# Key Concepts
+ `MVCC` (multi-version concurrency control)
+ `Timestamp`
  + Allocated from `Placement Driver` (for now)
  + `64` bits in total, `47` bits for physical `17` bits for logical
+ **`start_ts`**
  + Timestamp when a transaction starts
+ **`commit_ts`**
  + Timestamp when a transaction commits
+ Primary key
  + A special key marks whether a transaction commits or rollbacks

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