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Http caching basics

The document provides an overview of HTTP caching fundamentals, detailing how caching works between browsers and origin servers. It covers important HTTP headers related to caching, expiration, revalidation, and the usage of intermediaries, along with various caching directives and best practices. Additionally, it discusses the implications of using the Vary header and how to optimize caching behavior.

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HTTP Caching Basics
Agenda HTTP Basics Cache-Related HTTP Headers Expiration Revalidation Variation
HTTP Basics
HTTP Basics - Browser and origin server Browser Origin Server
HTTP Basics - Resource Browser Origin Server Resource /index.html
HTTP Basics - Request and response Browser Origin Server Resource /index.html Request Response
HTTP Basics - Representation Browser Origin Server Resource /index.html Request Response Representation Cached Representation
HTTP Basics - Method and status code Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation HTTP Method HTTP Status Code
HTTP Basics - Request and response headers Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept: text/html ... Accept-Encoding: gzip, deflate, sdch, br User-Agent: Mozilla/5.0 … Cookie: PHPSESSID=12b239d7825bd8d21d5e60df56cbc3a5; Response HTTP/1.1 200 OK Content-Type: text/html Content-Encoding: gzip Date: Fri, 16 Sep 2016 12:13:45 GMT Representation Request header Response header
HTTP Basics - Intermediaries Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation Intermediary Representation Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Resource /index.html
Quiz: What kind of intermediaries can you think of?
HTTP Basics - Intermediate caches Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Resource /index.html Cached Representations
Cache-Related HTTP Headers
Quiz: Which cache-related HTTP headers do you know?
Cache-related HTTP headers Expiration 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 Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Expires: Fri, 16 Sep 2016 13:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700 Cached Representation Cached Representation
Quiz: Which Cache-Control directives do you know?
Expiration - Cache-Control directives Directives 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 Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Cached Representation RepresentationNever use stale representation (even if origin server is down)
Expiration - No-cache shortcut Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=0, must-revalidate Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache = Cached Representation Cached Representation
Expiration - No-cache vs. no-store Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-store Cached Representation Cached Representation
Expiration - Age specifies how long representation is cached already Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Age: 0 Intermediate Cache (Akamai, Varnish) Request GET /index.html HTTP/1.1 Resource /index.html 1st Request 2nd Request 15 Minutes Later Different Browser Browser Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Age: 900 Intermediate Cache (Akamai, Varnish)Resource /index.html Cached Representation Cached Representation
Revalidation
Revalidation - Based on what? Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Cached Representation Representation Revalidation based on what?
Revalidation - Weak vs. strong cache validator Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "3e86-410-3596fbbc" Cached Representation Cached Representation
Revalidation - Revalidation based on Last-Modified Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 If-Modified-Since: Fri, 15 Sep 2016 12:00:00 GMT Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 16 Sep 2016 13:00:00 GMT Browser Origin Server Resource /index.html Response HTTP/1.1 304 Not Modified Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Cached Representation Cached Representation OR
Revalidation - Revalidation based on ETag Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 If-None-Match: "3e86-410-3596fbbc" Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "6e35-240-2672fbbc" Browser Origin Server Resource /index.html Response HTTP/1.1 304 Not Modified Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "3e86-410-3596fbbc" Cached Representation Cached Representation OR
Quiz: How does revalidation behave for different Cache-Control setups?
Expiration - No-store means no revalidation possible Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-store Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Cached Representation Cached Representation
Variation
Variation - Most common is Vary on Accept-Encoding Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Response HTTP/1.1 200 OK Content-Encoding: gzip Vary: Accept-Encoding Representation Means different zipped and unzipped variants of representation are possible.
Response HTTP/1.1 200 OK Vary: Accept-Encoding Variation - Used to control behavior of intermediate caches Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Response HTTP/1.1 200 OK Vary: Accept-Encoding Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Resource /index.html Cached Representations Accept-Encoding: gzip Empty Accept-Encoding
Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Variation - Also used in combination with User-Agent to deliver device type specific content Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-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 Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Variation - Wrong usage of Vary header can lead to low hit rates Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-Agent: Chrome … Accept-Encoding: gzip User-Agent: Edge …
Combinations are endless and hit rate will be really low Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Variation - Don’t do it like that Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-Agent: Chrome … Accept-Encoding: gzip User-Agent: Edge …
Response HTTP/1.1 200 OK Vary: Accept-Encoding, X-Device Variation - Create generic header to improve hit rate and return private in Cache-Control to avoid caching Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome ... Response HTTP/1.1 200 OK Cache-Control: private Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip X-Device: Mobile Resource /index.html Cached Representations Accept-Encoding: gzip X-Device: Mobile Accept-Encoding: gzip X-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?

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Http caching basics

  • 1.
  • 2.
    Agenda HTTP Basics Cache-Related HTTPHeaders Expiration Revalidation Variation
  • 3.
  • 4.
    HTTP Basics -Browser and origin server Browser Origin Server
  • 5.
    HTTP Basics -Resource Browser Origin Server Resource /index.html
  • 6.
    HTTP Basics -Request and response Browser Origin Server Resource /index.html Request Response
  • 7.
    HTTP Basics -Representation Browser Origin Server Resource /index.html Request Response Representation Cached Representation
  • 8.
    HTTP Basics -Method and status code Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation HTTP Method HTTP Status Code
  • 9.
    HTTP Basics -Request and response headers Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept: text/html ... Accept-Encoding: gzip, deflate, sdch, br User-Agent: Mozilla/5.0 … Cookie: PHPSESSID=12b239d7825bd8d21d5e60df56cbc3a5; Response HTTP/1.1 200 OK Content-Type: text/html Content-Encoding: gzip Date: Fri, 16 Sep 2016 12:13:45 GMT Representation Request header Response header
  • 10.
    HTTP Basics -Intermediaries Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation Intermediary Representation Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Resource /index.html
  • 11.
    Quiz: What kindof intermediaries can you think of?
  • 12.
    HTTP Basics -Intermediate caches Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Resource /index.html Cached Representations
  • 13.
  • 14.
    Quiz: Which cache-related HTTPheaders do you know?
  • 15.
  • 16.
    Quiz: Which arerequest and which are response headers? Header Request Response Cache-Control Expires Age Pragma ETag Last-Modified If-Modified-Since If-None-Match Vary
  • 17.
    Cache-related HTTP headersin 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
  • 18.
  • 19.
    Expiration - Absolutevs. relative expiration date Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Expires: Fri, 16 Sep 2016 13:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700 Cached Representation Cached Representation
  • 20.
    Quiz: Which Cache-Controldirectives do you know?
  • 21.
    Expiration - Cache-Controldirectives Directives 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
  • 22.
    Expiration - Enforcerevalidation with origin server Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Cached Representation RepresentationNever use stale representation (even if origin server is down)
  • 23.
    Expiration - No-cacheshortcut Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=0, must-revalidate Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache = Cached Representation Cached Representation
  • 24.
    Expiration - No-cachevs. no-store Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-store Cached Representation Cached Representation
  • 25.
    Expiration - Agespecifies how long representation is cached already Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Age: 0 Intermediate Cache (Akamai, Varnish) Request GET /index.html HTTP/1.1 Resource /index.html 1st Request 2nd Request 15 Minutes Later Different Browser Browser Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Cache-Control: max-age=2700 Age: 900 Intermediate Cache (Akamai, Varnish)Resource /index.html Cached Representation Cached Representation
  • 26.
  • 27.
    Revalidation - Basedon what? Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Cached Representation Representation Revalidation based on what?
  • 28.
    Revalidation - Weakvs. strong cache validator Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "3e86-410-3596fbbc" Cached Representation Cached Representation
  • 29.
    Revalidation - Revalidationbased on Last-Modified Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 If-Modified-Since: Fri, 15 Sep 2016 12:00:00 GMT Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 16 Sep 2016 13:00:00 GMT Browser Origin Server Resource /index.html Response HTTP/1.1 304 Not Modified Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Cached Representation Cached Representation OR
  • 30.
    Revalidation - Revalidationbased on ETag Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 If-None-Match: "3e86-410-3596fbbc" Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "6e35-240-2672fbbc" Browser Origin Server Resource /index.html Response HTTP/1.1 304 Not Modified Date: Fri, 16 Sep 2016 13:00:00 GMT Cache-Control: max-age=2700, must-revalidate ETag: "3e86-410-3596fbbc" Cached Representation Cached Representation OR
  • 31.
    Quiz: How doesrevalidation behave for different Cache-Control setups?
  • 32.
    Expiration - No-storemeans no revalidation possible Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-cache Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Response HTTP/1.1 200 OK Date: Fri, 16 Sep 2016 12:15:00 GMT Cache-Control: no-store Last-Modified: Fri, 15 Sep 2016 12:00:00 GMT Cached Representation Cached Representation
  • 33.
  • 34.
    Variation - Mostcommon is Vary on Accept-Encoding Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Response HTTP/1.1 200 OK Content-Encoding: gzip Vary: Accept-Encoding Representation Means different zipped and unzipped variants of representation are possible.
  • 35.
    Response HTTP/1.1 200 OK Vary:Accept-Encoding Variation - Used to control behavior of intermediate caches Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Response HTTP/1.1 200 OK Vary: Accept-Encoding Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip Resource /index.html Cached Representations Accept-Encoding: gzip Empty Accept-Encoding
  • 36.
    Response HTTP/1.1 200 OK Vary:Accept-Encoding, User-Agent Variation - Also used in combination with User-Agent to deliver device type specific content Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-Agent: Chrome … Accept-Encoding: gzip User-Agent: Edge …
  • 37.
    Quiz: What isthe problem with that setup?
  • 38.
    Combinations are endlessand hit rate will be really low Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Variation - Wrong usage of Vary header can lead to low hit rates Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-Agent: Chrome … Accept-Encoding: gzip User-Agent: Edge …
  • 39.
    Combinations are endlessand hit rate will be really low Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Variation - Don’t do it like that Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Response HTTP/1.1 200 OK Vary: Accept-Encoding, User-Agent Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome … Resource /index.html Cached Representations Accept-Encoding: gzip User-Agent: Chrome … Accept-Encoding: gzip User-Agent: Edge …
  • 40.
    Response HTTP/1.1 200 OK Vary:Accept-Encoding, X-Device Variation - Create generic header to improve hit rate and return private in Cache-Control to avoid caching Browser Origin Server Resource /index.html Request GET /index.html HTTP/1.1 Accept-Encoding: gzip User-Agent: Chrome ... Response HTTP/1.1 200 OK Cache-Control: private Representation Intermediate Cache (Akamai, Varnish) Representation Request GET /index.html HTTP/1.1 Accept-Encoding: gzip X-Device: Mobile Resource /index.html Cached Representations Accept-Encoding: gzip X-Device: Mobile Accept-Encoding: gzip X-Device: Desktop
  • 41.
  • 42.
    Conclusion Use Cache-Control headerin 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
  • 43.

Editor's Notes

  • #8 Representation - different based on request (mobile, desktop; uncompressed, compressed)
  • #11 Why? Caches, security, routing, ...
  • #12 Caching, Security, Routing, Logging Cross-cutting concerns
  • #13 Can cache representations Shorter response time Lower latency with edge caches (cache nodes near customer) Better scalability
  • #19 Why? What is the downside?
  • #24 -do not use without revalidation for subsequent requests -behaviour depends on browser -back-button might not qualify as subsequent request
  • #27 Why? What is the downside?