Ramblings of a web guy

So, one of master server’s disk filled up with replication logs last night. We had a permission issue due to our data center move. An oversight on my part. Anyhow, I got to the server and purged the old logs and the master server started responding with no problems. However, the slaves did not. Both slaves connected to the master would not respond to a slave stop. Nor would they respond to a mysqladmin shutdown. I had to kill -9 the daemons to get them to stop. When I restarted the daemon and started the slave, all was fine.

I am hoping someone knows what is going on or can tell me what to send in a bug report. I don’t want to just put that in a bug report. It is kind of lame and useless.

Update: This may be my problem. Bug #31024.  I certainly did not give the slave a long time to stop.

MySQL master servers won’t remove their logs automatically when slaves are done with them. You can set expire_logs_days to remove them after a certain number of days. But, you are not assured that a slave is done with the logs. So, we wrote a script to connect to slaves and then purge logs on the master servers. It works for us. Your mileage may vary. BSD style license. Enjoy.

flush_mysql_master.php.gz

I have seen it time and time again. And yet, every time, it irritates me to no end. You are on a professional web site. You are navigating around and at some point you hit the link for their forums. And just like that you feel transported to another place. The whole site design just changes. Colors, layout, navigation… everything. Here are some examples, including the new C7Y site from php|Architect which inspired this post. (I really do love you guys on the podcast I promise =)

• php|architect’s C7Y - main site - forums
• Zend’s Developer Zone - main site - forums
  Zend’s forums do at least use the Zend.com header, but you can’t get to the forums from the main Zend.com site. You have to go to the Developer Zone.
• TextPad (great windows editor) - main site - forums
  The header is kind of the same. Fonts and link colors change slightly though which is worse in some ways than a wholesale change. It looks like they just wedged in their HTML into the phpBB template.

I could continue to list some here, but you get the idea. So, what is the problem? Does most message board software make it too hard to edit their templates? Are forums an after thought and some underling is given the task to make them work and not allowed access to the main site’s templates?

Some people do better at it. MySQL for example. Theirs is still not perfect. An ad awkwardly appears in the forums in a way that makes it look like an error. However, thanks to Phorum (cha-ching), MySQL was able to make their own log in system work with their forums. Heck, even at dealnews I have not done that. Mostly because our forum logins predate our site accounts for email alerts and newsletters. I am not asking for perfection though. I would just like to feel like the company/entitiy gave some love to making their forums part of their site and not an afterthought.

So, I call for all web sites to start treating their forums like real pages. Give them the same love and attention you give that front page or any other page. And, if your message board software makes that hard, give Phorum a try.

I first saw it on dealnews, and read more on Google News.  So, selling lame stuff for really high prices doesn’t pay the bill?  Huh.  Who knew?

I had mentioned a while back that I submitted three proposals for the 2008 MySQL Conference.  Well, two were accepted.

From one server to a cluster

In the last 10 years, dealnews.com has grown from a single shared hosting account to an entire rack of equipment. Luckily, we started using PHP and MySQL very early in the company’s history.

From the early days of growing a forum to surviving Slashdotting, Digging and even a Yahoo! front page mention, we have had to adapt both our hardware and software many times to keep up with the growth.

I will discuss the traps, bottlenecks, and even some big wins we have encountered along the way using PHP and MySQL. From the small scale to using replication and even some MySQL Cluster.  We have done many interesting things to give our readers (and our content team) a good experience when using our web site.

MySQL hacks and tricks to make Phorum fast

Phorum is the message board software used by MySQL. One reason they chose Phorum was because of its speed. We have to use some tricks and fancy SQL to make this happen. Things we will talk about in this session include:

• Using temporary tables for good uses.
• Why PHP and MySQL can be a bad mix with large data sets.
• What mysqlnd will bring to the table with the future of PHP and MYSQL.
• How Phorum uses full text indexing and some fancy SQL to make our search engine fast.
• Forcing MySQL to use indexes to ensure proper query performance.

You can find my conference page here.  (as Terry would say, me, me, me!)

The PHP manual basically tells you not to use Apache 2 with a threaded MPM and PHP as an Apache module. In general, it may be good advice. But, at dealnews.com, we have found it very valuable.

Apache threaded MPMs

Well, first, what is an MPM? It stands for Multi-Processing Module. It is the process model that Apache uses for its children process. Each request that comes in is handed to a child. Apache 1 used only one model for this, the prefork model. That uses one process per Apache child. The most commonly used threaded MPM is the Worker MPM. In this MPM, you have several processes that run multiple threads within it. This is the one I will be talking about. You can read more on Apache MPMs at the Apache web site.

Huge memory savings

With the Apache prefork or even FastCGI, each apache/php process allocates its own memory. Most healthy sites I have worked on use about 15MB of memory per apache process. Code that has problems will use even more than this. I have seen some use as much as 50MB of RAM. But, lets stick with healthy. So, a server with 1GB of RAM will only realistically be able to run 50 Apache processes or 50 PHP children for FastCGI if each uses 15MB or RAM. That is 750MB total. That leaves just 256MB for the OS and other applications. Now, if you are Yahoo! or someone else with lots of money and lots of equipment, you can just keep adding hardware. But, most of us can’t do that.

As I wrote above, the worker MPM apache uses children (processes) and threads. If you configure it to use 10 child processes, each with 10 threads you would have 100 total threads or clients to answer requests. The good news is, because 10 threads are in one process, they can reuse memory that is allocated by other threads in the same process. At dealnews, our application servers use 25 threads per child. In our experience, each child process uses about 35MB of RAM. So, that works out to about 1.4MB per thread. That is 10% the usage for a prefork server per client.

Some say that you will run out of CPU way before RAM. That was not what we experienced before switching to worker. Machines with 2GB of RAM were running out of memory before we hit CPU as a bottleneck due to having just 100 Apache clients running. Now, with worker, I am happy to say that we don’t have that problem.

Building PHP for best success with Worker

This is an important part. You can’t use radical extensions in PHP when you are using worker. I don’t have a list of extensions that will and won’t work. We stick with the ones we need to do our core job. Mainly, most pages use the mysql and memcached extension. I would not do any fancy stuff in a worker based server. Keep a prefork server around for that. Or better yet, do funky memory sucking stuff in a cron job and push that data somewhere your web servers can get to it.

Other benefits like static content

Another big issue you hear about with Apache and PHP is running some other server for serving static content to save resources. Worker allows you to do this without running two servers. Having a prefork Apache/PHP process that has 15MB of RAM allocated serve a 10k jpeg image or some CSS file is a waste of resources. With worker, like I wrote above, the memory savings negate this issue. And, from my benchmarks (someone prove me wrong) Apache 2 can keep up with the lighttpds and litespeeds of the world in terms of requests per second for this type of content. This was actually the first place we used the worker mpm. It may still be a good idea to have dedicated apache daemons running just for that content if you have lots of requests for it. That will keep your static content requests from over running your dynamic content requests.

Some issues we have seen

Ok, it is not without problems (but, neither was prefork). There are some unknown (meaning undiagnosed by us) things that will occasionally cause CPU spikes on the servers running worker. For example, we took two memcached nodes offline and the servers that were connected to them spiked their CPU. We restarted Apache and all was fine. It was odd. We had another issue where a bug in my PHP code that was calling fsockopen() without a valid host name and a long timeout would cause a CPU spike and would not seem to let go. So, it does seem that bad PHP code makes the server more sensitive. So, your mileage may vary.

As with any new technology, you need to test a lot before you jump in with both feet. Anyone else have experience with worker and want to share?

One last tip

We have adopted a technique that Rasmus Lerdorf had mentioned. We decide how many MaxClients a server can run and we configure that number to always run. We set the min and max settings of the Apache configuration the same. Of course, we are running service specific servers. If you only have one or two servers and they run Apache and MySQL and mail and dns and… etc. you probably don’t want to do that. But, then again, you need to make sure MaxClients will not kill your RAM/CPU as well. I see lots of servers that if MaxClients was actually reached, they would be using 20GB of RAM. And, these servers only have 2GB of RAM. So, check those settings. If you can, configure it to start up more (all if you can) Apache process rather than a few and make sure you won’t blow out your RAM.

I have decided to try out Twitter.  Mostly it is a curiousity about what it does and how it works.  The recent outages baffle me.  It seems like a very simple application.  I am hoping to find the thing that makes it a tough thing to scale out and/or up.  (Other than it being built on Rails that is.)

http://twitter.com/brianlmoon

Call it paranoia.  Call it being prepared.  Whatever your stance, we are considering using more than one data center for dealnews.com.  It is not a capacity issue.  We can keep growing our current data center without a problem.  But, stories of power outages and power outages we have experience have us wanting to explore the idea.

Here is the problem.  No one in our company has experience with this.  And, there does not seem to be any resources on the internet talking about this.  Our problems are not so much with managing the data between the two.  The problem is failover and how to deal with one data center being out.  Here are some of the ideas that have been thrown on to the wall.

Round Robin DNS

This was the first idea.  It seems simple enough.  We have two data centers.  We publish different DNS for each data center and traffic goes to each one.  The problem here is that it is, well, random.

Global Traffic Management

There are devices that “balance” traffic  across multiple different locations.  But, I am unsure how those deal with outages at one of the locations.  It seems like there is still one point of failure.

BGP Routing

This is the biggest mystery to me.  I know what it is.  I know what it means.  I have no idea how to deploy this type of solution.  I understand that you can “move” your IP addresses with routing changes.  But, that means running routers.  Where are these routers?  Does this happen at some provider?  Is there a provider that handles this?  Does that mean that all of our data centers are with one provider?  I think one more peace of mind feature of this is that we would not be tied to just one vendor.  So, if one vendor had major issues or there was some legal troubles (we lived through the dot come boom and bust) we would have security in knowing we had other equipment that was not affected.

Is there something else?  Are we being way paranoid?  Maybe it is not cost effective in the end.  I/we have no idea really.  Anyone out there that has knowledge on this subject?

Luckily, I don’t have personal experience with them. But, based on the 2 to 3 users per week that come to the Phorum support forums and IRC, they have the worst support of any host on the internet.

Example 1

A user comes to the forums having trouble with his Phorum install. In the user’s words, GoDaddy tells him “they couldnt help me costumize my scripts because it wasnt their job”. In this case “customize” meant filling in the MySQL permissions into the Phorum config files. In the end, GoDaddy had to move him to a Linux hosting account. They claimed that the Windows hosting accounts do not support PHP. However, they are clearly wrong about their own hosting as this all started because the user received a PHP error about not connecting to MySQL.

Example 2

This user found that GoDaddy is using MySQL 4 on their servers. Their web site does not mention a version anywhere. So, users are locked in to a hosting plan before knowing this.

Example 3

This happened today in IRC. It was much like the first example. In this case, GoDaddy support told him “permissions are set via ftp”. Um, MySQL permissions are set via FTP? The user had a MySQL server name. For some reason it did not exist. So, either he typed it wrong or they gave him the wrong server name. Either way, their support should recognize this and be able to help their clients.

Example 4

This is less a support issue and just plain crappy of them IMO. GoDaddy does not allow the creation of temporary tables. The Phorum search engine makes use of them to save lots of CPU and memory on the PHP side. Luckily for their users, Thomas felt sorry enough for them to make a module that used good old fashioned slow LIKE queries. So, that will work until their account is shut off because they have search queries clogging up the database servers.

So, if you are on GoDaddy, I feel for you. From where we sit, it really seems like they do not provide very good support. We end up having to support their users for things that GoDaddy should be able to answer.

So, I checked my email this morning and found this jewel:

I might use Phorum if you brain deads knew how to upload or download your files via FTP. Your documentation has no order to it, its all a mess. I even dropped a release level to see if it was just that release. Ill give you a clue, DONT TRANSFER YOUR FILES VIA AUTO, EXPECIALY YOUR TXT FILES. TRANSFER THEM IN ASCII MODE ONLY, THIS INCLUDES YOUR PHP FILES. Then you just f—ing* MIGHT get readable files. Now you might say hey wait a min, we have full documentation on our web site, but you forget, someone has to open the sample.config.php file and read the crap that resides there.

* edited for content

Should I respond?  If so, how?  I decided to respond in as nice a way as I could.

 I normally don’t answer direct support emails.  Neither do I normally answer very angry emails.  However, I view this as an educational experience.

Judging by your email, I would say you are using Notepad on Windows to edit and read files.  That is mistake number one.  Notepad only reads one file format: Windows text files.  Windows natively uses a CRLF for it’s line endings.  It is the only operating system that does so. Notepad is the only application on the Windows platform that only reads that format.  If you would use Wordpad instead, this would not have been a problem for you.  For some reading on the subject, you may want to read:

http://en.wikipedia.org/wiki/Newline
http://www.cs.toronto.edu/~krueger/csc209h/tut/line-endings.html

Because PHP scripts are most commonly deployed on a Linux platform, the Unix line feed (LF or \n) is best for PHP applications.  Here are some suggestions for some great text editors for Windows.

TextPad - http://www.textpad.com/
Metapad - http://www.liquidninja.com/metapad/
PSPad   - http://www.pspad.com/en/

I hope this has helped educate you on the world of new lines and how real programming works.  In the future, a kind word in the forums would be much more appreciated than an email like this.  Not all people would be as kind as I am being and want to help you grow.

What do you think?  Should have just let this guy go?  Should have been as ugly to him as he was to me?