WEBVTT 00:00.440 --> 00:06.590 So in the last module we were discussing about the basics of the cache control header and the directives 00:06.830 --> 00:10.010 for the newer Http 1.1 protocol. 00:10.370 --> 00:17.750 So today we are going to talk in detail about the cache control header directives and two of the important 00:17.750 --> 00:20.810 directives are max age and max age. 00:22.010 --> 00:24.860 So revising our concepts on caching. 00:25.810 --> 00:32.830 So if a browser requests for a particular document and if there is an intermediary proxy in between, 00:32.860 --> 00:37.030 then the request first goes to the cache proxy over here. 00:37.120 --> 00:42.760 The cache proxy will try to validate if the file is present within the proxy memory. 00:43.030 --> 00:51.670 If it is not present, then the proxy will forward the request to the origin server where the file is 00:51.670 --> 00:52.420 present. 00:54.300 --> 00:59.970 Now, in response, the Origin server will send the file back to the cache server. 01:00.570 --> 01:06.090 Cache server will store the file depending upon the cache control headers, which origin server has 01:06.090 --> 01:10.290 sent, and it will also send the file back to the client. 01:11.660 --> 01:15.770 Now there are two important concepts that are important to know over here. 01:17.000 --> 01:20.240 The concepts of public and private cash. 01:21.350 --> 01:28.730 So if we talk about this intermediary cash server, it can happen that within an organization there 01:28.730 --> 01:36.980 is one intermediary cash server and that actually does the cashing of, say, 200 users within an organization. 01:37.540 --> 01:45.700 So essentially this cache server is being used by multiple users, so this can be considered as public 01:45.700 --> 01:46.360 cache. 01:47.350 --> 01:52.000 However, we know that even browser has its own cache memory. 01:52.840 --> 01:56.860 And that cash is private to a particular user. 01:56.860 --> 01:59.800 And this is why it is called as private cash. 02:00.250 --> 02:03.550 So again, revising we have two important concepts. 02:03.550 --> 02:06.550 One is public cash and one is private cash. 02:08.880 --> 02:15.540 So talking about the max age and max age directives within the cache control header. 02:17.110 --> 02:21.940 We have a in the cache control max age is equal to 120. 02:21.970 --> 02:25.850 So this 120 is basically the timeline in seconds. 02:25.870 --> 02:34.810 So if a nginx or if a web server sends along with a document, this directive which is cache-control 02:34.810 --> 02:36.730 max age is equal to 120. 02:36.760 --> 02:44.470 So it basically determines that the web server is saying to the cache proxy, both public and private, 02:44.500 --> 02:50.170 to consider this document as fresh for one 20s. 02:51.130 --> 03:00.130 After one 20s This document will not be considered fresh and it will ask the cache to revalidate it. 03:02.020 --> 03:06.610 Now there is a second header college s max age. 03:06.640 --> 03:11.830 Now s max age is specifically for the intermediary cache servers. 03:12.920 --> 03:19.190 So let's assume for a particular file there are two headers which are sent, which is Max H is equal 03:19.190 --> 03:22.220 to 120 and S max is is equal to 200. 03:22.490 --> 03:32.480 So what it will mean is it will mean that the public cache server should store this file for 200 seconds 03:32.480 --> 03:34.130 before it gets stale. 03:35.030 --> 03:41.690 However, for private cache, the file should be considered as fresh only for one 20s. 03:42.050 --> 03:46.100 So these are directives specifically for public and private cache. 03:47.690 --> 03:52.130 So let's go back to our links and see on how exactly it works. 03:54.230 --> 04:01.670 So let's let me do a curl on width to show the headers for http example.com. 04:03.570 --> 04:04.570 One dot HTML. 04:06.040 --> 04:11.710 So here, if we see there are no cache control headers which are present, even the expired header is 04:11.710 --> 04:12.690 not present. 04:12.700 --> 04:17.680 So the web server specifically is not saying when the document will expire. 04:18.790 --> 04:26.080 So during this cases, the browser will have its own heuristic method to determine the expiration time. 04:27.520 --> 04:31.270 So let's go to the Enginex configuration. 04:33.770 --> 04:37.550 And let's mention our first cache control header. 04:38.270 --> 04:41.030 So I'll say add header. 04:42.100 --> 04:44.680 So this is basically used for adding the header. 04:45.420 --> 04:46.710 Saying cash. 04:47.470 --> 04:52.450 Control followed by Max H is equal to 120. 04:53.860 --> 05:02.020 So this basically means that any document that ends with Dot HTML will have a cache control header with 05:02.020 --> 05:06.520 a directive max hyphen age is equal to one 20s. 05:07.510 --> 05:11.120 So I'll save this and let's reload the nginx. 05:13.780 --> 05:15.760 And if we do a curl again. 05:16.060 --> 05:16.720 Curl. 05:16.720 --> 05:25.480 I say example.com one dot HTML and now you have seen the cache control header with max. 05:25.480 --> 05:28.030 Age is equal to one 20s has been added. 05:29.290 --> 05:36.520 So let's open our favorite wireshark and let's clear this up and let's start a new packet capture. 05:37.240 --> 05:48.940 Okay, I'll go to my chrome and I'll say Http or I'll say example.com web one dot HTML. 05:52.230 --> 05:54.790 And this is a simple website which has been loaded. 05:54.810 --> 05:56.940 So if I go to about cash. 06:00.250 --> 06:08.650 And let's filter it on Web one dot HTML and let's click here and it will basically give us the. 06:10.070 --> 06:11.570 Headers that it has received. 06:11.690 --> 06:17.290 And here you can see cache control where Max hyphen age is one 20s. 06:17.300 --> 06:23.260 So this basically instructs Chrome that this document is valid only for one 20s. 06:23.270 --> 06:29.630 And after this time has expired, the Chrome should again revalidate with the server to check the freshness 06:29.630 --> 06:31.550 of a particular document. 06:32.550 --> 06:40.110 Okay, so now let's go back to the Nginx and let's look at. 06:44.000 --> 06:45.260 One more directive. 06:49.670 --> 06:52.790 So I'll comment the first one out. 06:54.290 --> 06:58.430 And as it's very interrelated with the. 07:00.630 --> 07:02.280 Cache-control Max age. 07:02.280 --> 07:02.690 I'll. 07:04.260 --> 07:05.940 At this in this video itself. 07:05.940 --> 07:08.730 So it's no store. 07:11.560 --> 07:18.800 So what does no store means is that do not store this particular document in cache. 07:18.820 --> 07:25.240 So this specifies both the public as well as private cache servers do not store this document at all. 07:25.990 --> 07:28.150 So let's see on how it works. 07:28.150 --> 07:30.340 So I'll save the Nginx. 07:30.490 --> 07:31.570 I'll do reload. 07:32.970 --> 07:34.740 Let's do a curl again. 07:37.510 --> 07:39.490 Let's open up one more document. 07:39.490 --> 07:46.930 And here, if we see the web server is sending no stored directives along with the web two dot HTML 07:46.930 --> 07:47.200 file. 07:48.370 --> 07:49.300 Now. 07:50.660 --> 07:53.440 Let's open the web two HTML. 07:55.310 --> 07:56.000 Over here. 08:01.160 --> 08:03.140 And the same website has loaded. 08:03.140 --> 08:06.080 So now if I go to about cash in the. 08:07.740 --> 08:11.430 And if I try to find web two dot HTML. 08:13.420 --> 08:16.870 This is just a Google search which had done previously. 08:17.050 --> 08:22.150 Here you will not find specifically example.com slash web2 dot HTML. 08:22.150 --> 08:23.620 So if I search over here. 08:24.710 --> 08:25.120 I. 08:26.680 --> 08:27.670 You see it is not finding. 08:27.670 --> 08:37.090 But if I type web one dot HTML which had the cache control header, you see it is present in the cache. 08:37.300 --> 08:45.640 So this is why it is important to know that if you have some document which you don't want a client 08:45.640 --> 08:53.950 to store or even a cache server to store, then you can add a no store header to the document or to 08:53.950 --> 08:59.890 the Http response, and this will instruct the browser as well as the cache server to not store the 08:59.890 --> 09:00.640 document. 09:02.650 --> 09:08.830 Now there is the third header which I added up. 09:09.670 --> 09:13.060 Add header cache-control. 09:14.440 --> 09:17.110 Max age is equal to 200. 09:19.460 --> 09:20.990 Let's save this. 09:22.910 --> 09:24.890 And if I do a call. 09:32.170 --> 09:34.490 You see there are three headers which are added. 09:35.720 --> 09:38.180 One is max age, no store and max age. 09:38.180 --> 09:39.080 So let's see. 09:39.110 --> 09:40.610 Max Age and Max age. 09:40.940 --> 09:49.010 So if we have max, age is equal to 120 and Max is is equal to 200, then the browser will store the 09:49.430 --> 09:52.340 particular document for one 20s. 09:52.370 --> 09:59.570 However, the cache server or the public cache server will store the document for 200 seconds. 10:00.530 --> 10:10.790 Now, if the Max Age directive is not present and only the Max directive is present, then both the 10:10.790 --> 10:18.080 browser cache as well as the intermediary proxy cache will store the document for one 20s. 10:18.470 --> 10:26.990 So adding a max age allows us a lot of flexibility to determine on whether the document should be stored 10:26.990 --> 10:29.870 in the public cache or in the private cache. 10:29.870 --> 10:33.030 And if yes, then for how many seconds? 10:33.420 --> 10:41.430 So this type of flexibility was not provided in the Http 1.0 protocol where we just had the expires 10:41.430 --> 10:41.820 header. 10:43.260 --> 10:45.540 So this is it about this video. 10:45.570 --> 10:47.460 I hope this has been useful for you. 10:47.460 --> 10:49.410 And I'd like to thank you for viewing.