作者：张洛丹
原爱可生 DBA 团队成员，现陆金所 DBA 团队成员，对技术执著有追求！
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某天晚上，数据库 hang 住，现象是：
无奈之下通过强制 kill 掉进程，重启数据库恢复。
这里暂且不说 hang 住的原因，仅分析数据库 hang 住，但是 MHA 未触发切换。
先说下结论，MHA 默认使用长连接对数据库做 ping 健康检测（执行select 1 as Value），4次无法连接 MySQL 则触发切换。 前面数据库 hang 住只是新的连接无法建立，但是老连接却没有影响，且 MHA 的健康检测语句很简单，只在 server 层进行了检测，不涉及到 InnoDB 层，所以 MHA 认为 MySQL 是健康的，并没有作出任何决策。
MHA 从 0.53 版本开始支持 ping_type 参数设置如何检查 master 的可用性。支持3个 value ：
注意：此种情况，MHA 监控进程会 fork 出一个子进程进行检测
通过将 ping_type 修改设置为connect，MHA 每次进程状态检测，需要新建连接，新链接无法成功建立，就触发了切换。
三种检测机制代码：
##如果获取分布式锁失败返回2，正常返回0，异常返回1
sub ping_connect($) {
my $self = shift;
my $log = $self->{logger};
my $dbh;
my $rc = 1;
my $max_retries = 2;
eval {
my $ping_start = [gettimeofday];
# 连接max_retries次，连接失败则退出
while ( !$self->{dbh} && $max_retries-- ) {
eval { $rc = $self->connect( 1, $self->{interval}, 0, 0, 1 ); };
if ( !$self->{dbh} && $@ ) {
die $@ if ( !$max_retries );
}
}
# 调用ping_select
$rc = $self->ping_select();
# To hold advisory lock for some periods of time
$self->sleep_until( $ping_start, $self->{interval} - 1.5 );
$self->disconnect_if();
};
if ($@) {
my $msg = "Got error on MySQL connect ping: $@";
undef $@;
$msg .= $DBI::err if ($DBI::err);
$msg .= " ($DBI::errstr)" if ($DBI::errstr);
$log->warning($msg) if ($log);
$rc = 1;
}
return 2 if ( $self->{_already_monitored} );
return $rc;
}
# 正常返回0，异常返回1
sub ping_select($) {
my $self = shift;
my $log = $self->{logger};
my $dbh = $self->{dbh};
my ( $query, $sth, $href );
eval {
$dbh->{RaiseError} = 1;
$sth = $dbh->prepare("SELECT 1 As Value");
$sth->execute();
$href = $sth->fetchrow_hashref;
if ( !defined($href)
|| !defined( $href->{Value} )
|| $href->{Value} != 1 )
{
die;
}
};
if ($@) {
my $msg = "Got error on MySQL select ping: ";
undef $@;
$msg .= $DBI::err if ($DBI::err);
$msg .= " ($DBI::errstr)" if ($DBI::errstr);
$log->warning($msg) if ($log);
return 1;
}
return 0;
}
# 正常返回0，异常返回1
sub ping_insert($) {
my $self = shift;
my $log = $self->{logger};
my $dbh = $self->{dbh};
my ( $query, $sth, $href );
eval {
$dbh->{RaiseError} = 1;
$dbh->do("CREATE DATABASE IF NOT EXISTS infra");
$dbh->do(
"CREATE TABLE IF NOT EXISTS infra.chk_masterha (`key` tinyint NOT NULL primary key,`val` int(10) unsigned NOT NULL DEFAULT '0')"
);
$dbh->do(
"INSERT INTO infra.chk_masterha values (1,unix_timestamp()) ON DUPLICATE KEY UPDATE val=unix_timestamp()"
);
};
if ($@) {
my $msg = "Got error on MySQL insert ping: ";
undef $@;
$msg .= $DBI::err if ($DBI::err);
$msg .= " ($DBI::errstr)" if ($DBI::errstr);
$log->warning($msg) if ($log);
return 1;
}
return 0;
}
MHA 配置文件
[server default]
manager_log=/Data/mha/log/workdir/my3306tst.log
manager_workdir=/Data/mha/workdir/my3306tst
remote_workdir=/Data/mysql/my3306/mha
master_binlog_dir=/Data/mysql/my3306/log
password=xxx
ping_interval=5
repl_password=xxx
repl_user=xxx
ssh_user=mysql
ssh_port=xxx
user=mha
master_ip_online_change_script="/usr/local/bin/master_ip_online_change"
master_ip_failover_script="master_ip_failover"
[server1]
hostname=xxx
port=3306
candidate_master=1
[server2]
hostname=xxx
port=3306
candidate_master=1
注意：在测试的时候将ping_interval设置成5，便于快速观测到切换，实际生产中，可根据业务对故障的容忍能力进行调整。
模拟服务器CPU满负载，数据库无法建立新连接 编写一个简单的c程序，如下：
# include <stdio.h>
int main()
{
while(1);
return 0;
}
编译：
gcc -o out test_cpu.c
执行：
for in in `seq 1 $(cat /proc/cpuinfo | grep "physical id" | wc -l)`; do ./out & done
另外再跑两个 mysqlslap 压测程序：
mysqlslap -c 30000 -i 100 --detach=1 --query="select 1 from dual" --delimiter=";" -uxxx -pxxx -S /xxxx/xxx.sock
Got error on MySQL connect: 2013 (Lost connection to MySQL server at 'waiting for initial communication packet',system error: 110)
有兴趣的同学可自行测试一下
调用链路：
MasterMonitor.pm|MHA::MasterMonitor::main()
-->
MasterMonitor.pm|MHA::MasterMonitor::wait_until_master_is_dead()
-->
MasterMonitor.pm|MHA::MasterMonitor::wait_until_master_is_unreachable()
-->
MHA::HealthCheck::wait_until_unreachable();
-->
HealthCheck.pm|MHA::HealthCheck::ping_select(或者)
HealthCheck.pm|MHA::HealthCheck::ping_insert(或者)
HealthCheck.pm|MHA::HealthCheck::ping_connect(或者)
MHA 监控进程启动后，会持续监控主节点的状态，主要的健康检测函数是 wait_until_unreachable()。
PS：MHA 监控进程启动过程中，会读取配置文件，对配置文件中的服务器进行一系列检查，包括存活状态、版本信息、从库配置（read_only,relay_log_purge,log-bin,复制过滤等），ssh状态等，若检查不通过，则无法启动
在这个函数中会有一个死循环，持续地进行健康检测
1.首先，测试连接，连接正确返回0，否则返回1。
our @ALIVE_ERROR_CODES = (
1040, # ER_CON_COUNT_ERROR
1042, # ER_BAD_HOST_ERROR
1043, # ER_HANDSHAKE_ERROR
1044, # ER_DBACCESS_DENIED_ERROR
1045, # ER_ACCESS_DENIED_ERROR
1129, # ER_HOST_IS_BLOCKED
1130, # ER_HOST_NOT_PRIVILEGED
1203, # ER_TOO_MANY_USER_CONNECTIONS
1226, # ER_USER_LIMIT_REACHED
1251, # ER_NOT_SUPPORTED_AUTH_MODE
1275, # ER_SERVER_IS_IN_SECURE_AUTH_MODE
);
2.测试连接成功后，则进行健康状态检测（前面说的3种方式）；如果连续4次连接失败，则在第4次的时候会使用第二脚本进行检测（如果定义了的话），如果检测通过，则认为 master 挂掉
关键函数 wait_until_unreachable()代码：
# main function
sub wait_until_unreachable($) {
my $self = shift;
my $log = $self->{logger};
my $ssh_reachable = 2;
my $error_count = 0;
my $master_is_down = 0;
eval {
while (1) {
$self->{_tstart} = [gettimeofday];
## 判断是否需要建立连接
if ( $self->{_need_reconnect} ) {
my ( $rc, $mysql_err ) =
$self->connect( undef, undef, undef, undef, undef, $error_count );
if ($rc) {
if ($mysql_err) {
# 错误代码在ALIVE_ERROR_CODES中时，不触发切换，常见的有用户密码不正确，不会切换
if (
grep ( $_ == $mysql_err, @MHA::ManagerConst::ALIVE_ERROR_CODES )
> 0 )
{
$log->info(
"Got MySQL error $mysql_err, but this is not a MySQL crash. Continue health check.."
);
# next直接进入下次循环
$self->sleep_until();
next;
}
}
$error_count++;
$log->warning("Connection failed $error_count time(s)..");
$self->handle_failing();
if ( $error_count >= 4 ) {
$ssh_reachable = $self->is_ssh_reachable();
# 返回1表示主库down，0表示主库没有down
$master_is_down = 1 if ( $self->is_secondary_down() );
# 主库down则跳出循环
last if ($master_is_down);
$error_count = 0;
}
$self->sleep_until();
next;
}
# connection ok
$self->{_need_reconnect} = 0;
$log->info(
"Ping($self->{ping_type}) succeeded, waiting until MySQL doesn't respond.."
);
}
# 如果ping_type为connect，则断开连接
$self->disconnect_if()
if ( $self->{ping_type} eq $MHA::ManagerConst::PING_TYPE_CONNECT );
# Parent process forks one child process. The child process queries
# from MySQL every <interval> seconds. The child process may hang on
# executing queries.
# DBD::mysql 4.022 or earlier does not have an option to set
# read timeout, executing queries might take forever. To avoid this,
# the parent process kills the child process if it won't exit within
# <interval> seconds.
my $child_exit_code;
eval {
# 调用检测函数
if ( $self->{ping_type} eq $MHA::ManagerConst::PING_TYPE_CONNECT ) {
$child_exit_code = $self->fork_exec( sub { $self->ping_connect() },
"MySQL Ping($self->{ping_type})" );
}
elsif ( $self->{ping_type} eq $MHA::ManagerConst::PING_TYPE_SELECT ) {
$child_exit_code = $self->fork_exec( sub { $self->ping_select() },
"MySQL Ping($self->{ping_type})" );
}
elsif ( $self->{ping_type} eq $MHA::ManagerConst::PING_TYPE_INSERT ) {
$child_exit_code = $self->fork_exec( sub { $self->ping_insert() },
"MySQL Ping($self->{ping_type})" );
}
else {
die "Not supported ping_type!n";
}
};
if ($@) {
my $msg = "Unexpected error heppened when pinging! $@";
$log->error($msg);
undef $@;
$child_exit_code = 1;
}
if ( $child_exit_code == 0 ) {
#ping ok
## ping成功的话，则更新状态，并将计数器置为0
$self->update_status_ok();
if ( $error_count > 0 ) {
$error_count = 0;
}
$self->kill_sec_check();
$self->kill_ssh_check();
}
elsif ( $child_exit_code == 2 ) {
$self->{_already_monitored} = 1;
croak;
}
else {
## 创建连接失败
# failed on fork_exec
$error_count++;
$self->{_need_reconnect} = 1;
$self->handle_failing();
}
$self->sleep_until();
}
$log->warning("Master is not reachable from health checker!");
};
if ($@) {
my $msg = "Got error when monitoring master: $@";
$log->warning($msg);
undef $@;
return 2 if ( $self->{_already_monitored} );
return 1;
}
return 1 unless ($master_is_down);
return ( 0, $ssh_reachable );
}
1;
