As a software developer, you certainly have a high-level picture of how web apps work and what kinds of technologies are involved: the browser, HTTP, HTML, web server, request handlers, and so on.

In this article, we will take a deeper look at the sequence of events that take place when you visit a URL.

1. You enter a URL into the browser

It all starts here:

image

2. The browser looks up the IP address for the domain name

 image

The first step in the navigation is to figure out the IP address for the visited domain. The DNS lookup proceeds as follows:

  • Browser cache – The browser caches DNS records for some time. Interestingly, the OS does not tell the browser the time-to-live for each DNS record, and so the browser caches them for a fixed duration (varies between browsers, 2 – 30 minutes).
  • OS cache – If the browser cache does not contain the desired record, the browser makes a system call (gethostbyname in Windows). The OS has its own cache.
  • Router cache – The request continues on to your router, which typically has its own DNS cache.
  • ISP DNS cache – The next place checked is the cache ISP’s DNS server. With a cache, naturally.
  • Recursive search – Your ISP’s DNS server begins a recursive search, from the root nameserver, through the .com top-level nameserver, to Facebook’s nameserver. Normally, the DNS server will have names of the .com nameservers in cache, and so a hit to the root nameserver will not be necessary.

Here is a diagram of what a recursive DNS search looks like:

500px-An_example_of_theoretical_DNS_recursion_svg 

One worrying thing about DNS is that the entire domain like wikipedia.org or facebook.com seems to map to a single IP address. Fortunately, there are ways of mitigating the bottleneck:

  • Round-robin DNS is a solution where the DNS lookup returns multiple IP addresses, rather than just one. For example, facebook.com actually maps to four IP addresses.
  • Load-balancer is the piece of hardware that listens on a particular IP address and forwards the requests to other servers. Major sites will typically use expensive high-performance load balancers.
  • Geographic DNS improves scalability by mapping a domain name to different IP addresses, depending on the client’s geographic location. This is great for hosting static content so that different servers don’t have to update shared state.
  • Anycast is a routing technique where a single IP address maps to multiple physical servers. Unfortunately, anycast does not fit well with TCP and is rarely used in that scenario.

Most of the DNS servers themselves use anycast to achieve high availability and low latency of the DNS lookups.

3. The browser sends a HTTP request to the web server

image

You can be pretty sure that Facebook’s homepage will not be served from the browser cache because dynamic pages expire either very quickly or immediately (expiry date set to past).

So, the browser will send this request to the Facebook server:

GET http://facebook.com/ HTTP/1.1
Accept: application/x-ms-application, image/jpeg, application/xaml+xml, [...]
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; [...]
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Host: facebook.com
Cookie: datr=1265876274-[...]; locale=en_US; lsd=WW[...]; c_user=2101[...]

The GET request names the URL to fetch: “http://facebook.com/”. The browser identifies itself (User-Agent header), and states what types of responses it will accept (Accept and Accept-Encoding headers). The Connection header asks the server to keep the TCP connection open for further requests.

The request also contains the cookies that the browser has for this domain. As you probably already know, cookies are key-value pairs that track the state of a web site in between different page requests. And so the cookies store the name of the logged-in user, a secret number that was assigned to the user by the server, some of user’s settings, etc. The cookies will be stored in a text file on the client, and sent to the server with every request.

There is a variety of tools that let you view the raw HTTP requests and corresponding responses. My favorite tool for viewing the raw HTTP traffic is fiddler, but there are many other tools (e.g., FireBug) These tools are a great help when optimizing a site.

In addition to GET requests, another type of requests that you may be familiar with is a POST request, typically used to submit forms. A GET request sends its parameters via the URL (e.g.: http://robozzle.com/puzzle.aspx?id=85). A POST request sends its parameters in the request body, just under the headers.

The trailing slash in the URL “http://facebook.com/” is important. In this case, the browser can safely add the slash. For URLs of the form http://example.com/folderOrFile, the browser cannot automatically add a slash, because it is not clear whether folderOrFile is a folder or a file. In such cases, the browser will visit the URL without the slash, and the server will respond with a redirect, resulting in an unnecessary roundtrip.

4. The facebook server responds with a permanent redirect

image

This is the response that the Facebook server sent back to the browser request:

HTTP/1.1 301 Moved Permanently
Cache-Control: private, no-store, no-cache, must-revalidate, post-check=0,
      pre-check=0
Expires: Sat, 01 Jan 2000 00:00:00 GMT
Location: http://www.facebook.com/
P3P: CP="DSP LAW"
Pragma: no-cache
Set-Cookie: made_write_conn=deleted; expires=Thu, 12-Feb-2009 05:09:50 GMT;
      path=/; domain=.facebook.com; httponly
Content-Type: text/html; charset=utf-8
X-Cnection: close
Date: Fri, 12 Feb 2010 05:09:51 GMT
Content-Length: 0

The server responded with a 301 Moved Permanently response to tell the browser to go to “http://www.facebook.com/” instead of “http://facebook.com/”.

There are interesting reasons why the server insists on the redirect instead of immediately responding with the web page that the user wants to see.

One reason has to do with search engine rankings. See, if there are two URLs for the same page, say http://www.igoro.com/ and http://igoro.com/, search engine may consider them to be two different sites, each with fewer incoming links and thus a lower ranking. Search engines understand permanent redirects (301), and will combine the incoming links from both sources into a single ranking.

Also, multiple URLs for the same content are not cache-friendly. When a piece of content has multiple names, it will potentially appear multiple times in caches.

5. The browser follows the redirect

image

The browser now knows that “http://www.facebook.com/” is the correct URL to go to, and so it sends out another GET request:

GET http://www.facebook.com/ HTTP/1.1
Accept: application/x-ms-application, image/jpeg, application/xaml+xml, [...]
Accept-Language: en-US
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; [...]
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Cookie: lsd=XW[...]; c_user=21[...]; x-referer=[...]
Host: www.facebook.com

The meaning of the headers is the same as for the first request.

6. The server ‘handles’ the request

image

The server will receive the GET request, process it, and send back a response.

This may seem like a straightforward task, but in fact there is a lot of interesting stuff that happens here – even on a simple site like my blog, let alone on a massively scalable site like facebook.

  • Web server software
    The web server software (e.g., IIS or Apache) receives the HTTP request and decides which request handler should be executed to handle this request. A request handler is a program (in ASP.NET, PHP, Ruby, …) that reads the request and generates the HTML for the response.

    In the simplest case, the request handlers can be stored in a file hierarchy whose structure mirrors the URL structure, and so for example http://example.com/folder1/page1.aspx URL will map to file /httpdocs/folder1/page1.aspx. The web server software can also be configured so that URLs are manually mapped to request handlers, and so the public URL of page1.aspx could be http://example.com/folder1/page1.

  • Request handler
    The request handler reads the request, its parameters, and cookies. It will read and possibly update some data stored on the server. Then, the request handler will generate a HTML response.

One interesting difficulty that every dynamic website faces is how to store data. Smaller sites will often have a single SQL database to store their data, but sites that store a large amount of data and/or have many visitors have to find a way to split the database across multiple machines. Solutions include sharding (splitting up a table across multiple databases based on the primary key), replication, and usage of simplified databases with weakened consistency semantics.

One technique to keep data updates cheap is to defer some of the work to a batch job. For example, Facebook has to update the newsfeed in a timely fashion, but the data backing the “People you may know” feature may only need to be updated nightly (my guess, I don’t actually know how they implement this feature). Batch job updates result in staleness of some less important data, but can make data updates much faster and simpler.

7. The server sends back a HTML response

image

Here is the response that the server generated and sent back:

HTTP/1.1 200 OK
Cache-Control: private, no-store, no-cache, must-revalidate, post-check=0,
    pre-check=0
Expires: Sat, 01 Jan 2000 00:00:00 GMT
P3P: CP="DSP LAW"
Pragma: no-cache
Content-Encoding: gzip
Content-Type: text/html; charset=utf-8
X-Cnection: close
Transfer-Encoding: chunked
Date: Fri, 12 Feb 2010 09:05:55 GMT

2b3
��������T�n�@����[...]

The entire response is 36 kB, the bulk of them in the byte blob at the end that I trimmed.

The Content-Encoding header tells the browser that the response body is compressed using the gzip algorithm. After decompressing the blob, you’ll see the HTML you’d expect:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"   
      "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" 
      lang="en" id="facebook" class=" no_js">
<head>
<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-language" content="en" />
...

In addition to compression, headers specify whether and how to cache the page, any cookies to set (none in this response), privacy information, etc.

Notice the header that sets Content-Type to text/html. The header instructs the browser to render the response content as HTML, instead of say downloading it as a file. The browser will use the header to decide how to interpret the response, but will consider other factors as well, such as the extension of the URL.

8. The browser begins rendering the HTML

Even before the browser has received the entire HTML document, it begins rendering the website:

 image

9. The browser sends requests for objects embedded in HTML

image

As the browser renders the HTML, it will notice tags that require fetching of other URLs. The browser will send a GET request to retrieve each of these files.

Here are a few URLs that my visit to facebook.com retrieved:

  • Images
    http://static.ak.fbcdn.net/rsrc.php/z12E0/hash/8q2anwu7.gif
    http://static.ak.fbcdn.net/rsrc.php/zBS5C/hash/7hwy7at6.gif
  • CSS style sheets
    http://static.ak.fbcdn.net/rsrc.php/z448Z/hash/2plh8s4n.css
    http://static.ak.fbcdn.net/rsrc.php/zANE1/hash/cvtutcee.css
  • JavaScript files
    http://static.ak.fbcdn.net/rsrc.php/zEMOA/hash/c8yzb6ub.js
    http://static.ak.fbcdn.net/rsrc.php/z6R9L/hash/cq2lgbs8.js

Each of these URLs will go through process a similar to what the HTML page went through. So, the browser will look up the domain name in DNS, send a request to the URL, follow redirects, etc.

However, static files – unlike dynamic pages – allow the browser to cache them. Some of the files may be served up from cache, without contacting the server at all. The browser knows how long to cache a particular file because the response that returned the file contained an Expires header. Additionally, each response may also contain an ETag header that works like a version number – if the browser sees an ETag for a version of the file it already has, it can stop the transfer immediately.

Can you guess what “fbcdn.net” in the URLs stands for? A safe bet is that it means “Facebook content delivery network”. Facebook uses a content delivery network (CDN) to distribute static content – images, style sheets, and JavaScript files. So, the files will be copied to many machines across the globe.

Static content often represents the bulk of the bandwidth of a site, and can be easily replicated across a CDN. Often, sites will use a third-party CDN provider, instead of operating a CND themselves. For example, Facebook’s static files are hosted by Akamai, the largest CDN provider.

As a demonstration, when you try to ping static.ak.fbcdn.net, you will get a response from an akamai.net server. Also, interestingly, if you ping the URL a couple of times, may get responses from different servers, which demonstrates the load-balancing that happens behind the scenes.

10. The browser sends further asynchronous (AJAX) requests

image

In the spirit of Web 2.0, the client continues to communicate with the server even after the page is rendered.

For example, Facebook chat will continue to update the list of your logged in friends as they come and go. To update the list of your logged-in friends, the JavaScript executing in your browser has to send an asynchronous request to the server. The asynchronous request is a programmatically constructed GET or POST request that goes to a special URL. In the Facebook example, the client sends a POST request to http://www.facebook.com/ajax/chat/buddy_list.php to fetch the list of your friends who are online.

This pattern is sometimes referred to as “AJAX”, which stands for “Asynchronous JavaScript And XML”, even though there is no particular reason why the server has to format the response as XML. For example, Facebook returns snippets of JavaScript code in response to asynchronous requests.

Among other things, the fiddler tool lets you view the asynchronous requests sent by your browser. In fact, not only you can observe the requests passively, but you can also modify and resend them. The fact that it is this easy to “spoof” AJAX requests causes a lot of grief to developers of online games with scoreboards. (Obviously, please don’t cheat that way.)

Facebook chat provides an example of an interesting problem with AJAX: pushing data from server to client. Since HTTP is a request-response protocol, the chat server cannot push new messages to the client. Instead, the client has to poll the server every few seconds to see if any new messages arrived.

Long polling is an interesting technique to decrease the load on the server in these types of scenarios. If the server does not have any new messages when polled, it simply does not send a response back. And, if a message for this client is received within the timeout period, the server will find the outstanding request and return the message with the response.

Conclusion

Hopefully this gives you a better idea of how the different web pieces work together.

148 Comments to “What really happens when you navigate to a URL”

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  2. SM says:

    Nice explanation! Thanks :)

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    I asked about LINQ performance on Channel 9 and you replied. Now I found your blog and it’s very cool. Keep on blogging!

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  10. Eli says:

    Nice article, Igor. Well done

  11. [...] What really happens when you navigate to a URL [...]

  12. Karl Rosaen says:

    Nice post! Though isn’t what you describe not a recursive dns search, but an iterative one? E.g, if the root server were performing a recursive search, it would ask org.nameserver itself and return the result directly to the client instead of telling it to check with org.nameserver

  13. @Karl: I think that it is called “recursive” because it is logically recurses down the URI. So, you could represent the lookup of “my.example.com” as follows:


    IpAddress resolved =
    GetIpAddress(
    "my",
    GetIpAddress(
    "example",
    GetIpAddress(
    "com",
    rootDNS)))

    (Sorry, the commenting system won’t let me format the code sample).

  14. Karl Rosaen says:

    Makes sense, seems to just be a matter of terminology. I seem to remember my computer networking textbook from years back, http://www.aw-bc.com/kurose_ross, making a distinction between recursive and iterative name queries based on what server was making the queries, similar to this article http://technet.microsoft.com/e.....637(WS.10).aspx

    but in any case, really great overview of the complete end to end, thanks again.

  15. Joshua says:

    Igor, this was a nice article, I found the IP address load balancing on big traffic websites to be very interesting. Thankyou from Josh

  16. Jeffrey.Dan says:

    That’s a great article““Hmmmm“`like it.Thanks

  17. Álvaro Del Hoyo says:

    Igor,
    Nice post.
    Was wondering if you could explain in that simple way cookies usage: regular cookies, Google Analytics cookies, third party cookies.
    Guess it will be realli nice complement for the post
    Thanks
    Regards

  18. [...] Este artículo es una adaptación y traducción al español de What really happens when you navigate to a URL. [...]

  19. scorpion1206 says:

    thanks for clear explanation

  20. Mithun says:

    привет Igor….
    Very Nice tutorial…amazing what and all happens..when we type https://www.facebook.com:)

  21. precelek says:

    Interesting post. Bookmarked for future referrence

  22. Thanks for writing this article. Now everything is clear for me.

  23. Bearings says:

    I have a problem, my homepage fetch as google shows the following informtion, how shall I do?

    HTTP/1.1 200 OK
    Connection: close
    Pragma: no-cache
    cache-control: no-cache
    Content-Type: text/html
    Content-Length: 65

  24. Bearings: And what is the problem?

  25. Bearings says:

    The above information is from google webmasters when I use the tool “Fetch as google”, now the Http Header information is as follows:

    HTTP/1.1 302 Found
    Connection: close
    Pragma: no-cache
    cache-control: no-cache
    Location: /SLXVQ/

    I think the problem maybe was due to the URL changes two days ago, I will wait some more days and check what will happpen.

    Thanks

  26. Sreeraj says:

    Good very usefull….
    Thanks

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    Very good post. A lot of interesting articles and comments.Thanks a lot for this information. If you feel up to it, check this out for a really inexpensive hosting offer. If you use this coupon they give you like 2 months free hosting: “eww994off2″.

  28. Prince says:

    Fantastic Article on this topic. It did explained very very well.

    Thanks Igore.

  29. LaoWang says:

    It’s a Wonderful article,Thank you!

  30. qqq says:

    Good!It’s a Wonderful article,Thank you!

  31. psychic says:

    I navigate to a url every day, so this is helpful

  32. Travel says:

    A Pocket Travel Guide – find some fresh travel notes about the most famouse destinations in the world!

  33. qi gong says:

    This information about things that happen when we navigate to a URL taught me several things about http and about browsers,router cache,browser cache,ISP DNS cache,OS cache etc.

  34. Dev says:

    Thank you so much, Igor

  35. This is a great post, I was always curious as to the exact sequence of events after clicking on a URL.

  36. b makowsky says:

    Awesome information over again. I am looking forward for your next post!

  37. This is boring and everyone knows this.
    And, surprisingly, for a Microsoft-employed smartass you forgot to mention the Windows OS (pretty) unique feature of a “Negative DNS cache”.
    For everyone else – if this elaborate “technology” sounds `c00l’ to you, then you’re wrong – it’s all old and ugly, and needs redesign from the very beginning.

  38. bdaniel7 says:

    It’s 2011!

    Internet Explorer is no longer the default browser so the “e” for browser is rather obsolete.

  39. Peter says:

    I’m really surprised if this is new info for any software developer… It is only a superficial intro to how this works. In fact, you’ve only covered part of DNS translation and redirection. You haven’t covered how the IP addresses found are used for routing to the machine having that address, how connections are setup, maintained and closed (e.g. TCP,…) There’s so much more happening and I should say a developer, and certainly one who develops apps to use a TCP/IP network, should be aware of this.

  40. JD says:

    Igor,

    An informative post.

    Thank you!

  41. Shrike says:

    It’d be nice to add more low level info. i.e. what’s happening when the browser sends GET-request. IP/TCP stuff I mean.

  42. JMB says:

    Igor,
    another reason for the redirect is make the cookie Path more specific.

    As for developers knowing how all this works – many do not know this level of detail.

  43. S2 says:

    Very good article, in addition I’d really like to see how OS’s like Windows delegate between the different instances of the browser. I.e. all my windows will come from the same IP address so how does it know a response is directed to it and not another window? I can see the cookie can help here on ‘page sessions’ but I expect there is another way this information is redirected to the right instance as it must have to do this automatic redirection for proxies too which isn’t ‘cookie’d.

  44. S2: The article mostly talks about what happens at the HTTP layer. The HTTP response does not need to specify which process the response should go to. That kind of stuff is handled by lower levels of the communication stack (TCP/IP).

  45. arijit says:

    and one question is where is this DNS? can you give me an example ? I mean how I am using this DNS? this is not in my PC ..as I am using broadband service so is it in the router of the company which is giving me broad band service? or is it in my computer? if so then where in the browser?

  46. sophie says:

    waw, it was not as easy as imagined. the process of calling a web, through several stages of the complicated

  47. [...] What really happens when you navigate to a URL [...]

  48. Rushabh says:

    Very informative & detailed explanation! I’d like to read more such lucid articles teaching how these broadly used technologies are working from you.

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