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A short presentation about HTTP caching at the Rome PHP User Group.
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HTTP cache
Alessandro Nadalin
March, 29th 2011
The Webis
inconsistent
get over it
Why?
Because it scales to billions of users
Because it scales to billions of users
WWW data traffic won't be possible without being
inconsistent
How does it scales?
Cache
What is caching?
Storing data so that future requests for that data can be served faster
( general purpose )
Process of storing copies of your most accessed resources near to a/some client/s
( the WWW )
Caching is not saving a webpage, or a fragment of it, on the disk, to serve it to the next client
Caching is not saving a webpage, or a fragment of it, on the disk, to serve it to the next client
This is a caching mechanism
Disk-persisted or in-memory
Can be distributed
Can be distributed with a NoSQL database
Cache implementation can be really funny
Don't reinvent the wheel
Goals
Goals
evolve
Goals
evolveloose coupling
Goals
Work lessevolve
loose coupling
Evolve Because you want your platform to extensible
Evolve Because you want your platform to extensible
Loose coupling Because you want it to be easy to integrate with, evolve, plug and mantain
Evolve Because you want your platform to extensible
Loose coupling Because you want it to be easy to integrate with, evolve, plug and mantain
Work less
Because every LoC is bug-prone and our man-day is a hard to scale cost
enters our Hero #1
enters our Hero #2
http://www.lullabot.com/articles/a-beginners-guide-to-caching-data
Goals
Work lessevolve
loose coupling
Goals
Work lessevolve
loose coupling
everything is done for us!
:)
but....
tmp files, cache tables, procedural crap...
mmmmh....
gotta be something better
Frameworks
Cache is used for compiling routes, autoloading, ...
Cache is used for compiling routes, autoloading, ...
...but also for storing the view
Goals
Work lessevolve
loose coupling
Goals
Work lessevolve
loose coupling
at least because we use a framework
but, probably
PHP sucks
Let's call a hero, from the East
on Rails
Goals
Work lessevolve
loose coupling
gosh :-(
hey, what aboutthe latest
products?
both Symfony2 andRuby's Rack embrace
HTTP
(here comes the nice part)
Local
Shared/proxy
Shared/reverse proxy
Local
Shared/proxy
Shared/reverse proxy
Local
Shared/proxy
Shared/reverse proxy
Local
Shared/proxy
Shared/reverse proxy
on the server side
Proxy Reverse proxy
http://www.codeproject.com/KB/aspnet/ExploringCaching.aspx
There is something cool about caching softwares.
Free as free beer
...but the server should take advantage of them
( the boring part were you need to write code )
and here are a few ways to do so, using
and here are a few ways to do so, using
expiration
and here are a few ways to do so, using
expiration
validation
and here are a few ways to do so, using
expiration
validation
invalidation
and here are a few ways to do so, using
expiration
validation
invalidationwe will se why
Expiration
GET / HTTP/1.1Host: www.example.comExpires: 0
GET / HTTP/1.1Host: www.example.comExpires: 0
GET / HTTP/1.1Host: www.example.comExpires: Tue, 15 Nov 1994 01:00 GMT
GET / HTTP/1.1Host: www.example.comCache-Control: max-age=60, public
GET / HTTP/1.1Host: www.example.comCache-Control: max-age=60, public
GET / HTTP/1.1Host: www.example.comCache-Control: max-age=60, public
Cacheable for 60 seconds
GET / HTTP/1.1Host: www.example.comCache-Control: max-age=60, public
Cacheable by both local and shared caches
GET / HTTP/1.1Host: www.example.comCache-Control: stale-if-error=600, stale-while-revalidate=600
GET / HTTP/1.1Host: www.example.comCache-Control: stale-if-error=600, stale-while-revalidate=600
fault-tolerant
GET / HTTP/1.1Host: www.example.comCache-Control: stale-if-error=600, stale-while-revalidate=600
available during downtime
GET / HTTP/1.1Host: www.example.comCache-Control: stale-if-error=600, stale-while-revalidate=600
available during revalidation
Validation
GET / HTTP/1.1Host: www.example.comEtag: 1234
GET / HTTP/1.1Host: www.example.comEtag: 1234
an identifier for your response
GET / HTTP/1.1Host: www.example.comIf-None-Match: 1234
the browsers asks you if it has been modified
Conditional requests
Relax
Calculating an Etag is cheaper than generating a full MVC response
HTTP/1.1 304 Not Modified
GET / HTTP/1.1Host: www.example.comLast-Modified: Tue, 15 Jan 2011 12:00:00 GMT
GET / HTTP/1.1Host: www.example.comLast-Modified: Tue, 15 Jan 2011 12:00:00 GMT
tell the client about the latest change
GET / HTTP/1.1Host: www.example.comIf-Modified-Since: Tue, 15 Jan 2011 12:00:00 GMT
the client asks you if it has been modified since the last time
Conditional requests
Relax
Calculating a date is cheaper than retrieving an entire object
HTTP/1.1 304 Not Modified
Invalidation
The web is not meant for invalidating data.
Server should not be able to keep clients' state, otherwise they wont scale well.
That's why long-polling and endless connections haven't had big success dealing with caching.
but hey, you say
HTTP's cache fails when dealing with really dynamic pages, because consumers will always have to hit the origin server, although a part of the page would be cacheable ( header and
footer, for example )
Nope
Nope
and hey, Varnish is a reverse proxy implementing what you need of the ESI specification
take 2, pay for 1
So what does HTTP cache is meant to solve?
Less work
because the hard work is delegated to the browser/proxy
http://www.flickr.com/photos/snakphotography/5004775320/sizes/o/in/photostream/
evolve
because cache is abstracted from the application
loose coupling
because caching is bound to the protocol, HTTP, not to your implementation ( Sf, RoR, Django )
The presentations lacks on technical stuff
So here are a few resource for who wants documentation:
http://tomayko.com/writings/things-caches-dohttp://www.slideshare.net/fabpot/caching-on-the-edgehttp://www.odino.org/301/rest-better-http-cachehttp://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
Deep version of the same talk,about Sf2 cache internals and ESI
May, 12/14 2011 in Verona
Early bid ends on April, 6th
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