http caching basics

HTTP Caching Basics

Agenda

HTTP Basics

Cache-Related HTTP Headers

Expiration

Revalidation

Variation

HTTP Basics

HTTP Basics - Browser and origin server

Browser OriginServer

HTTP Basics - Resource


Resource/index.html

HTTP Basics - Request and response


Resource/index.html

Request

Response

HTTP Basics - Representation


Resource/index.html

Request

Response

Representation

Cached Representation

HTTP Basics - Method and status code


Resource/index.html

RequestGET /index.html HTTP/1.1

ResponseHTTP/1.1 200 OK

Representation

HTTP Method

HTTP Status Code

HTTP Basics - Request and response headers


Resource/index.html

RequestGET /index.html HTTP/1.1Accept: text/html ...Accept-Encoding: gzip, deflate, sdch, brUser-Agent: Mozilla/5.0 …Cookie: PHPSESSID=12b239d7825bd8d21d5e60df56cbc3a5;

ResponseHTTP/1.1 200 OKContent-Type: text/htmlContent-Encoding: gzipDate: Fri, 16 Sep 2016 12:13:45 GMT

Representation

Request header

Response header

HTTP Basics - Intermediaries


Resource/index.html



Representation

Intermediary

Representation



Resource/index.html

Quiz: What kind of intermediaries can you think of?

HTTP Basics - Intermediate caches


Resource/index.html



Representation

Intermediate Cache

(Akamai, Varnish)

Representation



Resource/index.html

CachedRepresentations

Cache-Related HTTP Headers

Quiz: Which cache-related HTTP headers do you know?

Cache-related HTTP headersExpiration

Cache-Control

Expires

Age

Pragma

Variation

Vary

Revalidation

ETag

Last-Modified

If-Modified-Since

If-None-Match

Quiz: Which are request and which are response headers?

Header Request Response

Cache-Control

Expires

Age

Pragma

ETag

Last-Modified

If-Modified-Since

If-None-Match

Vary

Cache-related HTTP headers in requests and responses

Header Request Response

Cache-Control x x

Expires x

Age x

Pragma x x

ETag x

Last-Modified x

If-Modified-Since x

If-None-Match x

Vary x

Expiration

Expiration - Absolute vs. relative expiration date


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTExpires: Fri, 16 Sep 2016 13:00:00 GMT


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: max-age=2700

CachedRepresentation


Quiz: Which Cache-Control directives do you know?

Expiration - Cache-Control directivesDirectives Request Response

no-cache x x

no-store x x

max-age x x

s-maxage x

max-stale x

min-fresh x

no-transform x x

only-if-cached x

public x

private x

must-revalidate x

proxy-revalidate x

Expiration - Enforce revalidation with origin server


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: max-age=2700, must-revalidate


RepresentationNever use stale representation(even if origin server is down)

Expiration - No-cache shortcut


Resource/index.html




Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: no-cache

=Cached

Representation


Expiration - No-cache vs. no-store


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: no-cache


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: no-store



Expiration - Age specifies how long representation is cached already

ResponseHTTP/1.1 200 OKCache-Control: max-age=2700


Resource/index.html


ResponseHTTP/1.1 200 OKCache-Control: max-age=2700Age: 0

Intermediate Cache

(Akamai, Varnish)


Resource/index.html

1st Request

2nd Request 15 Minutes Later Different Browser

Browser


ResponseHTTP/1.1 200 OKCache-Control: max-age=2700Age: 900

Intermediate Cache

(Akamai, Varnish)Resource

/index.html



Revalidation

Revalidation - Based on what?


Resource/index.html




Representation

Revalidation based on what?

Revalidation - Weak vs. strong cache validator


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: max-age=2700, must-revalidateLast-Modified: Fri, 15 Sep 2016 12:00:00 GMT


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: max-age=2700, must-revalidateETag: "3e86-410-3596fbbc"



Revalidation - Revalidation based on Last-Modified


Resource/index.html

RequestGET /index.html HTTP/1.1If-Modified-Since: Fri, 15 Sep 2016 12:00:00 GMT

ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 13:00:00 GMTCache-Control: max-age=2700, must-revalidateLast-Modified: Fri, 16 Sep 2016 13:00:00 GMT


Resource/index.html

ResponseHTTP/1.1 304 Not ModifiedDate: Fri, 16 Sep 2016 13:00:00 GMTCache-Control: max-age=2700, must-revalidateLast-Modified: Fri, 15 Sep 2016 12:00:00 GMT



OR

Revalidation - Revalidation based on ETag


Resource/index.html

RequestGET /index.html HTTP/1.1If-None-Match: "3e86-410-3596fbbc"

ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 13:00:00 GMTCache-Control: max-age=2700, must-revalidateETag: "6e35-240-2672fbbc"


Resource/index.html

ResponseHTTP/1.1 304 Not ModifiedDate: Fri, 16 Sep 2016 13:00:00 GMTCache-Control: max-age=2700, must-revalidateETag: "3e86-410-3596fbbc"



OR

Quiz: How does revalidation behave for different Cache-Control setups?

Expiration - No-store means no revalidation possible


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: no-cacheLast-Modified: Fri, 15 Sep 2016 12:00:00 GMT


Resource/index.html


ResponseHTTP/1.1 200 OKDate: Fri, 16 Sep 2016 12:15:00 GMTCache-Control: no-storeLast-Modified: Fri, 15 Sep 2016 12:00:00 GMT



Variation

Variation - Most common is Vary on Accept-Encoding


Resource/index.html

RequestGET /index.html HTTP/1.1Accept-Encoding: gzip

ResponseHTTP/1.1 200 OKContent-Encoding: gzipVary: Accept-Encoding

Representation

Means different zipped and unzipped variants of representation are possible.

ResponseHTTP/1.1 200 OKVary: Accept-Encoding

Variation - Used to control behavior of intermediate caches


Resource/index.html


ResponseHTTP/1.1 200 OKVary: Accept-Encoding

Representation

Intermediate Cache

(Akamai, Varnish)

Representation


Resource/index.html


Accept-Encoding: gzip Empty Accept-Encoding

ResponseHTTP/1.1 200 OKVary: Accept-Encoding, User-Agent

Variation - Also used in combination with User-Agentto deliver device type specific content


Resource/index.html

RequestGET /index.html HTTP/1.1Accept-Encoding: gzipUser-Agent: Chrome …


Representation

Intermediate Cache

(Akamai, Varnish)

Representation


Resource/index.html


Accept-Encoding: gzipUser-Agent: Chrome …

Accept-Encoding: gzip User-Agent: Edge …

Quiz: What is the problem with that setup?

Combinations are endless and hit rate will be really low


Variation - Wrong usage of Vary header can lead to low hit rates


Resource/index.html



Representation

Intermediate Cache

(Akamai, Varnish)

Representation


Resource/index.html



Accept-Encoding: gzipUser-Agent: Edge …

Combinations are endless and hit rate will be really low


Variation - Don’t do it like that


Resource/index.html



Representation

Intermediate Cache

(Akamai, Varnish)

Representation


Resource/index.html



Accept-Encoding: gzipUser-Agent: Edge …

ResponseHTTP/1.1 200 OKVary: Accept-Encoding, X-Device

Variation - Create generic header to improve hit rate and return private in Cache-Control to avoid caching


Resource/index.html

RequestGET /index.html HTTP/1.1Accept-Encoding: gzipUser-Agent: Chrome ...

ResponseHTTP/1.1 200 OKCache-Control: private

Representation

Intermediate Cache

(Akamai, Varnish)

Representation

RequestGET /index.html HTTP/1.1Accept-Encoding: gzipX-Device: Mobile

Resource/index.html


Accept-Encoding: gzipX-Device: Mobile

Accept-Encoding: gzipX-Device: Desktop

Conclusion

Conclusion

Use Cache-Control header in general to control caching behaviour

Implement ETag and/or Last-Modified to allow for revalidation

Do not use User-Agent in Vary header and rather extract required attributes and work with X-... headers

Questions?

http caching basics

Technology