如何在Microsoft SQL Server中显式锁定表(寻找黑客 - 不合作的客户端)

Question

这是我最初的问题:

我试图弄清楚如何在SQL Server中强制执行EXCLUSIVE表锁.我需要解决不合作的读者(我无法控制,闭源的东西),明确地将他们的ISOLATION LEVEL设置为READ UNCOMMITTED.结果是,无论我在执行插入/更新时指定了多少锁和什么样的隔离,客户端只需要设置正确的隔离并返回读取我正在进行的垃圾.

答案结果很简单 -

虽然无法触发显式锁定,但任何DDL更改都会触发我正在寻找的锁定.

虽然这种情况并不理想(客户端阻塞而不是目击可重复读取),但它比让客户端覆盖隔离并读取脏数据要好得多.这是带有虚拟触发器锁定机制的完整示例代码

赢了!

#!/usr/bin/env perl

use Test::More;

use warnings;
use strict;

use DBI;

my ($dsn, $user, $pass) = @ENV{ map { "DBICTEST_MSSQL_ODBC_$_" } qw/DSN USER PASS/ };

my @coninf = ($dsn, $user, $pass, {
  AutoCommit => 1,
  LongReadLen => 1048576,
  PrintError => 0,
  RaiseError => 1,
});

if (! fork) {
  my $reader = DBI->connect(@coninf);
  $reader->do('SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED');

  warn "READER $$: waiting for table creation";
  sleep 1;

  for (1..5) {
    is_deeply (
      $reader->selectall_arrayref ('SELECT COUNT(*) FROM artist'),
      [ [ 0 ] ],
      "READER $$: does not see anything in db, sleeping for a sec " . time,
    );
    sleep 1;
  }

  exit;
}

my $writer = DBI->connect(@coninf);

eval { $writer->do('DROP TABLE artist') };
$writer->do('CREATE TABLE artist ( name VARCHAR(20) NOT NULL PRIMARY KEY )');
$writer->do(do('DISABLE TRIGGER _lock_artist ON artist');

sleep 1;

is_deeply (
  $writer->selectall_arrayref ('SELECT COUNT(*) FROM artist'),
  [ [ 0 ] ],
  'No rows to start with',
);

$writer->begin_work;

$writer->prepare("INSERT INTO artist VALUES ('bupkus') ")->execute;
# this is how we lock
$writer->do('ENABLE TRIGGER _lock_artist ON artist');
$writer->do('DISABLE TRIGGER _lock_artist ON artist');

is_deeply (
  $writer->selectall_arrayref ('SELECT COUNT(*) FROM artist'),
  [ [ 1 ] ],
  'Writer sees inserted row',
);

# delay reader
sleep 2;

$writer->rollback;

# should not affect reader
sleep 2;

is_deeply (
  $writer->selectall_arrayref ('SELECT COUNT(*) FROM artist'),
  [ [ 0 ] ],
  'Nothing committed (writer)',
);

wait;

done_testing;

结果:

READER 27311: waiting for table creation at mssql_isolation.t line 27.
ok 1 - READER 27311: does not see anything in db, sleeping for a sec 1310555569
ok 1 - No rows to start with
ok 2 - Writer sees inserted row
ok 2 - READER 27311: does not see anything in db, sleeping for a sec 1310555571
ok 3 - READER 27311: does not see anything in db, sleeping for a sec 1310555572
ok 3 - Nothing committed (writer)
ok 4 - READER 27311: does not see anything in db, sleeping for a sec 1310555573
ok 5 - READER 27311: does not see anything in db, sleeping for a sec 1310555574

Answer 1

一个hack hack hack方法是强制对表进行操作,该操作对表进行SCH-M锁定,即使在READ UNCOMMITTED隔离级别也会阻止对表的读取.例如,作为操作的一部分,执行类似ALTER TABLE REBUILD的操作(可能在特定的空分区上以减少性能影响)将阻止对表的所有并发访问,直到您提交为止.

Answer 2

添加锁定提示到您的SELECT:

SELECT COUNT(*) FROM artist WITH (TABLOCKX)

并把你INSERT进入交易.

如果您的初始语句是在显式事务中,则SELECT它将在处理之前等待锁定.