There is a good video to describe a algorithms of TiDB:

Also, there are a few documents related to online DDL

• The Online DDL design is descrbed in this design document
• Google F1 algorithm, which is refered from TiDB online schema change, is described in this research paper

Online Schema change

It seems it’s unable to confirm the schema version on TiKV node. Inside a PD node, it’s stored in the /tidb/ddl/global_schema_version in the etcd according to this article.