Fig 1 Illustrates How a Signal Can Be Decomposed Into Sine and Cosine

Sub: Web Tech. Batch: BCA V Notes by: Prayas Shrestha Advanced Server Side Issues

Server-side refers to operations that are performed by the server in a client-server relationship in computer networking. Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user's local computer or workstation. Operations may be performed server-side because they require access to information or functionality that is not available on the client, or require typical behavior that is unreliable when it is done client-side.

Server-side operations also include processing and storage of data from a client to a server, which can be viewed by a group of clients.

WEB SERVER The term Web server can mean one of two things:

1. A computer that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are Web pages such as HTML documents and linked objects (images, etc.).

2. A computer program that provides the functionality described in the first sense of the term.

Although Web server programs differ in detail, they all share some basic common features.

1. HTTP: responds to HTTP requests: every Web server program operates by accepting HTTP requests from the network, and providing an HTTP response to the requester. The HTTP response typically consists of an HTML document, but can also be a raw text file, an image, or some other type of document; if something bad is found in client request or while trying to serve the request, a Web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.

2. Logging: usually Web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the Webmaster to collect statistics by running log analyzers on log files.

In practice many Web servers implement the following features too.

1. Configurability of available features by configuration files or even by an external user interface.

2. Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.

3. Handling of not only static content (file content recorded in server's filesystem(s)) but of dynamic content too by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).

4. Module support, in order to allow the extension of server capabilities by adding or modifying software modules which are linked to the server software or that are dynamically loaded (on demand) by the core server.

5. HTTPS support (by SSL or TLS) in order to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.

6. Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).

7. Virtual Host to serve many web sites using one IP address.

8. Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.

9. Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

PATH TRANSLATION Web servers usually translate the path component of a Uniform Resource Locator (URL) into a local file system resource. The URL path specified by the client is relative to the Web server's root directory. Consider the following URL as it would be requested by a client: http://www.example.com/path/file.html The client's Web browser will translate it into a connection to www.example.com with the following HTTP 1.1 request: GET /path/file.html HTTP/1.1 Host: www.example.com The Web server on www.example.com will append the given path to the path of its root directory. On Unix machines, this is commonly /var/www/htdocs. The result is the local file system resource: /var/www/htdocs/path/file.html The Web server will then read the file, if it exists, and send a response to the client's Web browser. The response will describe the content of the file and contain the file itself.

PERFORMANCES Web servers (programs) are supposed to serve requests quickly from more than one TCP/IP connection at a time. Main key performance parameters (measured under a varying load of clients and requests per client), are:

 number of requests per second (depending on the type of request, etc.);

 latency time in milliseconds for each new connection or request;

 throughput in bytes per second (depending on file size, cached or non cached content, available network bandwidth, etc.). Above three parameters vary noticeably depending on the number of active connections, so a fourth parameter is the concurrency level supported by a Web server under a specific configuration. Last but not least, the specific server model used to implement a Web server program can bias the performance and scalability level that can be reached.

LOAD LIMITS A web server (program) has defined load limits, because it can handle only a limited number of concurrent client connections (usually between 2 and 60,000, by default between 500 and 1,000) per IP address (and IP port) and it can serve only a certain maximum number of requests per second depending on:  its own settings;  the HTTP request type;  content origin (static or dynamic);  the fact that the served content is or is not cached;  the hardware and software limits of the OS where it is working. When a web server is near to or over its limits, it becomes overloaded and thus unresponsive

Overload causes A daily graph of a web server's load, indicating a spike in the load early in the day. At any time Web servers can be overloaded because of:  too much legitimate Web traffic (i.e. thousands or even millions of clients hitting the Web site in a short interval of time);  DDoS (Distributed Denial of Service) attacks;  Computer worms that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them);  Internet web robots traffic not filtered / limited on large web sites with very few resources (bandwidth, etc.);  Internet (network) slowdowns, so that client requests are served more slowly and the number of connections increases so much that server limits are reached;  Web servers (computers) partial unavailability, this can happen because of required / urgent maintenance or upgrade, HW or SW failures, back-end (i.e. DB) failures, etc.; in these cases the remaining web servers get too much traffic and of course they become overloaded.

Overload symptoms The symptoms of an overloaded Web server are:  requests are served with noticeably (long) delays (from 1 second to a few hundreds of seconds);  500, 502, 503, 504 HTTP errors are returned to clients (sometimes also unrelated 404 error or even 408 error may be returned);  TCP connections are refused or reset before any content is sent to clients.

Anti-overload techniques To partially overcome above load limits and to prevent the overload scenario, most popular Web sites use common techniques like:  managing network traffic, by using: o Firewalls to block unwanted traffic coming from bad IP sources or having bad patterns; o HTTP traffic managers to drop, redirect or rewrite requests having bad HTTP patterns; o Bandwidth management and Traffic shaping, in order to smooth down peaks in network usage;  deploying Web cache techniques;  using different domain names to serve different (static and dynamic) content by separate Web servers, i.e.: o http://images.example.com o http://www.example.com  using different domain names and / or computers to separate big files from small and medium sized files; the idea is to be able to fully cache small and medium sized files and to efficiently serve big or huge (over 10 - 1000 MB) files by using different settings;  using many Web servers (programs) per computer, each one bound to its own network card and IP address;  using many Web servers (computers) that are grouped together so that they act or are seen as one big Web server, see also: Load balancer;  adding more HW resources (i.e. RAM, disks) to each computer;  tuning OS parameters for HW capabilities and usage;  using more efficient computer programs for Web servers, etc.;  using other workarounds, specially if dynamic content is involved.

WEB HOSTING SERVICE A web hosting service is a type of Internet hosting service that allows individuals and organizations to provide their own websites accessible via the World Wide Web. Web hosts are companies that provide space on a server they own for use by their clients as well as providing Internet connectivity, typically in a

© 2000 College of Information Technology & Engineering. All rights reserved, Circulation copy for CITians only. 3 Sub: Web Tech. Batch: BCA V Notes by: Prayas Shrestha data center. Webhosts can also provide data center space and connectivity to the Internet for servers they do not own to be located in their data center, called colocation.

Types of Hosting  Free web hosting service: is free, (sometimes) advertisement-supported web hosting, and is extremely limited when compared to paid hosting.  Shared web hosting service: one's Web site is placed on the same server as many other sites, ranging from a few to hundreds or thousands. Typically, all domains may share a common pool of server resources, such as RAM and the CPU.  Reseller web hosting: allows clients to become web hosts themselves. Resellers could function, for individual domains, under any combination of these listed types of hosting, depending on who they are affiliated with as a provider.  Virtual Dedicated Server: slicing up a server into virtual servers. each user feels like they're on their own dedicated server, but they're actually sharing a server with many other users.  Dedicated hosting service: the user gets his or her own Web server and gains full control over it (root access for Linux/administrator access for Windows); however, the user typically does not own the server.  Colocation web hosting service: similar to the dedicated web hosting service, but the user owns the server; the hosting company provides physical space that the server takes up and takes care of the server. This is the most powerful and expensive type of the web hosting service. In most cases, the colocation provider may provide little to no support directly for their client's machine, providing only the electrical, Internet access, and storage facilities for the server.  Clustered hosting: having multiple servers hosting the same content for better resource utilization. Wikipedia's own servers are a good example of this.

WEB LOG ANALYSIS SOFTWARE

Web log analysis software (also called a web log analyzer) is software that parses a log file from a web server (like Apache), and based on the values contained in the log file, derives indicators about who, when and how a web server is visited. Indicators reported by most web log analyzers  Number of visits and number of unique visitors  Visits duration and last visits  Authenticated users, and last authenticated visits  Days of week and rush hours  Domains/countries of host's visitors  Hosts list  Most viewed, entry and exit pages  Files type  OS used  Browsers used  Robots  Search engines, key phrases and keywords used to find the analyzed web site  HTTP errors

STATISTICS The most popular Web servers, used for public Web sites, are tracked by Netcraft Web Server Survey, with details given by Netcraft Web Server Reports. According to this site, Apache has been the most popular Web server on the Internet since April of 1996. The January 2007 Netcraft Web Server Survey found that about 60% of the Web sites on the Internet were using Apache, followed by IIS with about 30% share. Another site providing statistics is SecuritySpace, which also provides a detailed breakdown for each version of Web server