I’m at WordCamp London - live-blogging some of the talks
photo of Mark Jaquith by Dave Pullig
- Ultimately failing to cache could take your site down: run out of connections
- Caching principles:
- Do less work - By default WP is Totally dynamic - there’s a lot of complex stuff going on and it all gets done on every single request
- Hold onto data as long as it makes sense to do so: this is very specific to each individual site (at least for certain kinds of content)
- Make output generic: the more specific it is to a particular user, the less cachable it is
- Page caching:
- Batcache - if a page gets requested more than 3 times in 3 mins it caches that whole page for 6 mins
- Used by WP.com
- Unobtrusive - only caches the most frequently requested stuff
- low-configuration
- W3TotalCache
- Extremely complex, lots of layers
- WPSuperCache
- Somewhere in-between
- Varnish or Nginx
- Cache before it hits the webserver
- CDN
- geographically distributed caches: people access the cache closest to them
- Very popular for media files, but can be used for the whole site.
- Will obey cache headers
- Nginx Cache Purging
- This is tricky
- There’s a third party package which can be compiled from source which enables this. But :-P
- Approach: Configure a ‘magic header’ ;) - when it’s passed with a request, it skips getting, but not setting, so overwrites whatever is currently in the cache
- Variable cache lengths + proactive cache refreshing:
- Warm the cache (on cron) more frequently than it’s getting expired, to make sure that your content is always cached
- Does being logged-in mean you can’t cache the page?
- It’s a pain if you have 1000’s of logged in users/active commenters etc.
- Think about how the page varies based on who you are logged in as: toolbar, private posts - etc.
- Choice: remove those things OR make them generic
- Comment forms: hide them by default, show them with ajax - either when hitting a button, or when scrolling to the bottom of the page (!)
- Remove toolbar for subscribers etc. - it’s not very useful!
- Remove private post fucntionality if it’s not used: it makes the queries much more complicated anyway
- Replace moderated comments with a generic “your comment is being moderated” message
- Approach if you really need to show logged-in data:
- Use cookies: by default WP renders user-specific content using php, but that’s very odd
- Set JS-readable cookies and use them to populate the content
- He’s just written Cache Buddy - a plugin to do this
- Object Caching
- APCu, Memcache, or (recommended:) Redis
- put them in
object-cache.php- they need to be loaded before plugins
- Transients + Object Cache
- For just a few values use transients - stores data in the options table
- For 1000s of values use the object cache
- Object cache can organise cache keys into groups
- Cache stampedes: What happens when cache expires?
- One user triggers the remote request
- then the next user also triggers it (before it’s completed)
- and another… STAMPEDE!
tlc_transient- handles this problem: soft expiration - data gets marked as expired but not deleted until the new data is available (!)
- Html fragment caching
- e.g. A complex footer that’s the same on every page
CWS_Fragment_Cache