WEBVTT 00:00.410 --> 00:03.170 So welcome back to the Knowledge Portal Video series. 00:03.350 --> 00:08.570 So in the last module, we have already discussed the two directives within the cache control header, 00:08.600 --> 00:11.930 which were the max age and max age. 00:13.400 --> 00:19.190 Today we are going to talk about the no cash and must revalidate directives. 00:20.390 --> 00:30.290 So taking our previous example again, so we have already seen this diagram in the last module where 00:31.010 --> 00:34.010 a client is requesting for a particular file. 00:35.940 --> 00:38.370 And the cash is getting the response. 00:38.370 --> 00:45.150 It is storing the file within the cache memory and also it is sending it to the browser. 00:46.120 --> 00:55.960 Now, we have also seen where we mentioned no store directive, which specifies that this particular 00:55.960 --> 01:02.350 file or document associated with a no store directive should not be stored either in the public cache 01:02.350 --> 01:04.420 or in the private cache. 01:05.580 --> 01:09.390 So there is one more important directive calls no cash. 01:10.160 --> 01:16.850 So no cash, as the name indicates, does not mean that do not store the cash. 01:17.150 --> 01:23.270 It just means that you are free to store the file in your cash. 01:23.270 --> 01:29.780 But whenever you serve it to the client, just revalidate it with the origin server, whether the file 01:29.780 --> 01:30.830 is fresh or not. 01:31.400 --> 01:35.150 So this is the only thing which no cache means. 01:35.780 --> 01:39.290 So let me show you in the. 01:40.210 --> 01:43.450 Enginex server so that it becomes much more clearer. 01:44.590 --> 01:52.630 So if we go into the Nginx configuration here, we have one header which is Cache-control, no store. 01:53.200 --> 01:55.150 So if we do a curl. 01:58.190 --> 02:03.230 And here you see the web server is responding with no store header. 02:04.680 --> 02:07.440 So if I go to example.com. 02:08.790 --> 02:11.040 Slash web three dot HTML. 02:12.300 --> 02:14.010 You see, the website has opened. 02:14.690 --> 02:17.750 Now if I go to about cash in the browser. 02:19.820 --> 02:22.010 And go to web three dot HTML. 02:22.820 --> 02:30.200 It will not find because the no store header has been introduced and this is the reason why Chrome is 02:30.200 --> 02:34.160 not storing this particular document inside its memory. 02:35.300 --> 02:36.380 Now. 02:39.300 --> 02:47.550 Let's comment out this header and let's add a new header which is cache-control. 02:48.030 --> 02:49.050 No cache. 02:53.390 --> 02:55.970 I'll save this and I'll reload the engines. 02:57.080 --> 03:00.580 Let's do a curl again to just verify. 03:00.590 --> 03:04.310 And now here we can see we have no cache header. 03:05.330 --> 03:10.370 So if we refresh the page, let's, let's open this page in one more tab. 03:11.510 --> 03:12.870 And the page has opened. 03:12.950 --> 03:15.350 So if we go to about cache now. 03:16.660 --> 03:19.090 And search for web three dot HTML. 03:19.210 --> 03:25.240 Now you see the Chrome has actually saved this particular document in its cache. 03:26.670 --> 03:34.290 Now, if I open this document, Chrome will have to revalidate it with the server again to make sure 03:34.290 --> 03:39.520 that this is the updated document and this is what no cache means. 03:39.540 --> 03:48.930 So every time I open this document, Chrome has to send a validation query back to the server asking 03:48.930 --> 03:54.030 whether this is the latest and the most fresh document which is available on the server. 03:55.810 --> 04:02.500 Now, there is one more important thing that we should remember that let's say a server has send a no 04:02.500 --> 04:08.800 cache and the document is stored both in the intermediary proxy as well as the browser cache. 04:09.070 --> 04:12.640 Now what happens if the origin server goes down? 04:13.780 --> 04:15.250 Then how will. 04:16.560 --> 04:21.270 A cash revalidate, whether the document is the latest or not. 04:21.810 --> 04:23.520 So that is a big question. 04:23.520 --> 04:29.730 So during that time, what happens is the cash will send a header college age with a warning saying 04:29.730 --> 04:33.390 that this is a stale document and I am serving you. 04:33.390 --> 04:34.350 The stale document. 04:34.350 --> 04:40.770 Stale basically means older version of the document, which cash is not very aware if that is the latest 04:40.770 --> 04:41.130 one. 04:43.010 --> 04:50.000 So many times requirement comes that you should not serve stale document at any cost. 04:50.390 --> 04:55.730 So during that time there is a header called as Must Revalidate, which comes into the picture. 04:56.660 --> 04:57.530 So. 05:01.750 --> 05:04.000 So talking about must revalidate. 05:04.000 --> 05:05.380 I'll add a header. 05:07.400 --> 05:11.630 See Cache control must revalidate. 05:13.580 --> 05:22.190 So this basically indicates that cash in no circumstances should send a stale document. 05:23.120 --> 05:28.670 And it should revalidate every time for a document freshness. 05:28.880 --> 05:34.550 So if a origin server is down due to some reason, then it should throw an error back and it should 05:34.550 --> 05:36.140 not serve a stale response. 05:36.260 --> 05:39.140 So this is very important thing to know. 05:41.270 --> 05:43.250 One more directive. 05:43.250 --> 05:50.060 We have already talked about the public and private, so let's see on how we can use this in conjunction 05:50.060 --> 05:51.470 with other headers. 05:51.530 --> 05:54.710 So I'll say add header cache control. 05:56.410 --> 05:56.600 I. 05:56.960 --> 05:58.360 I'll say private. 06:00.160 --> 06:03.040 Followed by Max H is equal to 200. 06:04.420 --> 06:06.790 So before we do this, let's just. 06:08.710 --> 06:10.300 Delete the older headers. 06:13.590 --> 06:14.160 Okay. 06:14.160 --> 06:17.970 So this basically means that. 06:20.160 --> 06:25.350 Cash control should be private, followed by max age of 200 seconds. 06:25.590 --> 06:26.950 I'll add one more header. 06:26.970 --> 06:29.850 This will be specific for the public proxies. 06:30.120 --> 06:39.780 I'll say cache control public followed by s max age is equal to let's assume 500 seconds. 06:42.260 --> 06:44.990 So this basically means that. 06:46.480 --> 06:52.120 For private cash, which basically can indicate browser related cash. 06:52.570 --> 06:58.690 Max age should be 200 seconds, and for public cash max, it should be 500 seconds. 06:58.990 --> 07:01.360 So this is how you can actually. 07:03.760 --> 07:05.350 Add multiple header fields. 07:05.350 --> 07:09.340 You can also add, say, cache control. 07:10.710 --> 07:14.310 First should be no cash as well as must revalidate. 07:20.770 --> 07:29.470 So this basically means that the browser or a cache server are free to store the document in its cache, 07:29.680 --> 07:35.410 but it should revalidate every time a request is received. 07:35.500 --> 07:40.750 So if the server is down, then you should not serve stale responses. 07:40.960 --> 07:48.790 So if we would have not included the revalidate header then a lot of cache servers will serve the stale 07:48.790 --> 07:50.710 responses if the server is down. 07:51.360 --> 07:54.510 So it is very important to remember this. 07:54.870 --> 08:00.630 So this is it about the non-cash as well as the revalidate header. 08:00.930 --> 08:04.630 So I hope this basic concept has been clear to you. 08:04.650 --> 08:07.860 I hope this has been informative for you and I'd like to thank you for.