morgamic.com stuff and things, according to Mike Morgan

Scalable PHP with APC, memcached and LVS (Part 2)

In part 1 of this post I talked about some of the challenges we encountered when trying to scale a LAMP application. It's pretty much what you'd read on danga's memcached site, just dumbed down.

So after some discussion, caffeine and Googling, you'll probably end up knowing you'll need:

• Memcached!
• An internal cache to speed up and optimize PHP across requests.
• To continue to find ways to slim down your application.
• Get more caffeine.

I had originally intended this post to be a summary of test results, but I am beginning to realize that what you get out of apache bench or httperf isn't really as important as how much thought you put into your application. If you think about it, all of these perf tests are just trying to quantify something qualitative, and the tests themselves are nowhere near as important as how you get there.

So instead of showing a lot of Excel or Omnigraffle graphs that won't help you very much, I'd much rather spend this time talking about the process. That way, you might be able to learn from our mistakes, and not make them yourself.

Together with the Mozilla infra team we worked together to put a lot of thought into this application, and that is what really made the biggest impact. In the end, the big win is just snappy pages in production -- and we've achieved that. And since I'm a massive tool, I'll draw a comparison between scalability and basketball.

For one, it takes teamwork and unselfishness to succeed. You need the sysadmins to be involved with the application developers from an early point, because they always ask the right questions -- and often times the obvious ones developers miss. You need good coaches who know the game, and can direct on both sides of the coin. And after all is said and done on the performance side of things, you need your fans -- the community -- to gauge your overall success.

You hope along the way that when the game's over, the score is in your team's favor and the fans are cheering.

So when you're planning your app, the best thing you can do is minimize your code by not including massive libraries or classes. Not to knock PEAR or overgeneralize things, but anytime you include a PEAR class, you have to be very careful. PEAR classes are often times bloated and written to "just make it work". They work well for your blog, or some weekend project, but if you need some serious performance, including a PEAR class is typically a bad decision.

Includes in PHP are a bit like interest rates -- it may seem like a small sacrifice to just include something, but over time and over a lot of requests, it can amount to a huge loss. Imagine if you had a 1% fee every time you hit the ATM. Seems like a minor sacrifice, it's just 1%, but everybody knows that you'd lose a lot over time. So why would you give up 1% over millions of PHP transactions? You should follow some simple rules when dealing with PHP includes in your application:

• Make your includes modular. You should allow yourself the ability to mix-and-match includes or class definitions. Some may have dependencies, that's fine, but you shouldn't limit yourself by making everything dependent on your DBI, for example. You should think about what you'd do if you had a page that didn't pull from the DB, and how your app would serve it up.
• Use only what you need. It's easy to throw everything into one init script, but you should only include what your page actually needs to compile. It's like importing java.util.* instead of just java.util.List. Doesn't make sense.
• Make the most use of what PHP has to offer built-in, and when that fails, write your own wrappers if PECL doesn't already have a solution. If you're adventurous and have C experience, you could write your own PHP extension to avoid run-time compilation of common functions. We didn't necessarily need to do this, but you might consider it if you have a specific need that isn't addressed with any available PECL extension.
• Ask yourself if you really need DB layer abstraction. DBI's are great, but hey are also huge. PEAR::DB is massive, and if your app isn't going to be ported to other RDBMS's, then you should really consider using your own wrapper for the mysql/mysqli functions built-in to PHP. In my experience, people hardly ever switch their DB layer over, and even if they did, if you write a clear and concise DB class, it is easy to switch out anyway. Abstraction here isn't worth the overhead.
• Ask yourself if you really need a template language with more rules and more syntax to mess up. PHP itself is a scripting language made to write web pages -- so how much sense does Smarty make? Having been down the Smarty path, I've given it a shot, and I don't think it's worth it to replicate everything PHP does. If you're doing it for design purposes, PHP syntax is already pretty simple, and most WYSIWYG editors have built-in escaping/tokenization for PHP's syntax. If you're using Smarty for separation of your view component, you can do the same thing in just PHP using app logic. And if you're doing it so you can cache or precompile output, you're just duplicating what memcached and APC would already offer you. If we could do it again, Smarty would not be worth the performance loss. So be wary of templating languages in such a high-level language. It's usually a lose-lose.

At the app level, before you even get into server configuration or caching, you need to avoid violating the rules above. In our journey with addons.mozilla.org (AMO) we made some interesting group decisions a year ago that we regretted later:

• Smarty
• PEAR::DB

PEAR::DB was unnecessarily large, and Smarty is just not worth it -- it confuses the issue and redoes things PHP is already good at using arbitrarily complicated syntax. Any quick run through with something like the Zend Profiler or APD will tell you how much of a dog these things can be. If you haven't already, I highly recommend profiling your app to see where you're losing performance -- I bet it's mostly in includes.

For caching, we looked at:

• Page/output caching
  • Smarty (bad, bad, bad idea)
  • Cache_Lite (slow)
  • memcached (w00t)
• Internal caching / optimization
  • phpa (meh, and turning into a proprietary solution -- double meh)
  • APC 3.0.10 (w00t)
  • A handful of other outdated and lesser internal PHP caches

For external caching, the clear choice was memcached. Used and designed for LiveJournal.com, it is a pretty standard way to provide key-based caching of any serialized data. It has APIs for almost every language used in web development, so it was an easy choice. It gave the other caching methods an ass whooping.

Based on user comments in my previous post, we punted phpa and went for APC 3.0.x and we liked the results. Initially, using the default settings in APC.ini, we faced some performance losses. After some tweaking, though, APC showed about a 40% increase over the antiquated phpa. Just make sure the read the INSTALL doc.

AMO currently runs on a handful of memcached instances, feeding multiple LVS nodes configured to use APC 3.0.10. We can now easily handle release traffic and during peak hours the web app doesn't even break a sweat. The database bottleneck is history.

So we are happy with the results, but they were achieved using methods that are still less than ideal. There are so many more things we can do to improve performance:

• Remove SSL for non-sensitive pages
• Remove PEAR::DB and Smarty so when pages are compiled and set in the cache it is less expensive
• Move away from page-level caching and get into object-level caching to replace DB calls with queries against memached.
• Improve the memache implementation in the app to be truly decentralized with fallback. Currently it does not map a set key with a particular server. We still need to add a key->server hash so the app knows which server to try first per key. The trick there then becomes failover combined with the hash -- so the app could learn which server to hit if the first choice wasn't available and remember that choice. That is an interesting challenge in a stateless environment.
• Make certain high-load pages purely static and avoid PHP altogether.
• Additional tweaks and Apache config changes to improve performance.

Overall, I have to say it was a great ride and a good learning experience playing with these tools. Working with everyone on this was an exercise in open source development, and it showed us that with the right open source tools you can make some pretty decent enterprise-level web apps performance-wise. I hope that in reading this, you pick up a few things you can use in your next project. If you have any comments or suggestions I'd like to hear them.

Don't just learn as much as you can from what others have tried -- write and talk about it too.

AMO v2

The public rewrite of AMO was released today.

Fixed in this release:

• Stuff
• More stuff
• Scalability
• Other stuff

No, but seriously, you might find that your bookmarks are a little off, or _____. If so, find us on irc in #umo@irc.mozilla.org and let us know -- we aren't at the "file a bug if it's broke" stage yet.

Thanks to Wil Clouser, who worked on a large portion of the updates. Thanks to everyone who pointed out bugs and helped us fix stuff, and thanks most importantly to the reviewers who help us keep this ship afloat.

Speaking of which, please take some time to review our draft policy that we have been working on!

Better late than never, and better late than totally crappy.

Scalable PHP with phpa, memcached and LVS (Part 1)

One of the major pitfalls of the PHP LAMP stack is that run-time compilation does not scale very well. An experienced developer would lament over PHP bloat and the other downfalls that come with the ease and convenience of using a scripting language as the foundation for your application.

Over the last month or so, I have been working on creating a better version of addons.mozilla.org (AMO). It is a task that I enjoy, mostly because it isn't everyday you get the great opportunity to work on a project that touches so many people and means so much.

My journey is similar to the journey taken by Drupal, PHPBB or MediaWiki developers. Given a great PHP product, you not only have to fulfill usability requirements, you also have to make the thing work for an insanely large user base. In my journey I have found an appreciation for the delicate balance between usability/features/design and the pain of having to maintain them. It's the line between being a designer or marketing droid and being a core developer and a sysadmin.

I've learned that, in any successful project, you need to have someone who understands both sides. If you don't, you run the risk of having one of two things:

• A great looking website that meets your users' needs but doesn't run and is not secure
• A fast and secure web app that nobody uses

A successful project usually lands somewhere inbetween. It's a compromise between the best performance and the usability and flexibility most people demand -- albeit users or administrators.

So when you're faced with circumventing PHP bloat, you go through predictable steps:

• Try to run your site as-is, but then realize it fails miserably because of the database bottleneck
• Duplicate your database, splitting it between read/write and read-only -- add a slave or multiple slaves via RRDNS to serve the bits you need
• When this fails, you start looking for output caching mechanisms that give your database a break
• You then realize that full output caching is unrealistic because your cache hits are not high enough
• You find yourself stuck, looking for answers -- and you find ways to deliver features with the hardware you have, without making any large-scale app-level changes

When you're at this point, there are two solutions:

• Optimize PHP somehow to reduce redundant compilation between requests
• Use some sort of granular caching to give your DB a break

In the next part of this post, I'll talk about implementation of phpa/memcached across an LVS cluster and what a difference it made compared to altnernative solutions. I will post graphs explaining perf gains for AMO, and explain why we used phpa and memcached as the final solution.

Building scalable applications is challenging, but you're not alone.