The Bakery Everything CakePHP

It's been my experience that disk and memory has always been my bottleneck when scaling a disk backed db app.

Its easy to fix the memory bottlenecks by adding more memory or making code improvements that consume less memory per-request. Its not so easy to cut down on disk I/O - and furthermore in today's shift towards virtual machines (linode, EC2 etc) - even if you don't realize it, disk i/o is constrained MUCH more then ever before. Throw a disk backed DB in there and u get in trouble fast.

So anything that you can do to cut down on I/O is huge (IMO of course).

Use php optcode cache

I use http://eaccelerator.net/. there are tons out there, but I use this one primarily cuz its maintained in the ubuntu server package repo's. If you use an optcode cache that is not maintained in your package management repo - every time there is an update to php (or other dependent libs) you have to recompile. May be better ones out there, but not worth the hassle for the little (if any) improvement.

Use memcached wherever possible

If you don't use cake's caching functions - start. When its time to scale, fronting every database query you can with memcached is very important. Go turn on caching, and use the memcached one http://book.cakephp.org/view/772/Cache-config. It is a shame that view caching cant be backed by memcached (it can in other web frameworks) - but its still a no-brainer. There is a reason that facebook is the #1 consumer of memcached. Good read if your interested: http://www.facebook.com/note.php?note_id=39391378919
anyways, I have been unable to find any good docs on there on how to use cakes Cache class in a real life app. Things can get complex when you need to maintain a mapping of what cache entries need to be cleared when stuff in related (or unrelated) models are changed. If anyone knows of a nice tutorial on this topic please post in the comments....

Use the memcached session support built into php5

Ok so you've put up with my babbling - now some real code. Lets dive right in, then I'll explain. This is your php.ini:
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~$ cat /etc/php5/apache2/php.ini | grep -i session
[Session]
;session.save_handler = files
session.save_handler = memcache
session.save_path = "tcp://127.0.0.1:11211"

...

I've omitted lots of the php.ini, but the relevant lines are all here. Super simple, all we are doing here is telling the default php session handling to be backed by memcached. Comment out the files backed, and type up the next 2 lines. Slick/easy aint it?

Before doing this there are a few steps you have to setup first:

• in /config/core.php: set Configure::write('Session.save', 'php'); This tells cake to let PHP handle sessions.
• Setup a unique name for Session.cookie in core.php
• Change the Security.salt to something unique
• Install memcached. Use your package manager to install it (apt-get or whatever your distro's is)
• Start memcached (sudo /etc/init.d/memcached start). Note: some distros make you edit some memcached config file in /etc to tell it to allow it to start - not sure why they do it, but they do...
• Go make the php.ini changes above, then restart apache.

Thats it. You are really almost getting something for nothing here. Everything is on memory, not hitting your db and not sucking up precious I/O. Plus, if you end up scaling, and have a cluster of machines serving your web app - you can share this session info across workhorse nodes (memcached lets you pool up memory over boxes that r networked together). This comes in handy in sharing other cached data across nodes - again, too bad cake doesn't store view cache to memcache - otherwise you could really take advantage of it here (think of trying to run commands on every node to delete view cache files when something changes - not fun).

I use all these techniques in my startup (http://www.leaguelogix.com) so they are proven to work in a production environment.

Hopefully I did not forget anything here - if you run into problems leave a comment and i'll try to help you out. Good luck.