Mongodb——GridFS

本文内容纲要：

GridFS用于存储和恢复那些超过16M（BSON文件限制）的文件。

GridFS将文件分成大块，将每个大块存储为单独的文件.GridFS中限制chunk最大为256k。GridFS使用两个collection存储，一个存储chunks，一个存储元数据(metadata)。

fs.files和fs.chunks

WhenshouldIuseGridFS? http://docs.mongodb.org/manual/faq/developers/#faq-developers-when-to-use-gridfs

fileCollection

：具体形式如下

{

"_id":,

"length":,

"chunkSize":

"uploadDate":

"md5":

"filename":,

"contentType":,

"aliases":,

"metadata":,

}

Documentsinthefilescollectioncontainsomeorallofthefollowingfields.Applicationsmaycreateadditionalarbitraryfields:

files._id

TheuniqueIDforthisdocument.The_idisofthedatatypeyouchosefortheoriginaldocument.ThedefaulttypeforMongoDBdocumentsisBSONObjectID.

files.length

Thesizeofthedocumentinbytes.

files.chunkSize

Thesizeofeachchunk.GridFSdividesthedocumentintochunksofthesizespecifiedhere.Thedefaultsizeis256kilobytes.

files.uploadDate

ThedatethedocumentwasfirststoredbyGridFS.ThisvaluehastheDatetype.

files.md5

AnMD5hashreturnedfromthefilemd5API.ThisvaluehastheStringtype.

files.filename

Optional.Ahuman-readablenameforthedocument.

files.contentType

Optional.AvalidMIMEtypeforthedocument.

files.aliases

Optional.Anarrayofaliasstrings.

files.metadata

Optional.Anyadditionalinformationyouwanttostore.

ThechunksCollection

：举例如下

{

"_id":,

"files_id":,

"n":,

"data":

}

Adocumentfromthechunkscollectioncontainsthefollowingfields:

chunks._id

TheuniqueObjectIDofthechunk.

chunks.files_id

The_idofthe“parent”document,asspecifiedinthefilescollection.

chunks.n

Thesequencenumberofthechunk.GridFSnumbersallchunks,startingwith0.

chunks.data

Thechunk’spayloadasaBSONbinarytype.

GridFSIndex

GridFS使用chunks中files_id和n域作为混合索引，files_id是父文档的_id，n域包含chunk的序列号，该值从0开始。

GridFS索引支持快速恢复数据。

cursor=db.fs.chunks.find({files_id:myFileID}).sort({n:1});

如果没有建立索引，可以使用下列shell命令：

db.fs.chunks.ensureIndex({files_id:1,n:1},{unique:true});

ExampleInterface：

//returnsdefaultGridFSbucket(i.e."fs"collection)

GridFSmyFS=newGridFS(myDatabase);

//savesthefileto"fs"GridFSbucket

myFS.createFile(newFile("/tmp/largething.mpg"));

接口支持额外的GridFSbuckets

//returnsGridFSbucketnamed"contracts"

GridFSmyContracts=newGridFS(myDatabase,"contracts");

//retrieveGridFSobject"smithco"

GridFSDBFilefile=myContracts.findOne("smithco");

//savestheGridFSfiletothefilesystem

file.writeTo(newFile("/tmp/smithco.pdf"));

本文内容总结：

原文链接：https://www.cnblogs.com/bigbigtree/p/3156554.html