WEBVTT 00:00.260 --> 00:03.830 Hey everyone, and welcome to the Knowledge Portal Video series. 00:03.860 --> 00:11.090 Now, in the earlier lecture we were discussing about the Http caching and we had an overview about 00:11.090 --> 00:11.420 it. 00:11.420 --> 00:18.410 So continuing further today, we'll be speaking about the cache control headers in the Http protocol. 00:18.950 --> 00:21.050 So let's get started. 00:21.860 --> 00:23.030 Now. 00:24.290 --> 00:31.550 In the earlier lecture we discussed about this use case where you have a cache server and cache server 00:31.550 --> 00:38.910 has basically have the PNG file which was requested earlier by the client. 00:38.930 --> 00:47.600 So next time when the client refreshes the page, what would happen is that the request will go to the 00:47.600 --> 00:48.500 cache server. 00:48.530 --> 00:57.320 Now cache server already has this MC dot png in the local cache repository, so instead of sending the 00:57.320 --> 01:03.830 request back to the server, what it will do is it will fetch the response from the cache repository 01:03.830 --> 01:07.040 and it will send it back to the client. 01:07.780 --> 01:15.340 Now, there is one challenge over here is that what happens if this file, which is stored in the cache, 01:15.340 --> 01:18.040 gets updated with a newer file? 01:18.310 --> 01:23.530 So let's assume that this cache copy is sent to the client. 01:23.530 --> 01:30.590 But on the backend side, this PNG file got updated on the server side. 01:30.610 --> 01:37.960 So in that case, what really happens is that client has got a stale response or you can say client 01:37.960 --> 01:42.430 received a older response and not the newer response. 01:42.670 --> 01:45.800 Now there can be a lot of such cases. 01:45.820 --> 01:53.470 Now one example I can show you is of a stock market or a share market where what you want is you do 01:53.470 --> 02:01.690 not want anything to be cached because if you get an older response, then there can be a huge financial 02:01.690 --> 02:02.340 losses. 02:02.350 --> 02:10.970 So in the domain of stock markets or in banking systems, you do not want things to be cached. 02:11.240 --> 02:12.950 So how do you do that? 02:12.950 --> 02:21.140 How can you tell a browser or how can a web server tell a browser or a cache server to not cache the 02:21.140 --> 02:22.490 responses at all? 02:22.940 --> 02:26.150 And the answer is through the cache control headers. 02:26.690 --> 02:37.310 So cache control headers are basically used to specify various directives which are used for caching 02:37.310 --> 02:38.270 mechanisms. 02:38.270 --> 02:45.710 So there are certain directives of cache control headers, and these directives needs to be followed 02:45.710 --> 02:51.950 by the caching servers, be it in browser or be it a dedicated cache servers. 02:52.280 --> 02:59.510 So these headers are used to define various caching policies with various directives provided by the 02:59.510 --> 02:59.810 headers. 02:59.810 --> 03:07.150 So there can be various caching policies which can be dictated by the website. 03:07.160 --> 03:12.850 So one of the caching policies is like do not store any kind of cache at all. 03:12.860 --> 03:15.530 So this is one caching policy. 03:15.530 --> 03:23.060 So this can be followed by stock market or banking systems where they do not want client browsers or 03:23.060 --> 03:25.930 any caching proxies to store any cache. 03:25.940 --> 03:28.000 So this is one example. 03:28.010 --> 03:34.220 Second example is store the cache, but verify with the web server whether the file is modified or not. 03:34.310 --> 03:35.870 This is second kind. 03:35.900 --> 03:38.930 Third example is store the cache for 24 hours. 03:38.930 --> 03:42.560 So these are three different examples that we have looked. 03:42.800 --> 03:44.780 So let's do one thing. 03:44.780 --> 03:50.870 Let's go into our practical session and understand this in more detail. 03:51.080 --> 03:58.850 Now, what I have done is I have two files over here in our lab environment. 03:58.880 --> 04:03.440 One is no store dot PNG and one is my C dot jpg. 04:03.680 --> 04:05.690 So let's do one thing. 04:06.620 --> 04:12.500 Let's do a curl, I'll do a curl hyphen I on. 04:12.500 --> 04:19.190 Let's do Dexter dot labs dot in hyphen mike dot JPG. 04:21.680 --> 04:28.760 So now what will happen over here is that you don't really have any cache control headers. 04:29.180 --> 04:30.350 However 04:33.800 --> 04:35.510 if you do a no store. 04:38.070 --> 04:38.970 Dot PNG. 04:39.690 --> 04:47.280 Now you see there is one header called as cache control header and it says no hyphen store. 04:47.310 --> 04:51.900 However, for earlier file there was no cache control headers. 04:51.900 --> 05:01.380 So this cache control headers are the policies which are dictated by the web server to the caching servers. 05:01.800 --> 05:11.070 So caching server will read this headers and then it will determine what are the policies that it should 05:11.070 --> 05:13.920 apply for this specific file. 05:14.760 --> 05:15.780 Now. 05:16.940 --> 05:25.010 Again since this cache control header says no store, then the browser will actually not store this 05:25.010 --> 05:27.470 specific file in the cache. 05:27.620 --> 05:34.880 However, as far as my c dot jpg is concerned, since there is no caching policies which is explicitly 05:34.880 --> 05:41.690 dictated by the web server, the browser will go ahead and store this specific file in the cache. 05:41.960 --> 05:50.420 So we will be looking into this in great detail in the next lecture where we'll be speaking about the 05:50.450 --> 05:51.700 no store header. 05:51.710 --> 05:58.490 But for the time being I hope you got what the cache control headers are. 05:58.520 --> 06:06.980 Now going back to our PowerPoint presentation, now there are various cache control headers directives 06:06.980 --> 06:07.970 which are present. 06:07.970 --> 06:13.760 So this is a cache control header and there are various directives to cache control headers like no 06:13.760 --> 06:15.020 store, no cache. 06:15.050 --> 06:20.550 They can be used in combination together, like no store, comma, no cache, comma must revalidate. 06:20.970 --> 06:24.600 Along with that, there are other cache control headers like public private. 06:24.630 --> 06:31.290 There are more of them and we will be discussing about these headers with practical in the upcoming 06:31.290 --> 06:35.760 lectures to see which what each of these headers really do. 06:36.000 --> 06:37.860 So this is it. 06:37.860 --> 06:39.300 About this lecture. 06:39.300 --> 06:46.920 I hope you understood what cache control headers are all about and if you have any doubts, suggestions 06:46.920 --> 06:52.590 or questions, feel free to connect us at Twitter, Facebook or LinkedIn or mail us at instructors at 06:52.950 --> 06:53.580 KP Labs. 06:53.970 --> 06:55.200 Thanks for watching.