# Tso Assignment: Tso Concepts
__*TSO concept*__
+ `TSO = physical time logical time` 

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TSO is a global timestamp, which is the cornerstone of TiDB's implement of distributed transaction. Therefore, for PD, we first need to ensure that it can quickly and massively allocate TSO to transaction. At the same time, we also need to ensure that the allocated TSO must be monotonically increasing, and there is no possibility of falling rewind.

TSO is an int64 integer that consists of physical time plus logical time. Physical time is the current unix time in milliseconds, while logical time is a counter with a maximum of 1 << 18. In other words, for 1ms, PD can be allocated up to 262144 TSOs, which satisfies the vast majority of cases.