HTTP caching solutions such as Varnish are great for improving web application performance. Filling up the cache is easy, but how do you get rid of cache entries when they become stale? There are three ways: expiration, validation and invalidation. The first two are nice and simple, but have their limitations. The other option, invalidation, is frowned upon for its complexity. Still, it can be very useful. Case in point: an e-learning web platform we built that serves a large number of Dutch pupils and their teachers. I will tell you why and how we added an invalidation scheme to this platform. I will also show you our lightweight Symfony bundle that makes cache invalidation a little less complex.

1. Keeping it fresh
David de Boer

2. Hello
•
I’m David
•
Lead developer at Driebit, Amsterdam
•
Open source fanatic

3. The problem
•
E-learning platform
•
Now used by ~50% of Dutch primary and
secondary schools
•
Slow

4. So cache!
•
Keep coding fun
•
Keep users happy

5. Caching models
!
•
Expiration
•
Validation
•
Invalidation

6. What we had
•
REST API
•
Content changes at predictable moments
•
Rush hour

7. What we needed
•
Fresh data at push of button
•
Talk to Symfony as little as possible

8. Back to caching models
Quick
refresh
Don’t talk
to Symfony
Expiration
×
✔
Validation
✔
×
Invalidation
✔
✔

9. “There are only two hard things in
Computer Science: cache invalidation and
naming things.”
– Phil Karlton

10. “You should never need to invalidate
cached data…”
“Actually, all reverse proxies provide ways
to purge cached data, but you should
avoid them as much as possible.”
– Symfony2 documentation

11. Varnish
sub
vcl_hit
{
if
(req.request
==
"PURGE")
{
purge;
error
200
“Purged";
}
}

13. •
Easy to use
•
Configure, don’t code
•
Lightweight

14. Easy to use
•
Copy original route parameters
•
Note: don’t use getRouteCollection()!
•
$cacheManager
-­‐>invalidateRoute('my_route')
-­‐>flush();

15. Configure, don’t code
driebit_http_cache:
http_cache:
varnish:
ips:
[127.0.0.1,
123.123.123.1]
host:
yourapp.nl
invalidators:
exam_create:
origin_routes:
[
exams_post
]
invalidate_routes:
exam_get_collection:
~
#
/exams
exam_change:
origin_routes:
[
exam_put,
exam_del
]
invalidate_routes:
exam_get_collection:
~
#
/exams
exam_get:
~
#
/exams/{id}
current_exam:
~
#
/exams/current

16. Lightweight
•
Collect all routes that will be invalidated.
•
Send PURGE requests in kernel.terminate.
•
Send them in parallel.

17. Christmas wish list
•
Cache tags (X-­‐Id:
123)
•
Annotations (Expression Language)
•
Message queues

18. Thanks!
•
david@driebit.nl / @ddeboer_nl
•
driebit/http-cache-bundle
•
https://github.com/driebit
•
https://github.com/ddeboer
•
http://driebit.nl