Session VS File VS Memcache for a Cache in PHP?

Question

I have a social network

• The users table is around 60,000 rows

• The friends table is around 1 million rows (used to determine who is your
  friend)

I am wanting to do a friend feed, wall, whatever you like to call it, it will show things like user status post (twitter type posts), it will show a few different items but for the start it will just be friend status and maybe blog post.

Basicly you will only see content published with a user ID that is in your friend list.

I have been trying to come up with the best way and haven't gotten very far but here is my latest idea.

Currently to build this feed, I have to

1. Get the list of friend ID's on the large friend table

2. Get the stream data from the friend ids from the above result

3. JOIN the user table to get the publishers picture URL and username

4. Then JOIN the comments table to get comments posted on the feed items

That is one big task to build that feed

I have 3 ideas so far, this is where your help can come in.

Memcache Option:

1. Use memcache and cache a users friendlist as an array when the user logs into the site, also when the user approves a new friend request for a friend to be added to there list, it would rebuild there cache.
2. In addition to just getting there friends I could save there friends picture URL and username, this would speed up things again by eliminating this query when building the friend feed.

File cache Option:

1. Do the same as the memcache option does but save this data as an array to a cache file instead of memory, then include this cache file into the page.

2. I am not sure which is the best method for performance I understand memcache stores everything in memory so friends that have like 20,000 friends that could use a lot of memory and a file cache would only put it in memory when the users needs it if I am correct. Also if I did the file method, when a user logs out of the site, I would delete there cache file so the cache folder would never be too large of files

Session cache Option:

1. Same as file cache above, I just realized that session data is saved into a file so wouldn't that make it capable of being a cache?

Please give me your opinions or any advice or info you have on this as I don't have much knowledge of caching, I have read a lot but sometimes other peoples ideas help a lot

Answer 1

Memcache is your best bet for a lot of reasons:

1. It's REALLY fast - Everything's in memory, and it's highly optimized for situations just like yours (and caching in general :)
2. It's distributed - This means that if you have multiple web / app servers running, they can all access the same cache
3. You can pool multiple servers for memcache - If you've got a few servers that are relatively underutilized (or several dedicated cache servers), you can pool them all together into one big cache
4. It's super-scalable (for the reasons mentioned prior)
5. It's got great PHP support - The PECL package for memcache was recently updated with a lot of new goodness
6. You can even store your user sessions in memcache - just set it up in your php.ini file. This is much faster than storing sessions in databases, and allows your sessions to persist across multiple web hosts (if you're in a load balanced situation)... this will also give your site a bit of a performance boost as there's no need to hit the filesystem / database for session info on every request.

... and many more ;)

As to some of your concerns about memory footprint of individual cached items you've got a few options. My initial thought is to just give it a whirl, see how big these cache items really get (you can find several open-source things to monitor the actual cache usage, such as cacti). I think they'll be smaller than you'd think.

If they're not, I'd suggest re-thinking your cache strategy as far as what you actually cache, for how long, etc. Maybe you could build the feed from several things already in the cache (i.e. cache individual user data, and then build the feed for a person from all those individual items in cache). There are a lot of good articles out there on that front, just search 'em out :)