1
00:00:01,200 --> 00:00:05,726
Load a page twice, and watch the Network panel the second time.

2
00:00:05,867 --> 00:00:08,141
Some rows don't say 200.

3
00:00:08,166 --> 00:00:10,788
They say 304.

4
00:00:10,988 --> 00:00:14,005
Three oh four means Not Modified.

5
00:00:14,088 --> 00:00:17,511
It's the request you almost skipped.

6
00:00:18,211 --> 00:00:23,457
The first time, Demo Company sent the cat photo with two extra headers.

7
00:00:23,668 --> 00:00:28,472
One said: keep a copy, but check with me before you use it.

8
00:00:28,683 --> 00:00:33,418
The other, the ETag, is a fingerprint of the file.

9
00:00:33,618 --> 00:00:38,179
The second time, the browser asks a question instead of a demand.

10
00:00:38,541 --> 00:00:41,581
If none match this fingerprint, send it.

11
00:00:41,768 --> 00:00:49,498
The file hasn't changed, so the server answers three oh four: no body, just a few hundred bytes of headers.

12
00:00:49,570 --> 00:00:52,865
Forty-two thousand bytes stay home.

13
00:00:53,066 --> 00:00:57,685
Fun fact: the older version of the question uses a date.

14
00:00:57,896 --> 00:01:01,818
Last-Modified, then If-Modified-Since.

15
00:01:01,959 --> 00:01:04,604
The fingerprint is more exact.

16
00:01:05,304 --> 00:01:08,576
Every response can say how long to keep it.

17
00:01:08,717 --> 00:01:13,685
No-store: don't keep a copy, download every time.

18
00:01:13,885 --> 00:01:19,235
Max-age zero, must-revalidate: keep it, but ask first.

19
00:01:19,318 --> 00:01:21,070
That's the three oh four.

20
00:01:21,373 --> 00:01:25,702
It still costs a round trip, but almost no bytes.

21
00:01:25,902 --> 00:01:30,509
A long max-age: keep it for a year and don't even ask.

22
00:01:30,534 --> 00:01:32,111
No request at all.

23
00:01:32,322 --> 00:01:41,063
That's only safe when the URL changes whenever the file does, so build tools put a fingerprint in the file name.

24
00:01:41,762 --> 00:01:43,838
So choose per file.

25
00:01:43,863 --> 00:01:46,776
Pages that change: ask first.

26
00:01:46,917 --> 00:01:51,547
Fingerprinted scripts, styles and images: keep for a year.

27
00:01:51,851 --> 00:01:55,622
Private or one-time answers: don't store.

28
00:01:55,823 --> 00:02:02,532
Open Demo Company's cache page, fetch each card twice, and watch all three answers happen.