事务限制
本章将简单介绍 TiDB 中的事务限制。
隔离级别
TiDB 支持的隔离级别是 RC（Read Committed）与 SI（Snapshot Isolation），其中 SI 与 RR（Repeatable Read）隔离级别基本等价。
SI 可以克服幻读
TiDB 的 SI 隔离级别可以克服幻读异常 (Phantom Reads)，但 ANSI/ISO SQL 标准中的 RR 不能。
所谓幻读是指：事务 A 首先根据条件查询得到 n 条记录，然后事务 B 改变了这 n 条记录之外的 m 条记录或者增添了 m 条符合事务 A 查询条件的记录，导致事务 A 再次发起请求时发现有 n+m 条符合条件记录，就产生了幻读。
例如：系统管理员 A 将数据库中所有学生的成绩从具体分数改为 ABCDE 等级，但是系统管理员 B 就在这个时候插入了一条具体分数的记录，当系统管理员 A 改结束后发现还有一条记录没有改过来，就好像发生了幻觉一样，这就叫幻读。
SI 不能克服写偏斜
TiDB 的 SI 隔离级别不能克服写偏斜异常（Write Skew），需要使用 Select for update 语法来克服写偏斜异常。
写偏斜异常是指两个并发的事务读取了不同但相关的记录，接着这两个事务各自更新了自己读到的数据，并最终都提交了事务，如果这些相关的记录之间存在着不能被多个事务并发修改的约束，那么最终结果将是违反约束的。
举个例子，假设你正在为医院写一个医生轮班管理程序。医院通常会同时要求几位医生待命，但底线是至少有一位医生在待命。医生可以放弃他们的班次（例如，如果他们自己生病了），只要至少有一个同事在这一班中继续工作。
现在出现这样一种情况，Alice 和 Bob 是两位值班医生。两人都感到不适，所以他们都决定请假。不幸的是，他们恰好在同一时间点击按钮下班。下面用程序来模拟一下这个过程。
- Java
- Golang
Java 程序示例如下：
package com.pingcap.txn.write.skew;
import com.zaxxer.hikari.HikariDataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class EffectWriteSkew {
public static void main(String[] args) throws SQLException, InterruptedException {
HikariDataSource ds = new HikariDataSource();
ds.setJdbcUrl("jdbc:mysql://localhost:4000/test?useServerPrepStmts=true&cachePrepStmts=true");
ds.setUsername("root");
// prepare data
Connection connection = ds.getConnection();
createDoctorTable(connection);
createDoctor(connection, 1, "Alice", true, 123);
createDoctor(connection, 2, "Bob", true, 123);
createDoctor(connection, 3, "Carol", false, 123);
Semaphore txn1Pass = new Semaphore(0);
CountDownLatch countDownLatch = new CountDownLatch(2);
ExecutorService threadPool = Executors.newFixedThreadPool(2);
threadPool.execute(() -> {
askForLeave(ds, txn1Pass, 1, 1);
countDownLatch.countDown();
});
threadPool.execute(() -> {
askForLeave(ds, txn1Pass, 2, 2);
countDownLatch.countDown();
});
countDownLatch.await();
}
public static void createDoctorTable(Connection connection) throws SQLException {
connection.createStatement().executeUpdate("CREATE TABLE `doctors` (" +
" `id` int NOT NULL," +
" `name` varchar(255) DEFAULT NULL," +
" `on_call` tinyint DEFAULT NULL," +
" `shift_id` int DEFAULT NULL," +
" PRIMARY KEY (`id`)," +
" KEY `idx_shift_id` (`shift_id`)" +
" ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin");
}
public static void createDoctor(Connection connection, Integer id, String name, Boolean onCall, Integer shiftID) throws SQLException {
PreparedStatement insert = connection.prepareStatement(
"INSERT INTO `doctors` (`id`, `name`, `on_call`, `shift_id`) VALUES (?, ?, ?, ?)");
insert.setInt(1, id);
insert.setString(2, name);
insert.setBoolean(3, onCall);
insert.setInt(4, shiftID);
insert.executeUpdate();
}
public static void askForLeave(HikariDataSource ds, Semaphore txn1Pass, Integer txnID, Integer doctorID) {
try(Connection connection = ds.getConnection()) {
try {
connection.setAutoCommit(false);
String comment = txnID == 2 ? " " : "" + "/* txn #{txn_id} */ ";
connection.createStatement().executeUpdate(comment + "BEGIN");
// Txn 1 should be waiting until txn 2 is done.
if (txnID == 1) {
txn1Pass.acquire();
}
PreparedStatement currentOnCallQuery = connection.prepareStatement(comment +
"SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = ? AND `shift_id` = ?");
currentOnCallQuery.setBoolean(1, true);
currentOnCallQuery.setInt(2, 123);
ResultSet res = currentOnCallQuery.executeQuery();
if (!res.next()) {
throw new RuntimeException("error query");
} else {
int count = res.getInt("count");
if (count >= 2) {
// If current on-call doctor has 2 or more, this doctor can leave
PreparedStatement insert = connection.prepareStatement( comment +
"UPDATE `doctors` SET `on_call` = ? WHERE `id` = ? AND `shift_id` = ?");
insert.setBoolean(1, false);
insert.setInt(2, doctorID);
insert.setInt(3, 123);
insert.executeUpdate();
connection.commit();
} else {
throw new RuntimeException("At least one doctor is on call");
}
}
// Txn 2 is done. Let txn 1 run again.
if (txnID == 2) {
txn1Pass.release();
}
} catch (Exception e) {
// If got any error, you should roll back, data is priceless
connection.rollback();
e.printStackTrace();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
在 Golang 中，首先，封装一个用于适配 TiDB 事务的工具包 util，随后编写以下代码：
package main
import (
"database/sql"
"fmt"
"sync"
"github.com/pingcap-inc/tidb-example-golang/util"
_ "github.com/go-sql-driver/mysql"
)
func main() {
openDB("mysql", "root:@tcp(127.0.0.1:4000)/test", func(db *sql.DB) {
writeSkew(db)
})
}
func openDB(driverName, dataSourceName string, runnable func(db *sql.DB)) {
db, err := sql.Open(driverName, dataSourceName)
if err != nil {
panic(err)
}
defer db.Close()
runnable(db)
}
func writeSkew(db *sql.DB) {
err := prepareData(db)
if err != nil {
panic(err)
}
waitingChan, waitGroup := make(chan bool), sync.WaitGroup{}
waitGroup.Add(1)
go func() {
defer waitGroup.Done()
err = askForLeave(db, waitingChan, 1, 1)
if err != nil {
panic(err)
}
}()
waitGroup.Add(1)
go func() {
defer waitGroup.Done()
err = askForLeave(db, waitingChan, 2, 2)
if err != nil {
panic(err)
}
}()
waitGroup.Wait()
}
func askForLeave(db *sql.DB, waitingChan chan bool, goroutineID, doctorID int) error {
txnComment := fmt.Sprintf("/* txn %d */ ", goroutineID)
if goroutineID != 1 {
txnComment = "\t" + txnComment
}
txn, err := util.TiDBSqlBegin(db, true)
if err != nil {
return err
}
fmt.Println(txnComment + "start txn")
// Txn 1 should be waiting until txn 2 is done.
if goroutineID == 1 {
<-waitingChan
}
txnFunc := func() error {
queryCurrentOnCall := "SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = ? AND `shift_id` = ?"
rows, err := txn.Query(queryCurrentOnCall, true, 123)
if err != nil {
return err
}
defer rows.Close()
fmt.Println(txnComment + queryCurrentOnCall + " successful")
count := 0
if rows.Next() {
err = rows.Scan(&count)
if err != nil {
return err
}
}
rows.Close()
if count < 2 {
return fmt.Errorf("at least one doctor is on call")
}
shift := "UPDATE `doctors` SET `on_call` = ? WHERE `id` = ? AND `shift_id` = ?"
_, err = txn.Exec(shift, false, doctorID, 123)
if err == nil {
fmt.Println(txnComment + shift + " successful")
}
return err
}
err = txnFunc()
if err == nil {
txn.Commit()
fmt.Println("[runTxn] commit success")
} else {
txn.Rollback()
fmt.Printf("[runTxn] got an error, rollback: %+v\n", err)
}
// Txn 2 is done. Let txn 1 run again.
if goroutineID == 2 {
waitingChan <- true
}
return nil
}
func prepareData(db *sql.DB) error {
err := createDoctorTable(db)
if err != nil {
return err
}
err = createDoctor(db, 1, "Alice", true, 123)
if err != nil {
return err
}
err = createDoctor(db, 2, "Bob", true, 123)
if err != nil {
return err
}
err = createDoctor(db, 3, "Carol", false, 123)
if err != nil {
return err
}
return nil
}
func createDoctorTable(db *sql.DB) error {
_, err := db.Exec("CREATE TABLE IF NOT EXISTS `doctors` (" +
" `id` int NOT NULL," +
" `name` varchar(255) DEFAULT NULL," +
" `on_call` tinyint DEFAULT NULL," +
" `shift_id` int DEFAULT NULL," +
" PRIMARY KEY (`id`)," +
" KEY `idx_shift_id` (`shift_id`)" +
" ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin")
return err
}
func createDoctor(db *sql.DB, id int, name string, onCall bool, shiftID int) error {
_, err := db.Exec("INSERT INTO `doctors` (`id`, `name`, `on_call`, `shift_id`) VALUES (?, ?, ?, ?)",
id, name, onCall, shiftID)
return err
}
SQL 日志：
/* txn 1 */ BEGIN
/* txn 2 */ BEGIN
/* txn 2 */ SELECT COUNT(*) as `count` FROM `doctors` WHERE `on_call` = 1 AND `shift_id` = 123
/* txn 2 */ UPDATE `doctors` SET `on_call` = 0 WHERE `id` = 2 AND `shift_id` = 123
/* txn 2 */ COMMIT
/* txn 1 */ SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = 1 and `shift_id` = 123
/* txn 1 */ UPDATE `doctors` SET `on_call` = 0 WHERE `id` = 1 AND `shift_id` = 123
/* txn 1 */ COMMIT
执行结果：
mysql> SELECT * FROM doctors;
+----+-------+---------+----------+
| id | name | on_call | shift_id |
+----+-------+---------+----------+
| 1 | Alice | 0 | 123 |
| 2 | Bob | 0 | 123 |
| 3 | Carol | 0 | 123 |
+----+-------+---------+----------+
在两个事务中，应用首先检查是否有两个或以上的医生正在值班；如果是的话，它就假定一名医生可以安全地休班。由于数据库使用快照隔离，两次检查都返回 2，所以两个事务都进入下一个阶段。Alice 更新自己的记录休班了，而 Bob 也做了一样的事情。两个事务都成功提交了，现在没有医生值班了。违反了至少有一名医生在值班的要求。下图(引用自《Designing Data-Intensive Application》)说明了实际发生的情况：
现在更改示例程序，使用 SELECT FOR UPDATE 来克服写偏斜问题：
- Java
- Golang
Java 中使用 SELECT FOR UPDATE 来克服写偏斜问题的示例如下：
package com.pingcap.txn.write.skew;
import com.zaxxer.hikari.HikariDataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class EffectWriteSkew {
public static void main(String[] args) throws SQLException, InterruptedException {
HikariDataSource ds = new HikariDataSource();
ds.setJdbcUrl("jdbc:mysql://localhost:4000/test?useServerPrepStmts=true&cachePrepStmts=true");
ds.setUsername("root");
// prepare data
Connection connection = ds.getConnection();
createDoctorTable(connection);
createDoctor(connection, 1, "Alice", true, 123);
createDoctor(connection, 2, "Bob", true, 123);
createDoctor(connection, 3, "Carol", false, 123);
Semaphore txn1Pass = new Semaphore(0);
CountDownLatch countDownLatch = new CountDownLatch(2);
ExecutorService threadPool = Executors.newFixedThreadPool(2);
threadPool.execute(() -> {
askForLeave(ds, txn1Pass, 1, 1);
countDownLatch.countDown();
});
threadPool.execute(() -> {
askForLeave(ds, txn1Pass, 2, 2);
countDownLatch.countDown();
});
countDownLatch.await();
}
public static void createDoctorTable(Connection connection) throws SQLException {
connection.createStatement().executeUpdate("CREATE TABLE `doctors` (" +
" `id` int NOT NULL," +
" `name` varchar(255) DEFAULT NULL," +
" `on_call` tinyint DEFAULT NULL," +
" `shift_id` int DEFAULT NULL," +
" PRIMARY KEY (`id`)," +
" KEY `idx_shift_id` (`shift_id`)" +
" ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin");
}
public static void createDoctor(Connection connection, Integer id, String name, Boolean onCall, Integer shiftID) throws SQLException {
PreparedStatement insert = connection.prepareStatement(
"INSERT INTO `doctors` (`id`, `name`, `on_call`, `shift_id`) VALUES (?, ?, ?, ?)");
insert.setInt(1, id);
insert.setString(2, name);
insert.setBoolean(3, onCall);
insert.setInt(4, shiftID);
insert.executeUpdate();
}
public static void askForLeave(HikariDataSource ds, Semaphore txn1Pass, Integer txnID, Integer doctorID) {
try(Connection connection = ds.getConnection()) {
try {
connection.setAutoCommit(false);
String comment = txnID == 2 ? " " : "" + "/* txn #{txn_id} */ ";
connection.createStatement().executeUpdate(comment + "BEGIN");
// Txn 1 should be waiting until txn 2 is done.
if (txnID == 1) {
txn1Pass.acquire();
}
PreparedStatement currentOnCallQuery = connection.prepareStatement(comment +
"SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = ? AND `shift_id` = ? FOR UPDATE");
currentOnCallQuery.setBoolean(1, true);
currentOnCallQuery.setInt(2, 123);
ResultSet res = currentOnCallQuery.executeQuery();
if (!res.next()) {
throw new RuntimeException("error query");
} else {
int count = res.getInt("count");
if (count >= 2) {
// If current on-call doctor has 2 or more, this doctor can leave
PreparedStatement insert = connection.prepareStatement( comment +
"UPDATE `doctors` SET `on_call` = ? WHERE `id` = ? AND `shift_id` = ?");
insert.setBoolean(1, false);
insert.setInt(2, doctorID);
insert.setInt(3, 123);
insert.executeUpdate();
connection.commit();
} else {
throw new RuntimeException("At least one doctor is on call");
}
}
// Txn 2 is done. Let txn 1 run again.
if (txnID == 2) {
txn1Pass.release();
}
} catch (Exception e) {
// If got any error, you should roll back, data is priceless
connection.rollback();
e.printStackTrace();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
Golang 中使用 SELECT FOR UPDATE 来克服写偏斜问题的示例如下：
package main
import (
"database/sql"
"fmt"
"sync"
"github.com/pingcap-inc/tidb-example-golang/util"
_ "github.com/go-sql-driver/mysql"
)
func main() {
openDB("mysql", "root:@tcp(127.0.0.1:4000)/test", func(db *sql.DB) {
writeSkew(db)
})
}
func openDB(driverName, dataSourceName string, runnable func(db *sql.DB)) {
db, err := sql.Open(driverName, dataSourceName)
if err != nil {
panic(err)
}
defer db.Close()
runnable(db)
}
func writeSkew(db *sql.DB) {
err := prepareData(db)
if err != nil {
panic(err)
}
waitingChan, waitGroup := make(chan bool), sync.WaitGroup{}
waitGroup.Add(1)
go func() {
defer waitGroup.Done()
err = askForLeave(db, waitingChan, 1, 1)
if err != nil {
panic(err)
}
}()
waitGroup.Add(1)
go func() {
defer waitGroup.Done()
err = askForLeave(db, waitingChan, 2, 2)
if err != nil {
panic(err)
}
}()
waitGroup.Wait()
}
func askForLeave(db *sql.DB, waitingChan chan bool, goroutineID, doctorID int) error {
txnComment := fmt.Sprintf("/* txn %d */ ", goroutineID)
if goroutineID != 1 {
txnComment = "\t" + txnComment
}
txn, err := util.TiDBSqlBegin(db, true)
if err != nil {
return err
}
fmt.Println(txnComment + "start txn")
// Txn 1 should be waiting until txn 2 is done.
if goroutineID == 1 {
<-waitingChan
}
txnFunc := func() error {
queryCurrentOnCall := "SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = ? AND `shift_id` = ?"
rows, err := txn.Query(queryCurrentOnCall, true, 123)
if err != nil {
return err
}
defer rows.Close()
fmt.Println(txnComment + queryCurrentOnCall + " successful")
count := 0
if rows.Next() {
err = rows.Scan(&count)
if err != nil {
return err
}
}
rows.Close()
if count < 2 {
return fmt.Errorf("at least one doctor is on call")
}
shift := "UPDATE `doctors` SET `on_call` = ? WHERE `id` = ? AND `shift_id` = ?"
_, err = txn.Exec(shift, false, doctorID, 123)
if err == nil {
fmt.Println(txnComment + shift + " successful")
}
return err
}
err = txnFunc()
if err == nil {
txn.Commit()
fmt.Println("[runTxn] commit success")
} else {
txn.Rollback()
fmt.Printf("[runTxn] got an error, rollback: %+v\n", err)
}
// Txn 2 is done. Let txn 1 run again.
if goroutineID == 2 {
waitingChan <- true
}
return nil
}
func prepareData(db *sql.DB) error {
err := createDoctorTable(db)
if err != nil {
return err
}
err = createDoctor(db, 1, "Alice", true, 123)
if err != nil {
return err
}
err = createDoctor(db, 2, "Bob", true, 123)
if err != nil {
return err
}
err = createDoctor(db, 3, "Carol", false, 123)
if err != nil {
return err
}
return nil
}
func createDoctorTable(db *sql.DB) error {
_, err := db.Exec("CREATE TABLE IF NOT EXISTS `doctors` (" +
" `id` int NOT NULL," +
" `name` varchar(255) DEFAULT NULL," +
" `on_call` tinyint DEFAULT NULL," +
" `shift_id` int DEFAULT NULL," +
" PRIMARY KEY (`id`)," +
" KEY `idx_shift_id` (`shift_id`)" +
" ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin")
return err
}
func createDoctor(db *sql.DB, id int, name string, onCall bool, shiftID int) error {
_, err := db.Exec("INSERT INTO `doctors` (`id`, `name`, `on_call`, `shift_id`) VALUES (?, ?, ?, ?)",
id, name, onCall, shiftID)
return err
}
SQL 日志：
/* txn 1 */ BEGIN
/* txn 2 */ BEGIN
/* txn 2 */ SELECT COUNT(*) AS `count` FROM `doctors` WHERE on_call = 1 AND `shift_id` = 123 FOR UPDATE
/* txn 2 */ UPDATE `doctors` SET on_call = 0 WHERE `id` = 2 AND `shift_id` = 123
/* txn 2 */ COMMIT
/* txn 1 */ SELECT COUNT(*) AS `count` FROM `doctors` WHERE `on_call` = 1 FOR UPDATE
At least one doctor is on call
/* txn 1 */ ROLLBACK
执行结果：
mysql> SELECT * FROM doctors;
+----+-------+---------+----------+
| id | name | on_call | shift_id |
+----+-------+---------+----------+
| 1 | Alice | 1 | 123 |
| 2 | Bob | 0 | 123 |
| 3 | Carol | 0 | 123 |
+----+-------+---------+----------+
对 savepoint 和嵌套事务的支持
Spring 支持的 PROPAGATION_NESTED 传播行为会启动一个嵌套的事务，它是当前事务之上独立启动的一个子事务。嵌套事务开始时会记录一个 savepoint，如果嵌套事务执行失败，事务将会回滚到 savepoint 的状态。嵌套事务是外层事务的一部分，它将会在外层事务提交时一起被提交。下面案例展示了 savepoint 机制：
mysql> BEGIN;
mysql> INSERT INTO T2 VALUES(100);
mysql> SAVEPOINT svp1;
mysql> INSERT INTO T2 VALUES(200);
mysql> ROLLBACK TO SAVEPOINT svp1;
mysql> RELEASE SAVEPOINT svp1;
mysql> COMMIT;
mysql> SELECT * FROM T2;
+------+
| ID |
+------+
| 100 |
+------+
大事务限制
基本原则是要限制事务的大小。TiDB 对单个事务的大小有限制，这层限制是在 KV 层面。反映在 SQL 层面的话，简单来说一行数据会映射为一个 KV entry，每多一个索引，也会增加一个 KV entry。所以这个限制反映在 SQL 层面是：
最大单行记录容量为 120 MiB。
- TiDB v4.0.10 及更高的 v4.0.x 版本、v5.0.0 及更高的版本可通过 tidb-server 配置项
performance.txn-entry-size-limit调整，低于 TiDB v4.0.10 的版本支持的单行容量为 6 MiB。 - 从 v7.6.0 开始，你可以使用
tidb_txn_entry_size_limit系统变量动态修改该配置项的值。
- TiDB v4.0.10 及更高的 v4.0.x 版本、v5.0.0 及更高的版本可通过 tidb-server 配置项
支持的最大单个事务容量为 1 TiB。
- TiDB v4.0 及更高版本可通过 tidb-server 配置项
performance.txn-total-size-limit调整，低于 TiDB v4.0 的版本支持的最大单个事务容量为 100 MiB。 - 在 v6.5.0 及之后的版本中，不再推荐使用配置项
performance.txn-total-size-limit。更多详情请参考performance.txn-total-size-limit。
- TiDB v4.0 及更高版本可通过 tidb-server 配置项
另外注意，无论是大小限制还是行数限制，还要考虑事务执行过程中，TiDB 做编码以及事务额外 Key 的开销。在使用的时候，为了使性能达到最优，建议每 100 ～ 500 行写入一个事务。
自动提交的 SELECT FOR UPDATE 语句不会等锁
自动提交下的 SELECT FOR UPDATE 目前不会加锁。效果如下图所示：
这是已知的与 MySQL 不兼容的地方。
可以通过使用显式的 BEGIN;COMMIT; 解决该问题。