My opinion on the Mongodb--metadata

MongoDB Storage Engine

When it comes to MongoDB's metadata information, first introduce the storage category of MongoDB. mongoDB3.2.4 Support 5 kinds: Wiredtiger (default) MMAPv1 inmomery Encrypted 3rd party Engine metadata Information

From the file construct, the MongoDB metadata includes:
Collection namespace Index namespace $freelist

1 and 2 see name, focus on $freelist, which is stored in the deleted collection and index information.

From a functional standpoint, the MongoDB metadata includes:

1. Mapping relationship between Shard key and chunk range key

2. The mapping relationship between chunk and data key.

This information is stored in the config server. The general user's request comes over, after router server (mongos) gets metadata, carries on the resolution, starts addressing, makes the target data the mapping address, finally can the data to carry on the enhancement check.

PS: Originally painted a picture, the company can not upload pictures, later to add.