bxcodec · December 21, 2018 14:22
diff --git a/gistfile1.txt b/gistfile1.txt
 # ETags & If-None-Match headers: a dialogue

 ### 1st request.

 browser: **can haz /foo?**

    GET /foo HTTP/1.1

 ### 1st response.

 server: **o hai, dis page version is 12345.**

 note, there's always an empty line between headers & body.

    HTTP/1.1 200 OK
    ETag: "12345"

    <!doctype html><p>foo.

 ### 2nd request.

 browser: **hai. can haz latest? i haz 12345.**

    GET /foo HTTP/1.1
    If-None-Match: "12345"

 ### 2nd response if current version is unchanged

 server: **you haz latest!**

    HTTP/1.1 304 Not Modified

 ### 2nd response if current version has changed

 if page has changed + new version is 56789,

 server: **lol wut! herez latest**

    HTTP/1.1 200 OK
    ETag: "56789"

    <!doctype html><p>foo. too.

 ### Because the 304 has no response body, client and server save bytes & time.

 GET requests with ```If-None-Match``` headers are called **conditional GET** requests, since the server only returns a response body if there's new content. You can also use date-based validation to issue conditional GETs, saving transfer bandwidth without implementing ETags. The flow is the same, except the server sends down an ```Expires``` date, and the browser sends it back as a ```Last-Modified``` header.

 See the [HTTP 1.1 RFC](http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2) for more details. It's more readable than you'd think.

 Also, those are valid html5 documents in the examples; I just omitted [optional tags](http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html#syntax-tag-omission).
	# ETags & If-None-Match headers: a dialogue

	### 1st request.

	browser: can haz /foo?

	GET /foo HTTP/1.1

	### 1st response.

	server: o hai, dis page version is 12345.

	note, there's always an empty line between headers & body.

	HTTP/1.1 200 OK
	ETag: "12345"

	<!doctype html><p>foo.

	### 2nd request.

	browser: hai. can haz latest? i haz 12345.

	GET /foo HTTP/1.1
	If-None-Match: "12345"

	### 2nd response if current version is unchanged

	server: you haz latest!

	HTTP/1.1 304 Not Modified

	### 2nd response if current version has changed

	if page has changed + new version is 56789,

	server: lol wut! herez latest

	HTTP/1.1 200 OK
	ETag: "56789"

	<!doctype html><p>foo. too.

	### Because the 304 has no response body, client and server save bytes & time.

	GET requests with ```If-None-Match``` headers are called conditional GET requests, since the server only returns a response body if there's new content. You can also use date-based validation to issue conditional GETs, saving transfer bandwidth without implementing ETags. The flow is the same, except the server sends down an ```Expires``` date, and the browser sends it back as a ```Last-Modified``` header.

	See the [HTTP 1.1 RFC](http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2) for more details. It's more readable than you'd think.

	Also, those are valid html5 documents in the examples; I just omitted [optional tags](http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html#syntax-tag-omission).
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