# Causal Consistency Transaction

__*CAUSAL CONSISTENCY ONLY*__
+ Causal Consistency guarantees that if A causally-precedes B, then every user of the database observes A before observing B
+ Causal Consistency Transaction in TiDB
  + Gets `TSO` from `Placement Driver (PD)` at start of transaction
  + Does not get `TSO` from the `Placement Driver (PD)` at the end of the transaction
  + With lower commit latency
    + Statement **`START TRANSACTION WITH CAUSAL CONSISTENCY ONLY`**
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TiDB supports enabling causal consistent transaction. Causal consistent transaction do not need to get a timestamp from the PD when they are committed, so commit latency is lower. The syntax to turn on causal consistency transaction is:

```
START TRANSACTION WITH CAUSAL CONSISTENCY ONLY;
```

By default, TiDB guarantees linear consistency. In the case of linearity consistency, if Transaction 2 commits after Transaction 1 commits, it should logically occur after Transaction 1.

Causal consistency is weaker than linear consistency. In the case of causal consistency, the order in which transactions are committed is guaranteed to be consistent with the order in which transactions occur only if transaction 1 and transaction 2 lock or write data intersect.