How to scale a PHP application (servers, mysql, memcache)

Question

I'm currently creating a website for a social project in switzerland.

And before there is an overflow of user, I want to prepare the application to scale.

I answered by myself many questions but some are left.

I explain what I want to do.

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First

at the beginnning, the Application will have only one server (short time) with DNS, PHP, Mysql, Data, and memcache.

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Second

Then I will split them in two

1. DNS, Mysql, memcache
2. Data, PHP

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Third

Here is the problem, I don't know how to do it exactly here to keep the application running well.

I could do :

1. Front : Load Balancer, memcache, DNS
2. Web 1 : PHP, DATA
3. Web 2 : PHP, DATA
4. Mysql

This would be the scheme, all PHP sessions are kept in the DB.

BUT, how do I sync the data? do I run a Rsync to keep them up to date. do I put them on a separate disk (network disk) to be sure ? but in this case, how can I do in case of user uploads ?

and if the website gets more success and we have to go on greater structures, would'nt it create some latency on updates ?

or would it be a good thing to go directly to amazon's web services ?

some infos I use codeigniter as Framework. I use linux as webserver (distribution not chosen now, but should be Debian)

Thanks in advance for your answers.

Answer 1

According to Wikipedia, Switzerland has 4.6 million German speakers, 1.5 million French speakers, and .5 million speakers of Italian, Romansch and other languages. So I suspect you'll find that a single server will fit your needs. Guess what percentage of the population will visit your site every month or every day to get a sense of how big you can get before running into scaling issues.

So, I don't think you need to worry about scaling yet! Bonus: The time you don't spend worrying about this problem, you can use to solve other problems for your users.

Answer 2

There are a few common paths to scaling web services up, in order of what sites like Flickr and Facebook seem to use:

• Split servers based on concepts (API, login, media files, ads, static pages, dynamic pages)
• Split databases based on concepts that don't need to be JOINed (logins, long term reporting, page data, etc.)
• Compile/optimize your PHP and other resources (sprites, compiled css, zend)
• Add caching (front end, back end)
• Add delegation (round robin, etc.)

But, before scaling, measure. Set of tests, calculate your capacity, and don't optimize before you need to.