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# How NOT to test that mysqld is alive

MySQL Analysis Monitoring October 1, 2009

I had a call from a new customer last week. They had issues with their MySQL server. Apparently, it was repeatingly crashing, every few hours. To have their production system kept alive, they used a script to periodically see if MySQL was still alive, and if not – start it.

I was first thinking in directions of old installations; wrong memory allocations (too little memory for large content; to large memory allocation for weak machine). When I reached the customer’s premises, I quickly reviewed general settings, while explaining some basic configuration guidelines. There were peculiarities (e.g. query_cache_limit being larger than query_cache_size), but nothing to obviously suggest a reason for crash.

I then observed the error log. Took some time to find it, since the log_error parameter appeared twice in the my.cnf file; first one appearing at the very beginning, the second (overwriting the first) was far into the file.

Sure enough, there were a lot of startup messages. And… shutdown messages. In fact, the log looked something like:

>     090923 17:38:15  mysqld started
>     090923 17:38:16  InnoDB: Started; log sequence number 0 3707779031
>     090923 17:38:16 [Note] /usr/local/mysql/bin/mysqld: ready for connections.
>     Version: '5.0.77-log'  socket: '/tmp/mysql50.sock'  port: 3306  MySQL Community Server (GPL)
>     090923 19:53:41 [Note] /usr/local/mysql/bin/mysqld: Normal shutdown
>     090923 19:53:56  mysqld started
>     090923 19:53:56  InnoDB: Started; log sequence number 0 5288400927
>     090923 19:53:56 [Note] /usr/local/mysql/bin/mysqld: ready for connections.
>     Version: '5.0.77-log'  socket: '/tmp/mysql50.sock'  port: 3306  MySQL Community Server (GPL)
>     090929 22:38:48 [Note] /usr/local/mysql/bin/mysqld: Normal shutdown
>     090923 22:38:58  mysqld started
>     090923 22:38:58  InnoDB: Started; log sequence number 0 7102832776
>     090923 22:38:58 [Note] /usr/local/mysql/bin/mysqld: ready for connections.
>     Version: '5.0.77-log'  socket: '/tmp/mysql50.sock'  port: 3306  MySQL Community Server (GPL)
>     ...

(The above is just a sample, not the original file).

Well, the log says there’s a lot of normal shutdowns.

#### Looking at the script

Next it was time to look at the script which was supposed to verify MySQL was up and running – else wise start it. And it went something like this:

>     while [ 1 ];
>       if [`ps aux | grep mysqld | wc -l` -lt 2]
>         then /etc/init.d/mysql restart
>       ...
>       sleep 60

The script was looking for all processes, then grepping for mysqld, counting number of lines in output. It expected 2 lines: one for the mysqld process, one for the grep mysqld process itself.

If you don’t know what’s wrong with this, a very brief explanation about how pipelines work in unix work it at place:

Pipelined processes do not execute one after another, or one before another. They are all executed at once. So, “ps aux | grep mysqld | wc -l” immediately spawns ps, grep, wc, then sets the standard output of one to the standard input of the other (most simplified description I could think of).

It is likely that grep will outrun ps in the race for starting up. So grep is started, already waiting for input from ps, which then notices both mysqld is alive, but also grep mysqld, lists them both along with other processes, to be filtered by grep, to be counted by wc (returning two lines count).

But this is just because ps is heavier than grep. It doesn’t have to be like that.

#### The less common scenario

Every once and again, ps outruns grep in the race for starting up. It would list the active processes, and no “grep” would appear in the listing. grep would then run, filtering the result of ps, then to be counted by wc — oh, there is only one line now! The script assumes this means mysqld is down (since it assumed grep is always there), and restarts mysqld.

So, the script which was supposed to take care of MySQL crashes, was actually causing them (though no crash occurred).

#### Better ways to test that MySQL is alive

Look for the pid file. This is the standard (that’s how the mysql service works). Look for the unix socket file. Both require that you parse the my.cnf file to learn where these files are.

If you’re reluctant to read the configuration files, other options are at hand. OK, look for the process; but use pgrep mysqld. No need to list all processes, then grep them.

And best way, that will set your mind at ease even if you’re worried that “mysql is running but not responding; it is stuck”: connect to MySQL and issue SELECT 1, SELECT NOW(), SELECT something. This would be the ultimate test that MySQL is up, listening, responding and valid.

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Guest
Istvan POdor

ohh those good old times when were so beginners.. This article makes me remember my own mistakes 🙂

Guest
Tim Jinkerson

or
if [`ps -aux | grep mysqld | grep -v grep | wc -l` -lt 1]

Remove the grep from your output before you count it.

Guest
Mark Leith

“Just use mysqld_safe, huh?” 🙂

Author
shlomi

Mark,
Yes 😀

They had MySQL installed on Gentoo Linux. I’m not familiar with Gentoo at all; apparently the mysql package did not utilize mysqld_safe, but some Gentoo specific script to start/stop services.

On same note I usually prefer binary distribution to packaged distribution of MySQL; every linux distro wraps it up differently, uses different paths. conventions.
With binary I set all up manually from grounds up.

Guest
kedar

How about “mysqld status”?

Author
shlomi

@kedar,

thanks, yes. To elaborate, “mysqld status” will do the following:
look for the PID ;
try kill -0
attempt pidof mysqld

Really, connecting to MySQL and initiating a query is, in my opinion, the most reliable way.

Guest
kedar

Correct I just went through the /etc/rc.d/init.d/functions for “status”.

Author
shlomi

@kedar,
so did i 😉

Guest
Ronald Bradford

When I first read your article my initial though was , the number needs to be 2. As @mark wrote, where is mysqld_safe
It bothers me when distro’s think they can do stuff better.

It is sometimes of benefit to have multiple checks to determine or triangulate issues. Other checks include mysqladmin ping, netstat for 3306, however anything that checks for pid’s, ports, sockets etc are bound to having to support the variable nature etc.

mysqld_safe is designed to undertake exactly the situation your script attempts to describe. Seems the most logical solution is to implement mysqld_safe.

Author
shlomi

@Ronald,

Not my script 🙂

I’ve made a few recommendations for the customer.
The customer has no expert sys-admin. For some reason they prefer Gentoo to other, more popular distros; I therefore did not wish to complicate stuff by throwing in a binary distribution they didn’t know how to handle. Using the Gentoo repository was their familiar method of installing new apps.

Repeating my #4 comment, I personally prefer binary distribution (tgz download) and manual install over distro’s package; more control over which version; no accidental undesired upgrades etc.

Guest
Gerry

Need to know the socket (or any variable for that matter)? In bash:

my_print_defaults mysqld | grep socket | cut -d “=” -f 2

Wrap in $(…) and you can include it in any alias / script.

Another way to check if it’s up:

mysqladmin -u user -ppass version

The user could be one w/ only USAGE granted.

My $.02
G

Author
shlomi

@Gerry,

Cool $.02

Guest
mark

I do phone technical support. I never even considered this possibility. I will keep this in mind. Thanks for the knowledge drop.

BTW, I like Tim Jinkerson’s way of doing this. It’s probably the easiest way to do it.

Guest
cacdn

if you really must use grep for finding a process, try

| grep [m]ysqld

Guest
Mina

tangent:

To address the general problem of grepping theough ps’s output and grep finding itself, there are a couple of options:

* Switch to “pgrep”

* Make use of a clever trick: Grep supports regexes. If you switch the first letter of what you’re grepping for with a regex’s character class (by surrounding it with [ ]), it’ll still match what you want, but not the grep pattern itself: example: “grep [m]ysqld”

Guest
Chad Miller

The best thing to to is to make a regular expression that does not match itself.

ps x |grep fo[o]

The brackets of the character class containing the single letter are metacharacters that contribute to how the grep matches, and while the text that it matches is exactly the same as with no character class, it ensures that the characters that make it up do not match.

Guest
Josh

You can also use “ps -C mysqld > /dev/null” and test $?.

Guest
Simon

You could always use:

if [`ps -aux | grep [mM]ysqld | wc -l` -lt 1]
then blah blah blah
fi

Guest
DerekP

While there are better ways of doing this that have been noted, there are a couple ways they could’ve made their chosen method work better.

A workaround I’ve used in the past is to replace “grep mysqld” with “egrep [m]ysqld”, so that grep doesn’t find itself in the ps list.

Also, a lot of headache could have been saved if they made some type of log entry saying that the script was restarting the service instead of just issuing the command. This would also help finding trends.

Guest
Daddio

I have always used “mysqladmin ping” to test whether the service is alive and check for exit status of 0

mysqladmin ping
if [ $? -ne 0 ]
then
/etc/init.d/mysqld start
else
echo “All is well”
fi

Guest
JohnK

It all sounded very technical, your description of piped processes running simultaneously but in my experience earlier ones always feed into later ones. The issue was more likely a logic error somewhere or simply not testing for the right number. A cut and paste of the actual faulty script would have been good. On my Linux box a normal mysql install runs 2 processes and the grep process would take the minimum count to 3. So they should perhaps have been testing for less than 3 and not 2. However my main point is that I seriously doubt the accuracy… Read more »

Guest
Mihai Limbasan

Instead of connecting and running a query you can use mysqladmin ping then check the exit status (zero for alive, non-zero for dead.) That’s the “official” way to check it, won’t have the overhead and the potential problems of running a query, and will avoid false positives from other processes with names that include mysql.

Guest
Fred

If really using pgrep, at least do it correctly: pgrep “^mysql$” (or whatever the process is named, taking care of quoting any special meaning characters.)

Guest
RobA

Really the best way is the Query, When you are really old you will realize that even when a process is hung it still shows up in ps listings. Do the query because it confirms the process is handling requests and not overloaded.

Guest
sambeau

I wrote a little C program & an article on how to do exactly this at my last company..

http://knowledgehub.zeus.com/articles/2006/02/27/custom_monitors_for_mysql_in_c

Might be of help to anyone who as read this.

Cheers.

Guest
Lucas

Please, everyone, stop using these hacks. At their very best, they are inelegant and have race conditions. Instead, use something like daemontools or runit. They (a) automatically restart a daemon that dies, and (b) provide race-free reporting of PID and uptime without a kludge like “ps | grep”.

Guest
Chad

Never check for pid files. Never ever ever. It is entirely possible for a process to die abruptly (say, after getting a SIGKILL, after a kernel panic, power outage, the list is endless) and not have the opportunity to remove its pidfile. The single best test mechanism is to directly query the daemon with a short, low-intensity test.

Hell, or just do `ps aux | grep -v grep |grep mysqld | wc -l`

Guest
Adam Nelson

This is craziness. 1. Why is a company without a Sys Admin running Gentoo (not your fault – but somebody needs to tell them that the scenarios is worse than less than ideal). Ubuntu really is a far superior solution for them. 2. MySQL doesn’t just ‘go down’. I understand hacks are necessary sometimes, but auto-restarts usually cause more problems than they solve. Automatic failover (i.e. master-slave switch) is a better solution if maintaining uptime without intervention is necessary. 3. An uptime script should use app-specific code like ‘SELECT * FROM table1 LIMIT 1’ to confirm that the database isn’t… Read more »

Guest
Ammon Lauritzen

ps -Cmysqld –no-header

Will return exactly one line per running ‘mysqld’ process. No grep required.

Guest
Zach

This is how I do it.

sqlsocket=`grep socket /etc/my.cnf | awk -F = ‘{ print $2 } ‘`

count=0;
echo -n “Checking MySQL”

while [ ! -S “${sqlsocket}” ]; do
sleep 1;
echo -n “.”
count=$((count+1));
[ $count -gt 5 ] &done

Author
shlomi

@JohnK

I sincerely assure you that I’ve seen this happening with my own eyes, even while sitting at the customer’s office: by typing the script manually a few times, I did in fact stumbled on the rare case where it fails.
I can’t see how you mean “logic error somewhere or simply not testing for the right number”; while I can’t put my hands right now on the original script, I assure you it was all on the `ps aux | grep mysqld | wc -l` part.

Regards

Guest
originalgeek@mac.com

Your n00bness comes oozing out right through your proposed solutions. If you are going to tolerate a system where a SQL daemon is going offline, which you should not in the first place, the first solution is the correct one, with a modification: grep for the full path to mysqld. This is a superior technique to any of the other techniques you mentioned. Why? Let me enumerate: pgrep will tell you if any mysqld is running on your system, not necessarily the one you want running. same problem with looking for sockets. popping in to do a SELECT in mysql,… Read more »

Author
shlomi

@Adam (#28)
1. I agree, and I have.
2. That’s why they looked up a consultant; they couldn’t explain this behavior (albeit it was created by their own doing)
3. completely agree

“…SysAdmin community…whoever wrote that script needs to know that it’s totally unacceptable…”
The man who wrote the script is not a sys admin expert. Neither am I, for that matter. I am experienced in many fields, yet still make mistakes. When I’m tired, I can make newbie mistakes. It happens. I agree that a sys-admin review of one’s system in in place once in a while.

Author
shlomi

@Chad (#27)

Thanks. Yes. But take a look at the mysqld service source code, and see they actually use both methods…

Author
shlomi

Thanks everyone for your comments!

Guest
Jake

Salami,

I tried your suggestion but got the following error

mysql> SELECT something;
ERROR 1054 (42S22): Unknown column ‘something’ in ‘field list’

Please help!!

Author
shlomi

@originalgeek@mac.com (#32) In this case, allow me to further point out: You are right that more than one instance of mysqld might be runnign. But your proposal of checking for the full binary path is still dangerous: with proper configuration it’s possible to run many instances using the same binary. “…popping in to do a SELECT in mysql, when the daemon is torched, will take quite a few seconds for the connect request to timeout. …” Not sure what “torched” means (not a native English speaker). But I fail to see you reasoning: if the daemon is up yet non… Read more »

Guest
Sean

Ok, obviously this is a silly way to check if a process is running. A better way to do this, although still silly, is just to run a cron job every mnute that does /etc/init.d/mysql start

If mysql is already running, nothing will happen, But if it’s not, it will start, unlses there’s an actual problem with mysql.

Author
shlomi

@Sean,
Depending on your distribution, your “mysqld start” may do just the same thing: check of pid file, pidof mysqld etc.

Guest
JohnK

@schlomi I completely accept that there was something wrong with their script. However in my experience piped processes accept ALL of the stdin. They don’t somehow ‘lose’ input and return varying results. I have seen processes and grep being listed in different order but I have never seen any of the processes or grep NOT being listed whether they are executed simultaneously or not. I suggest that they simply tested for -lt 2 and it should have been -lt 3. No matter what everyone says about how it was a n00b error, everyone makes mistakes when writing scripts and the… Read more »

Guest
Ali

pgrep seems like the best idea, but a very quick way to fix the original is just to use `ps -e` since it won’t list process arguments.

Author
shlomi

@JohnK (#40) John, just now, on my laptop, see the following real output: shlomi@******:~$ ps aux | grep mysqld root 8708 0.0 0.0 4020 620 pts/0 S Oct04 0:00 /bin/sh ./bin/mysqld_safe --datadir=/usr/local/mysql/data --pid-file=/usr/local/mysql/data/piglet.pid mysql 8738 0.0 4.8 313584 150308 pts/0 Sl Oct04 1:21 /usr/local/mysql/bin/mysqld --basedir=/usr/local/mysql --datadir=/usr/local/mysql/data --user=mysql --pid-file=/usr/local/mysql/data/******.pid --skip-external-locking --port=3306 --socket=/tmp/mysql50.sock shlomi@******:~$ ps aux | grep mysqld shlomi 2175 0.0 0.0 7452 876 pts/5 S+ 22:28 0:00 grep mysqld root 8708 0.0 0.0 4020 620 pts/0 S Oct04 0:00 /bin/sh ./bin/mysqld_safe --datadir=/usr/local/mysql/data --pid-file=/usr/local/mysql/data/piglet.pid mysql 8738 0.0 4.8 313584 150308 pts/0 Sl Oct04 1:21 /usr/local/mysql/bin/mysqld --basedir=/usr/local/mysql --datadir=/usr/local/mysql/data --user=mysql --pid-file=/usr/local/mysql/data/******.pid --skip-external-locking --port=3306… Read more »

Guest
P

How about:

ps ax | grep mysql[d]

and do the necessary wc logic

Author
shlomi

@Jake (#36),

My mistake: should read as
SELECT that_other_thing;

Guest
JohnK

@shlomi. Ok. I see what you see now. However I only see it in the context of a statement within backticks.
Your laptop example shows a consistent 3 processes on mine. (Opensuse 11.0) I.e. ps aux | grep mysqld shows 2 mysqld processes and a grep process every time. An if statement with a backtick test statement though, drops the grep statement at random.
I’ve never seen that before as I’ve always filtered the grep process in scripts. (… and never tried to implemented such a stupid restart policy.) I stand corrected.

Author
shlomi

@JohnK

Interesting about only backticks on your distro/machine.

Cheers

Guest
Siddharth Bhattacharya

Here is the corrected Startup Script say
#start.sh
while [ 1 ];
if [ `ps aux | grep mysql | grep -v mysql | wc -l` -lt 1 ]
then /etc/init.d/mysql restart
…

Also add a cronjob using command:
crontab -e
* * * * * start.sh

Guest
sharks

Mihai @ 22: ‘mysqladmin ping’ wins 🙂
You win a … uh, something.

Everyone else discussing ps and various perverse grep misadventures, please don’t do it. All it takes is someone running
‘mysqldump’ or ‘man mysqld’ or ‘vi /tmp/why_is_mysqld_restarting.txt’ to generate a false positive.

pidfiles are also unreliable if the server has died unexpectedly.

Use ‘mysqladmin ping’. It goes away, parses the mysql config files for you, connects to the mysqld.sock and checks that mysqld is actually working.

Author
shlomi

@sharks
I agree, please re-read the last paragraph in my post.
Please also note the duality within mysql: “mysqladmin ping” conencts via socket etc., while “mysqld status” checks for pid file and process id.

Regards

Guest
FredV

looks like some sysadmin wannabe does not even know how to use awk and regular expressions, any kid should see the holes in that solution

for procf in /proc/[0-9]*; do
if [ “$(cat $procf/exename)” = “/usr/bin/mysqld” ]; then
echo “process is running”
fi
done

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