笔记-capped collection

1.      collection

1.1.    简介

集合分为固定与非固定collection，capped collection

1.1.1.   capped collections

Capped collections are fixed-size collections that support high-throughput operations that insert and retrieve documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.

Capped collections 是固定大小的collection。它支持高速吞吐，这是基于插入顺序实现的。它的工作原理类似于循环缓冲区，一旦接近预定的存储空间限制，它会覆盖集合中最老的文档。

1.2.    CURD

1.2.1.   create

capped colletions 需要显示创建；使用createcollection方法

db.createCollection(<name>, { capped: <boolean>,

autoIndexId: <boolean>,

size: <number>,

max: <number>,

storageEngine: <document>,

validator: <document>,

validationLevel: <string>,

validationAction: <string>,

indexOptionDefaults: <document>,

viewOn: <string>,

pipeline: <pipeline>,

collation: <document>,

writeConcern: <document>} )

参数释义：

capped：

size:文档空间限制，单位是bytes；只对capped collection生效

max:用于限制文档数，使用时需注意与size的配合。

下面是一些案例：

db.createCollection( "log", { capped: true, size: 100000 } )

db.createCollection("log", { capped : true, size : 5242880, max : 5000 } )

1.2.2.   Updates

If you plan to update documents in a capped collection, create an index so that these update operations do not require a collection scan.

1.2.3.   delete

You cannot delete documents from a capped collection. To remove all documents from a collection, use the drop() method to drop the collection and recreate the capped collection.

capped collection不支持删除文档操作，清除所有文档的方法是drop()集合然后重新创建。

1.2.4.   document size

如果更新或替换操作会导致文档大小改变，操作会失败。

If an update or a replacement operation changes the document size, the operation will fail.

1.3.    其它操作

检查是否为capped集合

db.collection.isCapped()

转换普通集合为限制集合

db.runCommand({"convertToCapped": "mycoll", size: 100000});