WEBVTT 00:00.260 --> 00:02.680 Welcome to the Knowledge Portal Video series. 00:02.690 --> 00:09.920 So today we are going to talk about two of the important Http headers which are useful specifically 00:09.920 --> 00:12.740 for the Http caching is concerned. 00:14.280 --> 00:18.630 So revising the caching concept that we have already learned. 00:19.080 --> 00:24.180 So if a client sends a request, the request goes to the cache. 00:24.660 --> 00:32.130 Now, if the cache does not have a response stored, then it will forward the request to the origin 00:32.130 --> 00:32.760 server. 00:34.050 --> 00:38.040 Now the origin server will send the response back to the cache. 00:38.850 --> 00:47.040 The cash will force store the response inside its cash memory and also send it back to the user. 00:48.870 --> 00:54.880 Now the question that comes is for how long the response should be stored in the cache. 00:54.900 --> 01:01.290 Should it be stored for one hour or maybe 24 hours, maybe 48 hours or maybe even one year? 01:01.740 --> 01:03.750 So you do not know that? 01:05.230 --> 01:10.060 And just touring based on probability is not the right way either. 01:10.930 --> 01:19.090 And this is one of the reasons why Http has introduced two of the important headers which are date and 01:19.090 --> 01:19.960 expiry. 01:24.490 --> 01:35.290 Now expire headers basically defines for how long a resource should be stored in the cache memory. 01:35.950 --> 01:45.820 So expire headers can tell the cache to store the response maybe for two hours, maybe for one hour, 01:45.850 --> 01:47.950 or to not store the response at all. 01:49.390 --> 01:57.070 So if we take a look into an example, this is one of the expires header that comes along with the Http 01:57.070 --> 02:02.890 response packet and it says expires followed by the date. 02:03.130 --> 02:09.850 Now the date that it is sending is in terms of GMT and not the local date of the server. 02:12.090 --> 02:12.990 Now. 02:14.410 --> 02:16.230 Talking about the date header. 02:16.440 --> 02:24.210 The server also sends the current date of the Http response when it was sent. 02:24.690 --> 02:30.180 Again, if you notice the date is in GMT and not in local. 02:31.330 --> 02:39.190 Now let's assume that the client is receiving this header. 02:39.370 --> 02:45.430 Now, it might be possible that the date in the client system is a bit out of sync. 02:46.150 --> 02:54.160 So it might be possible that during that time the response will not be properly cached. 02:54.760 --> 03:04.430 So it is very important that the cache response should be up to date and it should not serve stale responses. 03:04.450 --> 03:11.230 So this is one of the reasons why the date and expire headers are used in conjunction. 03:12.440 --> 03:18.350 So let's assume that this is the date and this is the expire, which the. 03:19.530 --> 03:22.530 Engineers sent in the response. 03:22.680 --> 03:31.440 So if we subtract this, you see the date is same 2353 and the expires is also 2353. 03:31.860 --> 03:33.000 However. 03:33.930 --> 03:36.810 The only difference is the. 03:38.670 --> 03:41.700 D, which is 17 and which is 18. 03:41.730 --> 03:43.650 So you see the. 03:44.610 --> 03:45.040 X. 03:45.270 --> 03:49.380 So if we subtract this data, then we get 24 hours. 03:49.680 --> 03:56.680 So now the cache knows that it should store the response for 24 hours. 03:56.700 --> 04:04.260 So the response will be particularly valid for 24 hours and after 24 hours cache should contact the 04:04.260 --> 04:09.390 Origin server again to get the fresh copy of the particular resources. 04:13.340 --> 04:14.300 Now. 04:16.050 --> 04:21.690 Let's go to the Nginx server and see on how exactly it works. 04:22.970 --> 04:26.120 So let me do a call. 04:27.860 --> 04:29.600 I followed by Http. 04:31.660 --> 04:34.270 example.com slash. 04:34.540 --> 04:36.640 I have a PNG file. 04:39.000 --> 04:41.870 And let me see. 04:41.880 --> 04:44.970 And here you see there is a date header which is introduced. 04:45.480 --> 04:53.250 So date header is something that it is mandatory to send in all the responses along with the date there 04:53.250 --> 04:55.260 is a expired header also. 04:57.660 --> 05:02.010 Now if we see the current date is 18th November. 05:03.010 --> 05:06.400 1237 and expires is. 05:07.210 --> 05:09.470 20th, November 1237. 05:09.490 --> 05:17.230 So if we subtract these two header fields, then we can get that this is a 48 hour time period. 05:17.680 --> 05:25.390 So that means that the web server is saying that this particular resource logo dot PNG should be cached 05:25.390 --> 05:28.960 or should be stored in cache for 48 hours. 05:32.060 --> 05:40.100 Now there is one more header called cache control followed by max hyphen H is equal to this particular 05:40.100 --> 05:40.620 value. 05:40.640 --> 05:45.350 Now this is in seconds, which also indicates 48 hours. 05:46.400 --> 05:56.480 Now we have already seen the difference between Http 1.0 and Http 1.1, and the expires header is specifically 05:56.480 --> 05:58.310 for Http 1.0. 05:58.730 --> 06:02.570 So for Http 1.1, there is. 06:03.610 --> 06:06.730 New directive, which is cash control, which was introduced. 06:06.730 --> 06:10.630 So we'll be talking about this in more detail in the upcoming lectures. 06:10.630 --> 06:14.950 But for the compatibility we have expires header. 06:15.560 --> 06:16.400 As well. 06:18.380 --> 06:26.990 Now this expires header or you can say max age header can be different for different kind of resources. 06:27.110 --> 06:29.870 So if I say sample dot text over here. 06:33.440 --> 06:39.800 And if you see over here, the max age, the reason why I'm saying is because it becomes much more easier 06:39.800 --> 06:40.400 to read. 06:40.430 --> 06:44.060 So this is 3600 seconds, which is one hour. 06:44.850 --> 06:53.430 So if you compare this zero zero 39 and 139, then it becomes one as well. 06:53.640 --> 07:02.790 And through this, you can actually calculate or a cash server can actually calculate on how long should 07:02.790 --> 07:10.680 it consider or for how long should it store this particular response before it considers the response 07:10.680 --> 07:11.790 as old? 07:13.860 --> 07:15.270 So this is it. 07:15.270 --> 07:20.940 About this module, I just wanted to introduce the two headers which are date and expired. 07:21.420 --> 07:28.290 Again, just remember, date header is sent with all the responses and expire header is something that 07:28.290 --> 07:33.480 we can configure depending upon the resources that we want to cache. 07:35.600 --> 07:38.400 The maximum amount that r.f.c.. 07:39.470 --> 07:46.130 Basically recommends that you should cash is one year, so you should not store or you should not tell 07:46.130 --> 07:50.300 the client to cash for a resource for more than one year. 07:51.300 --> 07:53.460 So this is it about this module. 07:53.490 --> 07:57.610 I hope this has been informative for you, and I'd like to thank you for viewing.