WEBVTT 00:00.320 --> 00:00.980 Hey, everyone. 00:00.980 --> 00:02.300 And welcome back. 00:02.300 --> 00:08.510 Now, in the earlier lectures we were discussing about the Http cache control headers and we looked 00:08.510 --> 00:11.150 into the store header in detail. 00:11.150 --> 00:17.240 So continuing our journey with the cache control headers today we'll be speaking about if modified since 00:17.240 --> 00:17.770 header. 00:17.780 --> 00:25.130 Now this is quite important header and this specific header is sent quite a lot by the browsers. 00:25.130 --> 00:29.480 So let's go ahead and understand more about this header. 00:29.720 --> 00:36.890 Now, we had looked into the scenarios where there can be multiple same requests by the browser, so 00:36.890 --> 00:45.590 if a user refreshes the page 5 or 10 times, then what Ideally a cache does is it sends the response 00:45.590 --> 00:49.190 from the cache itself instead of connecting to the web server. 00:49.190 --> 00:51.500 So let's look into the scenario. 00:52.350 --> 00:57.450 So let's assume there is a file which is already cached in the. 00:58.690 --> 00:59.830 Cash server. 00:59.830 --> 01:05.170 So what happens is a client sends a request of get my C dot PNG. 01:05.740 --> 01:14.320 Now since the cache already has this file in its local storage, what it will do is it will send the 01:14.320 --> 01:17.460 file from the local store itself. 01:18.160 --> 01:20.440 Now, this is quite good enough. 01:20.440 --> 01:27.400 But the problem is what happens if the file in the web server side is updated? 01:27.520 --> 01:33.640 Now, in that case, the caching server has sent a older response. 01:33.640 --> 01:42.960 That means the new updated file is not sent because of the cache servers misconfiguration related settings. 01:42.970 --> 01:52.840 So in order to solve this kind of a issue where a older response might be sent, a new header is generally 01:52.840 --> 01:55.240 used as if modified since header. 01:55.390 --> 02:04.510 So what really happens is let's assume you have a this is a my C dot PNG file which is there in the 02:04.540 --> 02:09.220 web server side and it is also there in the cache server side. 02:09.520 --> 02:19.910 Now when a client sends a request, a get request on my c dot PNG, although the cache server has this 02:19.910 --> 02:28.130 file, it will still verify whether the file which is there in its local cache is the latest updated 02:28.130 --> 02:28.640 one. 02:29.420 --> 02:35.900 So how can a cache server know how this file is updated or not? 02:35.900 --> 02:39.680 So what it can do is the cache server will send. 02:41.670 --> 02:51.510 A get request on my PNG asking whether this file is modified since this specific amount of time. 02:51.510 --> 03:01.830 So this time is basically the time associated with the last modified header of the specified PNG file. 03:02.070 --> 03:08.580 So the cache server will send a get request asking if this file is modified or not. 03:08.580 --> 03:10.350 After this specific time. 03:10.500 --> 03:22.050 If it is not modified then the web server will send a response saying 304 not modified and then the 03:22.050 --> 03:27.390 cache server will serve the file from the local cache itself. 03:27.600 --> 03:34.880 However, if this file is modified, then the web server will not send 304. 03:34.890 --> 03:37.410 It will send a 200 with a new file. 03:38.490 --> 03:39.120 Perfect. 03:39.120 --> 03:40.840 So let's do one thing. 03:40.840 --> 03:45.730 Let's go into the practical and let's understand this in great detail. 03:46.880 --> 03:54.470 So we have been looking into the practical and we'll try and keep the link same. 03:54.470 --> 03:58.790 So you have Dexter Labs dot in slash JPG. 04:00.710 --> 04:07.520 Now if you do a curl hyphen I on this specific URL 04:10.220 --> 04:20.120 you will see there is one specific header called as last modified and it has a modified or it has the 04:20.120 --> 04:24.320 time at which this file was specifically updated. 04:24.470 --> 04:25.880 My c dot jpg. 04:26.150 --> 04:28.520 Now let's. 04:29.790 --> 04:35.250 Take the example of a Docker instance so that it will become much more easier for us to do practical. 04:35.460 --> 04:41.170 I'll do a curl hyphen I and it contains the last modified time of this specific file. 04:41.190 --> 04:52.110 Now if you look into the I'll copy this key and in my Chrome browser if you look into the about cache. 04:55.370 --> 04:58.580 You see, this specific file is already cached. 04:58.700 --> 05:07.220 Now, whenever I enter this in my URL, the entry should be retrieved from the cache itself. 05:07.250 --> 05:17.990 However, in order to make sure that the older file is not sent, what browser will do, what the browser 05:17.990 --> 05:25.970 cache will do is it will send a get request to the server with a if modified since header. 05:26.750 --> 05:28.430 So let's try this out. 05:29.510 --> 05:36.650 Just remember that this file was modified on Sunday 18th October 2015. 05:37.040 --> 05:38.300 Quite old file. 05:39.650 --> 05:42.730 So this is basically a PNG file. 05:42.740 --> 05:46.640 We are actually recording this lecture at the end of 2017. 05:46.850 --> 05:51.410 But anyways, so let's do one thing. 05:51.980 --> 05:56.150 I'll do a tail on var log nginx access dot log. 05:59.350 --> 06:09.250 And now let's open this file up and let's refresh. 06:09.340 --> 06:11.920 Okay, Now, if we look into the 06:14.380 --> 06:16.580 Enginex access log, what do you see? 06:16.600 --> 06:18.330 You see A304. 06:18.340 --> 06:27.280 So the reason why you're seeing A304 is because my browser cache, my browser cache asked It sent a 06:27.310 --> 06:32.440 get request on the file name asking whether it was modified. 06:32.440 --> 06:39.520 Since now since the file was not modified, the web server sent A304 not modified response back. 06:40.540 --> 06:47.940 So this is the basic let let me show you this output in tcpdump. 06:47.950 --> 06:54.040 So I do a tcpdump hyphen, a hyphen I zero and on port 80. 06:54.910 --> 07:02.630 Now this will give you a much more clear idea just remember, this is the last modified time. 07:02.850 --> 07:07.310 Now I'll start my tcpdump capture. 07:11.090 --> 07:12.980 Okay, so the capture has started. 07:14.390 --> 07:15.380 I'll refresh. 07:15.380 --> 07:17.960 I'll refresh once and again. 07:17.960 --> 07:21.560 The browser must have sent a if modified since header. 07:22.070 --> 07:23.210 Let's see. 07:24.620 --> 07:30.830 And here if you see this is basically the let me go up. 07:36.350 --> 07:37.910 Let's go. 07:38.120 --> 07:38.540 Perfect. 07:38.540 --> 07:42.140 So this is basically the file. 07:42.140 --> 07:50.420 So now you see, this is the if modified since header where the browser is asking whether this file 07:51.380 --> 08:00.470 you see the get request is get Docker dot jpg because we actually requested for Docker dot jpg and now 08:00.950 --> 08:10.100 there is a header which browser has added saying whether this file has been modified since this specific 08:10.100 --> 08:10.610 time. 08:10.610 --> 08:17.480 So whenever a browser or a cache server stores a file, this specific file is stored in the cache. 08:17.480 --> 08:27.080 It will note the last modified date, so it will note this specific date and whenever the cache server 08:27.080 --> 08:35.280 will send a request to the web server in the if modified, it will append the date of the last modified. 08:35.280 --> 08:45.270 So since the last modified is 18th October 2015, it has appended that specific date and said whether 08:45.270 --> 08:48.780 this file is modified since this specific time. 08:48.780 --> 08:54.690 Now, if it is not modified, then the web server. 08:54.690 --> 09:02.310 If you look into the next response, the web server has sent that this specific file is not modified 09:02.310 --> 09:04.440 after this specific date. 09:04.440 --> 09:14.130 And now since my cache or a web browser now received a 304 response, what it will do is it will serve 09:14.160 --> 09:17.120 the file from the cache itself. 09:17.130 --> 09:23.340 Now, one important thing to remember is that whenever a web server sends a 304 not modified, it will 09:23.340 --> 09:24.300 not send a file. 09:24.300 --> 09:28.530 It will just send a response without a file. 09:29.130 --> 09:34.590 So this is the basics about the if modified since header. 09:34.590 --> 09:40.020 I hope you got the basic understanding on the importance of if modified since header. 09:40.200 --> 09:43.200 I hope this lecture has been informative for you. 09:43.200 --> 09:49.650 Again, go ahead and try this out because practical is very important and if you have any issues you 09:49.650 --> 09:51.810 can let us know in our forums. 09:51.810 --> 09:52.920 So this is it. 09:52.920 --> 09:58.560 If you have any questions, doubts, suggestions, feel free to connect us at Twitter, Facebook or 09:58.560 --> 10:02.760 LinkedIn or email us at instructors at the Rate Labs dot n. 10:02.790 --> 10:04.020 Thanks for watching.