How to force-drop a postgresql database by killing off connection processes

Ever need to drop a postgresql database, but it would not let you because there are open connections to it (from a webapp or whatever)?

Quite annoying.  If on a production server, and other databases are being used, restarting postgresql is a last resort, because it generates downtime for your site (even if small).

I finally took the time to scratch around and find the answer.

As a super user, to list all of the open connections to a given database:

select * from pg_stat_activity where datname='YourDatabase';

As a superuser, to drop all of the open connections to a given database:

select pg_terminate_backend(procpid) from pg_stat_activity where datname=’YourDatabase’;

Or for 9.x, change `procpid` to `pid`

select pg_terminate_backend(pid) from pg_stat_activity where datname='YourDatabase';

Here are some references to the functions:

Ubuntu Post-Install tips…

I received this from a friend, and thought I would post it here in case anyone would find it useful.

After Installing Ubuntu, basically I do this:

Go to:
System -> Administration -> Software Sources -> Other Sofware, and enable partner repository.

After that, we can this on a Terminal:

sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install ubuntu-restricted-extras
sudo /usr/share/doc/libdvdread4/

PostgreSQL Dump and Restore Notes

The pg_dump and pg_restore commands provide excellent flexibility in storing a compressed dump file, and selectively restoring any part of it.

I’ve found that dropping and re-creating the target database is the cleanest way to restore a dumpfile — no stray relations left to cause trouble.

Unless you own all of the objects being restored, you may need to be SUPERUSER in order to have a successful restore.

The custom dump format is quite useful.  Unlike the normal sequence of SQL statements you may be used to from mysqldump (and pg_dump as well), the –format=custom option will create a compressed archive file (internally a tar file) that can be selectivly read with pg_restore.  That flexibility could come in handy if you *just* need the schema from 1 table, or *just* the data from another table.

pg_dump –format=custom -U jason_super MyDatabase > MyDatabase.pgdump

pg_restore –exit-on-error –clean –dbname=MyDatabase MyDatabase.pgdump

Get all of the SQL
pg_restore TMTManage_2.pgdump | more

Get some of the SQL
pg_restore –schema=ACRM –table=Admin TMTManage_2.pgdump | more

nginx restart error

Was playing around with nginx on Centos 5 (EPEL package).

Most of the time I ran:

service nginx restart

I would get this message in the /var/log/nginx/error.log file:

panic: MUTEX_LOCK (22) [op.c:352].

After some hunting around, it appears to be a known bug in nginx (perhaps perl in nginx?)… Anyway, a simple workaround is to do this:

service nginx stop
service nginx start

Or, simply edit /etc/init.d/nginx, and add the sleep 1 line:

51 restart() {
52     configtest || return $?
53     stop
54     sleep 1
55     start
56 }

Nice workround!

Example Automated MySQL Backup Script

Rather than use --all-databases, which will prevent you from being able to selectively restore any single database, consider the following:

Ideally, you should have a daily backup, with some history. It should be bulletproof (--force), it should be logged (>> ...log), it should be compressed (| gzip), it should keep separate copies of each database, and it should automatically pick up any databases that are added.

Consider, rather, a shell script like this:


Dump="/usr/bin/mysqldump --skip-extended-insert --force"

Today=$(date "+%a")

# Get a list of all databases
Databases=$(echo "SHOW DATABASES" | $MySQL -h $Host)

for db in $Databases; do
        echo "Backing up '$db' from '$Host' on '$date' to: "
        echo "   $file"
        $Dump -h $Host $db | gzip > $file

Which is assuming that you have a file ~/.my.cnf (chmod 600), that has:

user = "BACKUP"
password = "SOMEPASS8342783492"

Make sure that whatever user you are using for BACKUP has this grant statement:

  TO BACKUP@localhost

So simply add this to a nightly cronjob, and you have a daily backup that rotates each 7 days week.

0 3 * * *   backup-mysql >> backup-mysql.log 2>> backup-mysql.log

The backup directory then contains:

-rw-r--r-- 1 backup backup 2217482184 Sep  3 13:35
-rw-rw-r-- 1 backup backup 2505876287 Dec 25 00:48
-rw-r--r-- 1 backup backup 2500384029 Dec 21 00:48
-rw-r--r-- 1 backup backup 2506849331 Dec 26 00:48
-rw-r--r-- 1 backup backup 2499859469 Dec 20 00:48
-rw-rw-r-- 1 backup backup 2505046147 Dec 24 00:48
-rw-rw-r-- 1 backup backup 2502277743 Dec 22 00:48
-rw-r--r-- 1 backup backup 2504169910 Dec 23 00:48
-rw-r--r-- 1 backup backup   76983829 Dec 25 00:49
-rw-r--r-- 1 backup backup   76983829 Dec 21 00:49
-rw-r--r-- 1 backup backup   76983829 Dec 26 00:49
-rw-r--r-- 1 backup backup   76983829 Dec 20 00:48
-rw-rw-r-- 1 backup backup   76983829 Dec 24 00:49
-rw-rw-r-- 1 backup backup   76983829 Dec 22 00:49
-rw-r--r-- 1 backup backup   76983829 Dec 23 00:49
-rw-r--r-- 1 backup backup  304803726 Dec 25 00:49
-rw-r--r-- 1 backup backup  303480087 Dec 21 00:49
-rw-r--r-- 1 backup backup  304710121 Dec 26 00:49
-rw-r--r-- 1 backup backup  303791294 Dec 20 00:49
-rw-rw-r-- 1 backup backup  305315415 Dec 24 00:49
-rw-rw-r-- 1 backup backup  302516217 Dec 22 00:49
-rw-r--r-- 1 backup backup  303314217 Dec 23 00:49
-rw-r--r-- 1 backup backup     135301 Dec 25 00:30
-rw-r--r-- 1 backup backup     135301 Dec 21 00:30
-rw-r--r-- 1 backup backup     135301 Dec 26 00:30
-rw-r--r-- 1 backup backup     135301 Dec 20 00:30
-rw-rw-r-- 1 backup backup     135301 Dec 24 00:30
-rw-rw-r-- 1 backup backup     135301 Dec 22 00:30
-rw-r--r-- 1 backup backup     135301 Dec 23 00:30

Freeky Bug

Ever have one of those bugs that customers complain about, but you just cannot reproduce it? Here is a good one…

Customers were complaining about being logged out when clicking a download link.

This particular setup is a Cisco CSS 11501 series load balancer with 2 Dell Poweredge web servers sitting behind it.  Each webserver is running apache, as well as an application server (python) which handles authentication and processing for THAT server.

For weeks, I could not reproduce this bug.  So tonight when I finally got bit by it (at home), I was clueless for a while.  The code is so simple.  A simple key lookup in a simple dictionary, yet it just was not making sense.

Here is the story:

A while ago, we were having problems with Internet Explorer downloading content over SSL.  This turns out to be a common problem with IE, so to fix it, I caused the downloads to not use SSL, which is more efficient anyway.

We use a cisco hardware load balancer which balances incoming requests to different backend servers.  It has a feature called STICKY SOURCE IP, which means that any connections routed from the same IP to the same site will be delivered to the same backend server.  This is nice, because you are always visiting the same server.

So as it turns out, by turning the download SSL off, the load balancer was using another “site” definition to handle the DOWNLOAD request.  STICKY SOURCE IP was out the window, and the request was being passed back to a “random” webserver.

About 50% of the time, users (like me tonight) were tossed to the other server, which knew nothing about the user login. That is why it was complaining about the “WB4_App::$DSEG and/or WB4_App::$AuthToken must be set in order to contact the     applications server.” error message, which is not one that should normally be shown.

To make matters worse, our IP address at work was apparently always using the same server, so I could not reproduce the problem.  I’m lucky that it happened to me at home, or I would still be banging my head against the desk…

Interesting Thoughts on Cloud Server Performance

Apache load testing on a Cloud Server – Jason – 7/31/2009

I recently created a cloud server for a wordpress blog, and configured it to the point that the blog was working OK.  Then I decided to check the performance aspects of the server, as it was a small 256 MB + 10GB machine.
Using apachebench (ab), I ran some load tests on the blog home page.  The server choked to death. It was swapping so bad, that RackSpace Cloud sent me this email:

This is an automatic notification to let you know that your Cloud Server,, is showing a considerable amount of consistent swapping activity. Quite often this is an indicator that your application or database are not as efficient as they could be. It also may indicate that you need to upgrade your Cloud Server for more RAM.

That’s strange…
I found that the response rate was:

4 requests per second, 10 concurrent connections

When the concurrency was raised to 50, the server died.  It took 10 minutes for it to calm down enough that I could LOG IN and KILL apache.
So upon further investingation, I found that the default httpd.conf configuration was WAY TOO LARGE:
We’re only working with 256 MB ram here, so if each apache process takes up any amount of memory at all, we have a low limit.

<IfModule prefork.c>
StartServers       8
MinSpareServers    5
MaxSpareServers   20
ServerLimit      256
MaxClients       256
MaxRequestsPerChild  4000

Only after drastically reducing the configuration to the following, did we get reasonable performance:

<IfModule prefork.c>
StartServers       4
MinSpareServers    2
MaxSpareServers   4
ServerLimit      4
MaxClients       4
MaxRequestsPerChild  4000

As it turns out, the performance went up considerably:

16 requests per second, 50 concurrent connections

Still, I thought that it could get better.  So I looked into installing some PHP opcode caching software.

The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code.

As it turns out, it was easy to install.

# yum install php-pecl-apc

And after restarting apache:

47 requests per second, 50 concurrent connections

Even during this load test, the site was still responsive from a web browser.
Not bad for a cheap little Cloud Server, eh?