the capped size specified for the capped collection is smaller than the size of the original uncapped collection, then MongoDB will overwrite documents in the capped collection based on insertion order, or first in, first out order.如果为封顶集合指定的capped size小于原始未封顶集合的大小，则MongoDB将根据插入顺序或先入先出顺序覆盖封顶集合中的文档。

• Internally, to convert the collection, MongoDB uses the following procedure在内部，为了转换集合，MongoDB使用以下过程

  • cloneCollectionAsCapped command creates the capped collection and imports the data.命令创建封顶集合并导入数据。
  • MongoDB drops the original collection.MongoDB删除了原始集合。
  • renameCollection renames the new capped collection to the name of the original collection.将新的封顶集合重命名为原始集合的名称。
• This holds a database exclusive lock for the duration of the operation. 这将在操作期间保持数据库独占锁定。Other operations which lock the same database will be blocked until the operation completes. 锁定同一数据库的其他操作将被阻止，直到操作完成。See What locks are taken by some common client operations? for operations that lock the database.查看一些常见的客户端操作使用了哪些锁？用于锁定数据库的操作。

Warning警告

The convertToCapped will not recreate indexes from the original collection on the new collection, other than the index on the _id field. 将不会在新集合上从原始集合重新创建索引，_id字段上的索引除外。If you need indexes on this collection you will need to create these indexes after the conversion is complete.如果需要此集合上的索引，则需要在转换完成后创建这些索引。

db.events.insertOne( { click: 'button-1', time: new Date() } )
db.events.stats()

{
        "ns" : "test.events",
        ...
        "capped" : false,
        ...
}

db.runCommand( { convertToCapped: 'events', size: 8192 } )
db.events.stats()

{
     "ns" : "test.events",
     ...
     "capped" : true,
     "max" : NumberLong("9223372036854775807"),
     "maxSize" : 8192,
     ...
}