Support Manual

Site Statistics

How It Works

The web server is a program running on a networked machine, waiting for connections from the outside world to serve certain documents on behalf of a request by a browser.

To communicate, the server and the browser use an asynchronous communication method called the HTTP (hypertext transaction) protocol. It works as follows:

the user starts the browser and types in an URL

the browser connects to the given host and requests the specified document.

The web server handles the request and sends out a response:

If this document exists, the web server delivers it.
If it does not exist or if access is not permitted, the web server sends back an error message instead.

The document delivered as an answer to this request may contain inline objects. Inline objects are simply URLs pointing to another resource, either a document, an image, an applet, a video/audio stream, or any other addressable HTML object.

The browser then requests all inline objects of the current page from the server using the steps 2 and 3 above, before it can display the content of that page.

This communication method is called asynchronous, because the browser sends out many requests for inline documents at once (without waiting for a response from the server before sending the next request) using different communication channels:

Since the browser's requests are often handled by different server processes or different threads of a server process, there is absolutely no relationship between the logfile entries caused by the responses from the server due to a request of a document and it's inline objects.

For example, the order in which the server logs the successful transmission of the document itself and the inline images contained therein is not predictable and depends on the type of documents, objects, server speed, system and network load, and many other parameters.

What is logged?

Each and every response from the server - whether it indicates success, an error, or even a timeout (i.e. no response) - gets logged in the server's logfile. Since the server was hit by a request, such a response is called a Hit. In other words, the total number of hits must equal the total number of lines in the logfile minus the number of corrupt and empty lines. A typical logfile entry in the Common Logfile Format looks like:

hostname-[01/Feb/1998:10:10:00 +0100]
"GET/index.html HTTP/1.0"200 4839

The hostname field contains the full qualified domain name (FQDN) of the site accessing your server (see ÈSpecial CasesÇ below). The next two fields usually contain a minus (`-') to indicate that those fields are empty. The date is surrounded by square brackets ('[' and ']'). The next field contains the request. It contains the request method ('GET' for example), the name of the requested document (URL), and the protocol specification ('HTTP/1.0').

The following field contains the servers response code ('200' stands for an 'OK', while '404' would mean 'Document not found', for example). The last field contains the size of the document (some servers log the number of bytes transferred actually, while other servers log the size of the document, which makes a difference if the user interrupts the transfer before the document could be transmitted completely.

There are two other logfile formats, the Combined or Extended Logfile Format. Those formats add the user-agent (browser type) and the referrer URL (the page, which contains a link to the requested document if this request for such document has been generated by following a link) to the logfile entry. Those Combined or Extended Logfile Format append following two fields to the Common Logfile Format (CLF) in one of two usual ways:

CLF Mozilla/2.0 (X11; IRIX 6.3; IP22) http://foo/bar.html
CLF "http://foo/bar.html" "Mozilla/2.0 (X11; IRIX 6.3; IP22)"

Note that in the second form, the user-agent and the referrer URL are surrounded by double quotes, which makes them ambiguous in certain cases such as erroneous referrer URLs, which contain double quotes. Therefore, the first form should be preferred if possible.

The entries shown above are the only information the server records in the logfile. There might be much more information being transferred from the browser to the server, but although this additional information is available through CGI-scripts running on your server, it gets not logged in the logfile. Therefore, http-analyze can only show you a summary of the information in the logfile - nothing more, nothing less.

Special Cases

Caching in the browser:

As soon as a page has been saved in a browser's disk cache, the browser might send out conditional requests for documents or inline objects. This conditional request ask the web server to only send a document/object if it has been modified since the last time the page has been requested (if the page is still in the browser's cache). This way, network traffic is reduced somewhat, since documents must be transferred only if they have changed recently. If such a conditional request arrives, the server will respond with a Code 304 (Not Modified) status to indicate that the document hasn't changed or with a Code 200 (OK) status if it has changed in the meantime. Since the browser may be configured (and usually is so by default) to only send out such conditional requests once per session and otherwise unconditionally use the copy from the cache, you may not even see a Code 304 response if this users visits your site again in the same session. Conditional requests are then sent out only if the user terminates the browser session and later restarts the browser.

Caching in a proxy server:

Organizations with a large number of users - such as companies, universities, or online providers - often use a so-called proxy server for mainly two reasons:

1. Often such organizations have a firewall to protect their internal network against intruders. This means, that their network is logically separated from the rest of the Internet and that they have to use such a proxy server, which is able to communicate with the inside and the outside of their local network.
    
2. To reduce network load somewhat, the proxy server acts as a local copy machine: As soon as a page is loaded into a browser through such a proxy server, the proxy saves a copy of this page in it's disk cache much like a browser does in the scenario above. This way, documents requested very often by users in the same local network need to be transferred to the proxy only once, which then answers future requests for the same page from it's local cache instead of connecting to the original web server the document originated from.

Both forms of caching make it technically impossible to count visitors or to track their way through your web site. All you see in the logfile of your server is only a few initial hits from the proxy or browser and probably some Code 304 responses resulting from conditional requests sent out by the proxy or browser, depending on the preferences settings of the proxy or browser.

 

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