Object IDs

Object IDs

Documents in MongoDB required a key, _id, which uniquely identifies them.(只有_id建立索引之后，各个doc的_id才会不同，否则有可能相同) The _id Field If a user tries to insert a document without providing an _id field, the database will automatically generate an _object id_ and store it the _id field.(当客户端提供不提供_id时，则服务器来产生。) The _id value may be of any type, other than arrays, so long as it is a unique. If your document has a natural primary key that is immutable we recommend you use that in _id instead of the automatically generated ids. Arrays are not allowed _ids because they areMultikeys. The BSON ObjectId Datatype  a special BSON datatype is provided for object ids.  All of the officially-supported MongoDB drivers use this type by default for _id values.  Also, the Mongo database itself uses this type when assigning _id values on inserts where no _id value is present. In the MongoDB shell, ObjectId() may be used to create ObjectIds.  ObjectId(string) creates an object ID from the specified hex string. BSON ObjectID Specification A BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), a 3-byte machine id, a 2-byte process id, and a 3-byte counter. Note that the timestamp and counter fields must be stored big endian(高字节在高位） unlike the rest of BSON. This is because they are compared byte-by-byte and we want to ensure a mostly increasing order. The format: 0 1 2 3 4 5 6 7 8 9 10 11 time machine pid inc  the server itself and almost all drivers use the format above.