PingCap's TiDB is a MySQL-compatible distributed database built on TiKV, and takes design inspiration from Google Spanner and Apache HBase. While its sharding and replication architecture are similar to that of Spanner, it follows a very different design for multi-shard transactions. TiDB uses Google Percolator as the inspiration for its multi-shard transaction design. This choice essentially makes TiDB unfit for deployments with geo-distributed writes since the majority of transactions in a random-access OLTP workload will now experience high WAN latency when acquiring a timestamp from the global timestamp oracle running in a different region. Additionally, TiDB lacks support for critical relational data modeling constructs such as foreign key constraints and Serializable isolation level.
Relevant blog posts
The following posts cover some more details around how YugabyteDB differs from TiDB.