Only a word comes to mind...

Peopo.org

to anonymous user

Cache-Control: public, max-age=21600 

• Only for anonymous visitor
• Tell browser take control of cache
• Tell browser this page will expire after 21600s
• Drupal already done in default: 

  // If the client sent a session cookie, a cached copy will only be served
  // to that one particular client due to Vary: Cookie. Thus, do not set
  // max-age > 0, allowing the page to be cached by external proxies, when a
  // session cookie is present unless the Vary header has been replaced or
  // unset in hook_boot().
  $max_age = !isset($_COOKIE[session_name()]) || isset($hook_boot_headers['vary']) ? variable_get('page_cache_maximum_age', 0) : 0;
  $default_headers['Cache-Control'] = 'public, max-age=' . $max_age;

read: drupal_serve_page_from_cache

• Only for anonymous visitor
• Etag:
  
• Add unique id to delivered page
• Last-Modified:
• Used to detect if page modified since last visit.
  
• Expires:
• Setting by "max-age" of heder cache-control
  

  // Entity tag should change if the output changes.
  $etag = '"' . $cache->created . '-' . intval($return_compressed) . '"';
  header('Etag: ' . $etag);

• Do not write CSS / Javascript in page
• drupal_add_css in ALL PAGE
  
• durpal_add_js should consider too

Cache Page
Without Hitting PHP

• Flood Protection / DOS protection
  
• Short lifetime ( 1s - 5s )
• Can used for logged user (use carefully)
• Never hit PHP when continue hitting

• Only for anonymous visitor
  
• Static HTML page generation when first hit on PHP
• Use rewrite rule (in Nginx or Apache) to redirect to HTML
• Expired by
• Cron
• Crawler ( drupal 6 only)
• Node update ( drupal 6 only )

 <!-- Page cached by Boost @ 2013-06-16 02:46:53, expires @ 2013-06-16 05:46:53 -->

Cache by 
PHP and Drupal

Warm up PHP in every request

1. Load system wide variables
2. Load menu to check user have permission to access page
3. Load localized data to prepare translation strings
4. Load modules to prepare work
5. Render forms when needed
6. Gethering content in Blocks of page
7. Deliver page rendered by template / theme system
8. Finally we got an page! So slow...

Before: 104MB per page

Memory used at: devel_boot()=5.43 MB, devel_shutdown()=104.15 MB

After: 49MB per page

Memory used at: devel_boot()=2.34 MB, devel_shutdown()=49.58 MB

1. Page ( only for anonymous user )
2. Variables
3. Session cache   ( 3 party module )
4. Localized string
5. Module info/registry
6. Template engine registry
7. Filtered content
8. Forms
9. Aggregated content (Block/Views/Panels)
10. Aggregated resource (CSS/JS/Image)
    

• Memcache API and Integration  or  Memcache Storage
  
• save cache into memcached (instead hitting database)
  
  
• Views Content Cache
  
• cache views based on content save
  more effective because not time period
  
  
• Panel Page Cache or Panel Hash Cache
• cache panels based on url / user / content type
  
  
• Cache Actions
• setting cache clear rules in simple clicks (with rules)
  
  
• Cache Warmer
• Claw specific pages you need.

• cache in Time Period
• cache per User
• cache per Page (URL)
• cache per User per Page
• cache per Role per Page
  
  
• others
• cache per content type
• cache per node
• cache per role
• ...

Query Cache by
MySQL

• Join , Join, Join, Join ....... 
• Eat 300 query a page easily
• Save config / variables in DB
• Save session in DB
• Save cache in DB
• Save localized string in DB
• Save search index in DB
• Un-optimized query from views
• Everything combined with node

Query Cache Hits    = 2800000
Quer Cache Insert   = 444400
Hits/(Hits+Inserts) = 86.30% 

• microcache of nginx
  
• 1-3s cache time is enough
• ajax needed for Drupal node page counting
• Best practice at perusio/ drupal-with-nginx
  
  
• Page Cache to memcached
• drupal core mechnism but save cache to memory
• Trouble-less, very stable, but still hitting PHP
• Handle session, status message, HTTP header correctly
• recommend set cache max lifetime
  
  
• Cache Warmer of drupal
• simple drush command to crawl specific page in cron

• Panels + Panel page cache
  
• Cache whole panel based on url
• Hack module to support per role per page
• Fix some personal string in ajax
• do not cache form
  
  
• Use views cache carefully
• can't run without this
  
• headache on Contextual / Exposed filter cache
  
  
• use Cache Actions
• finally, we can add "update - then - clear cache" rule
• got to have debug skill at fetch "cache id"

1. Page cache can verify by HTTP header
2. PHP-APC have watch script
3. Memcached have script, too
4. Use devel module to see page generate time
5. Profiling: XHProf
6. on large site, never "Clear All Cache"
   （this will save you life）
7. no cached page when anonymous have session
   （added at drupal 7）