我有一个将数据复制到外部磁盘上的应用程序。复制请求存储在MySQL数据库中，并将从多个复制计算机中读取，这些复制计算机运行bash脚本来接收请求。一旦请求被选中，它就会在数据库中设置为“inprogress”。但是，我试图避免多台机器一起读取请求并开始复制相同数据的情况。
我打算使用表锁定来执行此操作，但是我正在努力，因为当会话到期时表锁会过期，所以如果我这样做：
mysql="mysql -h dbhost -u user -pPassword diskcopydb"
echo "LOCK tables diskcopy WRITE;" | $mysql
echo "SELECT SQL_NO_CACHE * FROM diskcopy WHERE status=\"request\"" | $mysql
在获取请求时，锁实际上已经过期，因此竞争条件仍然存在。这里有一个问题：
MySQL from the command line - can I practically use LOCKs?
将SQL命令打包到一个块中并将它们一起传送到MySQL中，但我需要从MySQL中获取输出以获取请求。有人有这方面的食谱吗？它似乎应该是一个相当常见的用例...
答案 0 :(得分：1)
这个问题实际上已在这里得到解答：
Bulk insert of MySQL related tables from bash
为了解释，与Bash的双向通信对于Bash来说很棘手，并且使用像Perl或Python这样的“适当”编程语言更容易，你可以打开并保持连接。
答案 1 :(得分：1)
＆gt;但我需要从中途获取输出以获取请求。有人有这方面的食谱吗？
这是我的解决方案演示：
#!/bin/bash
################################################################################
# Bash synchronous client-server #
# or #
# Bash parallel processes with synchronized rendezvous points #
# or #
# Bash interprocess communication with named pipes #
################################################################################
#Declare your session 'globals'
args= #command line arguments are stored here
DIR= #script's current path is stored here
#Early store your bash args before you lose them
for arg in "$@"; do
args[i]=$arg
(( i+=1 ))
done
################################################################################
## Initialize constants ##
#------------------------------------------------------------------------------#
# Explain more... #
################################################################################
function init_vars() {
#absolute path of this script
DIR=$( cd "$( dirname "$0" )" && pwd )
#global MYSQL_PASS is defined in ~/.profile
#this is the file where we store MySQL root password
if [[ ! $MYSQL_PASS ]]; then
MYSQL_PASS='.mysqlpass'
fi
#do some other stuff here...
}
################################################################################
## Rotate over passed arguments ##
#------------------------------------------------------------------------------#
#Parse array 'args' based on the logic of your command line argument syntax #
#Say, '--db <database>' denotes the processing of one database, '--db all' of #
#all databases etc. #
#Linux utilities have contradicting rules for such kind of argument processing #
#(full of crap!), i.e. see tar: tar -xvf or tar --xfv or tar xvf, mysql -p #
#<pass> or mysql -p=<pass> or mysql -p'<pass>' (???) #
################################################################################
function parse_args() {
#your logic goes here
}
################################################################################
## Opens a MySQL client session ##
#------------------------------------------------------------------------------#
# For security reasons password is kept in a file given by MYSQL_PASS. #
################################################################################
function mysql_client() {
mysql --host=127.0.0.1 --port=3306 --default-character-set=utf8 -u root -p"$( cd $DIR; cat $MYSQL_PASS )"
}
################################################################################
## A job dispatcher ##
#------------------------------------------------------------------------------#
# This thread initiates the workers and also reads intermediate results from #
# the spawned jobs. We don't want to parallelize the whole part of a job but #
# rather to put some parts to run in parallel with the dispatcher and some #
# parts in a sequential order determined by what we call rendezvous points. #
# See the diagram: A, B, C, D, E and F are events or actions during the life of#
# a program. #
# #
# dispatcher #
# | start #
# A ----------------> worker #
# | | #
# B (read blocks) C #
# | signals | #
# B (unblocks)<--------------- D (write blocks) #
# | | #
# E (1st set of results) F #
# | | #
# We can only guarantee that E comes after D or, D->E in time but E->F or F->E #
# and B->C or C->B (we don't care) #
################################################################################
function dispatcher() {
#go to the directory of this script
cd $DIR
#make a temporary directory secured in 'time & space'
TMPDIR=$( mktemp -d XXXXXXXXXX )
#catch exit of this script and delete temporary folder
trap 'cd $DIR && rm -rf "$TMPDIR"' EXIT
#move into the temporary folder
cd ${TMPDIR}
#create named pipes-global to all functions of this script
TMPSQL=mysql-$RANDOM.$RANDOM.$RANDOM.$$
TMPSCRIPT=myscript-$RANDOM.$RANDOM.$RANDOM.$$
mkfifo $TMPSQL
mkfifo $TMPSCRIPT
#exec 3<> ${DIR}/${TMPDIR}/$TMPSQL || (echo 'error'; exit 1)
#exec 4<> ${DIR}/${TMPDIR}/$TMPSCRIPT || ( echo 'error'; exit 1)
echo '===1.PARENT=== Starting background jobs...'
echo 'We can guarantee the succession only of 2->3,4 and 5->6,7 but NOT 3->4 or 6->7!'
worker &
##################################
cat $TMPSQL #make a blocking read!
##################################
echo '===4.PARENT=== ...1st query has been read'
##################################
cat $TMPSQL #make a blocking read!
##################################
echo '===7.PARENT=== ...2nd query has been read'
#don't you dare to exit!
##################################
cat $TMPSQL #make a blocking read!
##################################
}
################################################################################
## A job spawned by the dispatcher ##
#------------------------------------------------------------------------------#
################################################################################
function worker() {
echo "===2.CHILD=== Executing 1st query......writing to $TMPSQL"
#the dash symbol '-' makes tabs at the beginning of line to be
#ignored inside the here-doc but improves formation!
mysql_client <<- QUERY
\! tee $TMPSQL
use test;
select * from customers;
\! echo '===3.CHILD=== End 1st query'
#####################################
#\. $TMPSCRIPT
#####################################
QUERY
echo "===5.CHILD=== Executing 2nd query......writing to $TMPSQL"
mysql_client <<- QUERY
\! tee $TMPSQL
use test;
select * from cars;
\! echo '===6.CHILD=== End 2nd query'
#####################################
#\. $TMPSCRIPT
#####################################
QUERY
echo '===8.CHILD=== End of background jobs'
echo > $TMPSQL
}
#parse command line arguments
parse_args
#initialize variables
init_vars
#call dispatcher
dispatcher
假设您有一个'test'数据库，那么您可以看到以下输出：
===1.PARENT=== Starting background jobs...
We can guarantee the succession only of 2->3,4 and 5->6,7 but NOT 3->4 or 6->7!
===2.CHILD=== Executing 1st query......writing to mysql-30627.1495.5394.7533
===4.PARENT=== ...1st query has been read
id_customer firstname secondname
1 John Pincolo
2 Mark Denonto
3 Ann Curtis
4 Jeny Wirth
===3.CHILD=== End 1st query
===5.CHILD=== Executing 2nd query......writing to mysql-30627.1495.5394.7533
===7.PARENT=== ...2nd query has been read
id_car type plate date_rent date_returned
1 fiat BG-457 2012-07-18 00:00:00 2012-07-20 00:00:00
2 renault AS-1234 2012-07-20 00:00:00 2012-07-25 00:00:00
3 fiat JYB-2856 2012-06-23 00:00:00 2012-06-24 00:00:00
===6.CHILD=== End 2nd query
===8.CHILD=== End of background jobs
正如您在2和5之间看到的那样，我的第一个数据库结果集！ 您希望在查询和“主”程序之间获得中间结果，并且拥有它们！
你可能想知道：“嘿，我想获得一个数据库锁，在冻结方面停止会话，而我的'主'程序做其他事情！” 嗯，也有一个答案。 函数'worker'有2个注释命令：
#####################################
#\. $TMPSCRIPT
#####################################
通过启用它们执行挂起!!!而已！现在，您可以在调度程序中开始执行操作，并在完成后发送“echo”，如下所示：
echo > $TMPSCRIPT
通过向我们的工作人员添加这行代码，我们转换为阻塞线程调度程序和工作者：现在两者都互相倾听。示意性地：
dispatcher
| start
A ----------------> worker
| |
B (read blocks) C
| signals |
B (unblocks)<--------------- D (write blocks)
| |
E (1st set of results) F
| |
| G (read blocks)
| signals |
H ----------------> G (unblocks)
| (write blocks) |
如果你需要更多的兴奋，那么发送
echo 'set @x=@x+1;' > $TMPSCRIPT
其中'@x'是用户定义的变量！我们在这里有什么？好吧，我们只是将“实时代码”注入我们的sql作业！不仅仅是伟大的!!!
现在，可以获取数据库的LOCk，进行逻辑备份，停止并继续mysql客户端，并在数据库LOCK仍处于活动状态时执行任何操作！
它可能更好：创建二进制备份怎么样？ （每年为您节省2,000-10,000美元，请参阅：https://shop.oracle.com）
它可能比更好更好：可以锁定一堆10个数据库并启动备份，第一个完成向调度程序发送确认，该调度程序在列表中添加另一个数据库，直到它变空！ 只记得每个工人我们都需要2把锁用于双向锁定。
我们甚至可以同时启动3个调度员，因为它们不可能面对命名管道的碰撞，因为它们在太空中是独一无二的。
这是我的2,000美元建议
甜！
（centurian）
答案 2 :(得分：0)
以相同的连接运行您的SQL：
mysql="mysql -h dbhost -u user -pPassword diskcopydb"
echo "LOCK tables diskcopy WRITE;SELECT SQL_NO_CACHE * FROM diskcopy WHERE status=\"request\"" | $mysql
否则，锁定用于第二次运行mysql时已经死的会话。