Percolator model. Despite promising snap- shot isolation, TiDB 2.1.7 through 3.0.0-beta.1-40 allowed read skew and lost updates by default, thanks to two auto-retry mechanisms which blindly re-applied updates TiDB also supports a select ... for update statement which mostly, but not entirely, prevents write skew. We found a minor race condition in table creation, reduced durability & fault tolerance in fresh dependence on CLOCK_MONOTONIC_RAW for safety, but we have not yet observed anomalies due to clock skew, possibly due to tooling limitations. PingCAP has published a companion blog post to this report.

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 502 Figure 58: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. 419 A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 427 Figure 18: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 516 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 509 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 496 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 495 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

SI cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 592 Figure 62: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. 495 A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 503 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;

cannot avoid write skew TiDB’s SI isolation level cannot avoid write skew exceptions. You can use the SELECT �→ FOR UPDATE syntax to avoid write skew exceptions. 498 A write skew exception occurs when same time. Let’s simulate this process with the following program: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource; import java.sql.Connection; import java.sql.PreparedStatement; happens. 506 Figure 61: Write Skew Now let’s change the sample program to use SELECT FOR UPDATE to avoid the write skew problem: package com.pingcap.txn.write.skew; import com.zaxxer.hikari.HikariDataSource;