1. Lecture 10: The Web Server Wednesday February 14, 2018 4/10/2019

2. Specifications Specify what will be done Scenarios
Lecture 10: The Web Server
Specifications
Specify what will be done
Scenarios
Lists of features to implement
Note optional versus required (priority)
Define the user experience
Sketches of web pages (not final design)
Basically list what should be there as a basis for the design
Identify interface to existing systems
Servers, databases, etc.
Outline of web site and pages
List of what pages are needed
4/10/2019

3. Specifications Detail what the application will do
Lecture 10: The Web Server
Specifications
Detail what the application will do
From the programmer’s point of view
Can talk about other systems, components, modules
More likely to talk about commands, inputs, outputs
WHAT not HOW
Define the inputs and outputs
What information is needed
What information is used
Where does this information come from
Where does this information go
Specifications Document due 2/25
4/10/2019

4. Specifications Are Not Designs
Lecture 10: The Web Server
Specifications Are Not Designs
HOW not WHY
Identify particular technologies to use
Unless mandated by outside requirements
Back ends, front ends, databases, …
Identify how tasks are done
Front end, back end, database
Particular algorithms or processing
Provide detailed web site designs
Will change as requirements change
4/10/2019

5. Web Applications Front End Web Browser HTTP Database Back End
Lecture 10: The Web Server
Web Applications
Web Browser
Front End
HTTP
Database
Web Server
Back End
Server
Mobile Platform
Front End
Today we want to preview the next section of the course.
4/10/2019

6. What Services Did You Guess
Lecture 10: The Web Server
What Services Did You Guess
What does a back end have to do for a web application?
Look at prior to class
4/10/2019

7. The Web Server Sits on the host machine
Lecture 10: The Web Server
The Web Server
Sits on the host machine
Listens for connections on a particular port (i.e. 80)
Gets HTTP requests sent to that port (via a socket connection)
Processes each request independently
URL tells it how to process a request
Sends a response back on the same socket
Basic requests
URL with a file specified
Find the file on disk and return it
Create an appropriate HTTP response (header)
Followed by the data in the file
4/10/2019

8. Web Server Game Volunteers (5) to act as clients making requests
Lecture 10: The Web Server
Web Server Game
Volunteers (5) to act as clients making requests
Can request a page of a given color
TAN, YELLOW, PURPLE, BLUE, PINK, GREEN
Volunteers (5) to act as HTTP connections
Interface between clients and server
Volunteer (1) to act as the web server
Pages reside on file system
Set up a “file system” near, but not adjacent to server. Server will probably move to the file system herself.
5 clients
Clients should go to web server (stand in line if you need to), ask for a certain color, take it back to their position, wait a random time, and then return color and ask again.
4/10/2019

9. Web Server Game Improvements
Lecture 10: The Web Server
Web Server Game Improvements
How can we speed this up?
Multiple web servers corresponds to multiple threads
Putting server near the data corresponds to web cache
4/10/2019

10. Dynamic Requests Static requests are static
Lecture 10: The Web Server
Dynamic Requests
Static requests are static
Don’t work for web applications
We need to get different data under different circumstances
Based on information passed in with the URL
Recall URLs have a query portion
With name-value pairs (or POST data)
Set up by HTML forms
Can involve interaction with JavaScript
Web server needs to return different results
Based on the query / data
4/10/2019

11. Modified Web Server Game
Lecture 10: The Web Server
Modified Web Server Game
Client asks for a color and a positive integer <= 100
Web server has to return a sheet giving the square of the number
Or ERROR (40x) if the input is illegal
Different computation strategies: recruit people to do the computation; do the computation yourself; precompute the values.
Ask the web server volunteer how he/she wants to deal with this.
4/10/2019

12. Web Server Game Improvements
Lecture 10: The Web Server
Web Server Game Improvements
How might we speed this up?
Someone else doing the computations (or multiple people for each server)
Multiple web servers
4/10/2019

13. Context-Based Requests
Lecture 10: The Web Server
Context-Based Requests
Most dynamic requests have a context
Shopping cart
Previous searches
Previous inputs and pages
User id
The web server needs know the context
Map users to contexts
Use the context in creating the resultant output
4/10/2019

14. Modified Web Server Game
Lecture 10: The Web Server
Modified Web Server Game
Client asks for a color and provides positive integer <= 100
Server provides the sum of their previous numbers plus the new one
Server provides the client with an ID
Same ID for same client
Client has to return the ID as part of their request
What’s wrong here. How will the server remember the client’s sum?
One way would be to pass the sum in – this is what a RESTful API might do.
Another is to just include some ID for the client and let the server look things up based on that.
Server keeps a record of ID->SUM (whiteboard?)
This corresponds to context.
Need to have an initial call that assigns an ID.
What does this break?
4/10/2019

15. Modified Web Server Game
Lecture 10: The Web Server
Modified Web Server Game
How might we speed this up?
How can we speed the server up now?
Why not let the client do their own processing? Possibly just ask the server initially for something and then at then end record final values.
Could pass in the previous sum as well as the new number. This corresponds to a RESTful interface.
Separate server to assign Ids.
Split by ID mod # servers. New ones assigned new ID.
4/10/2019

16. What the Web Server Does
Lecture 10: The Web Server
What the Web Server Does
Given a HTTP Request
Return a HTTP Response
Given a URL
Return the corresponding page
Given a URL plus parameters / data
Compute and return the resultant data
Compute and return a HTML page
END OF WEB SERVER GAME
4/10/2019

17. Web Server Issues Handling large numbers of clients
Lecture 10: The Web Server
Web Server Issues
Handling large numbers of clients
Multiple threads, caching, multiple servers
Managing context or state
Generating HTML output containing computed values
Doing the actual computations
We need to describe these
We need a program (and hence a language)
Where are the computations done
By the web server
Externally
4/10/2019

18. Web Servers General purpose servers
Lecture 10: The Web Server
Web Servers
General purpose servers
Handle static pages; designed to scale
Examples: Apache, NginX, Microsoft IIS
Extensions to handle Computation
Modules: PHP, Ruby, Python, Perl, FCGI, C#
External Calls: CGI
Special purpose servers
TOMCAT: Java servlets
NODE.JS: Event-based JavaScript
Django, Flask: Python; Ruby on Rails: Ruby
Embedded Servers
Nanohttpd.java; micro-httpd for arduino
NOTE THAT PHP/GWT/templating, etc are in 3/10 lecture. Ruby/Django in 3/13 – we have lots of time to cover these later. Don’t have to be covered here.
4/10/2019

19. Server Organization Server needs to handle multiple requests at once
Lecture 10: The Web Server
Server Organization
Server needs to handle multiple requests at once
Several alternative designs are possible for this
Use threads
Use multiple servers
Use asynchronous I/O
Combinations of these
4/10/2019

20. CGI Programs URL: http://host/cgi-bin/cmd?args
Lecture 10: The Web Server
CGI Programs
URL:
cgi-bin is a special directory on the web server
cmd is the name of a normal executable in that directory
Shell script, perl, php, python, java jar file, c/c++ binary, …
args are named arguments passed to command
The program ‘cmd’ is run on the web server
Any program output is passed back to client
Typical Use: Format a request and pass it on to server
Problems: efficiency, security, safety
Used in very limited applications
4/10/2019

21. PHP PHP is a simple string-oriented scripting language
Lecture 10: The Web Server
PHP
PHP is a simple string-oriented scripting language
Similar capabilities as Python, JavaScript
Designed to make string processing easy
Web server runs PHP internally
As a module or plug-in
Automatically when a page has a .php extension
We will cover this again later
4/10/2019

22. PHP and HTML What does the web server normally generate HTML pages
Lecture 10: The Web Server
PHP and HTML
What does the web server normally generate
HTML pages
With lots of HTML (text)
What’s different is based on query part of URL
Some fraction of the page
Most of the output is fixed text
Header, navigation, footer
Parts of the contents
Why should we write code to output this
In any language
4/10/2019

23. PHP Pages Normal URLS where the file has a .php extension
Lecture 10: The Web Server
PHP Pages
Normal URLS where the file has a .php extension
The plug in doesn’t run PHP directly on the file
The page is actually a mixture of text and code
HTML pages with embedded PHP code
PHP module reads the page
The HTML portion is passed on directly
The PHP code is embedded in <?php … ?> constructs
<? … ?>
Where the code appears, it is run & replaced by its output
PHP print or echo statements
This concept, templating, is very useful
Used to some extent in React, angular, vue, …
4/10/2019

24. Servlets and JSP Why add a new language Programmers know Java
Lecture 10: The Web Server
Servlets and JSP
Why add a new language
Programmers know Java
Back end applications are often written in Java
Use Java as the processing language
Not ideal for string processing, but acceptable
Multiple threads already accommodated
Servlet
Standard interface invoked directly by URL
Path name = class name, parameters accessible
Java Server Pages
Pages with embedded Java <? … ?>
4/10/2019

25. Java Servlets and JSP Handled by a separate web server
Lecture 10: The Web Server
Java Servlets and JSP
Handled by a separate web server
TOMCAT is the most common
Runs on a different port
URL: host:8080/servlet/class?parms
JSP handled by file extension
URL: host:8080/page.jsp
4/10/2019

26. ASP.Net Supported by Microsoft IIS
Lecture 10: The Web Server
ASP.Net
Supported by Microsoft IIS
Use C# (or C++) to write the back end
Web pages use templating
With embedded C#
4/10/2019

27. Node.JS Why learn a new language (PHP) We already know JavaScript
Lecture 10: The Web Server
Node.JS
Why learn a new language (PHP)
We already know JavaScript
PHP is too slow; JavaScript is now compiled and fast
It has most of what is needed
What’s wrong with Java (C#)
Too complex, not string-oriented
Too much baggage
Straight line code is inefficient
Querying database, servers, file system all take time
Multiple threads complicate processing
Difficult to load balance with diverse threads
4/10/2019

28. Node.JS JavaScript Web Server Separate server (like TOMCAT for Java)
Lecture 10: The Web Server
Node.JS
JavaScript Web Server
Separate server (like TOMCAT for Java)
App back end is written in JavaScript
Event-Based
Computation is done in small pieces
Complex interactions are done asynchronously
JavaScript code is associated with events
The code is executed when the event occurs
Code can initiate asynchronous computations with later events
Code supplies a continuation invoked when action completes
We’ll get back to this next time.
4/10/2019

29. Web Applications Front End Web Browser HTTP Database Back End
Lecture 10: The Web Server
Web Applications
Web Browser
Front End
HTTP
Database
Web Server
Back End
Server
Mobile Platform
Front End
4/10/2019

30. Databases Most web applications need to store information
Lecture 10: The Web Server
Databases
Most web applications need to store information
Much of what they do is information based
Shopping site as an example
The security, integrity, … of the information is important
The server code talks to a database system
All languages have code to make this relatively easy
Database operations
Setting up the database
Adding and removing information from the database
Getting (querying) information from the database
4/10/2019

31. Frameworks All this sounds complex to set up and operate
Lecture 10: The Web Server
Frameworks
All this sounds complex to set up and operate
A lot of the work is common and straightforward
Communications, setting up pages, database access, …
It can be simplified by extracting these
Leaving only the code specific to the particular application
Frameworks are attempts to do this
Provide common code to plug in the application
Provide all the glue code; simplify database access
Ruby on Rails, Django, Flask, GWT
Express for Node.JS (and other plug-ins)
4/10/2019

32. Next Time Node.JS Homework: Pre-Lab 4 Lecture 10: The Web Server
4/10/2019

33. Server Organization Internal processing Queue of tasks to be done
Lecture 10: The Web Server
Server Organization
Internal processing
Queue of tasks to be done
Thread pool to handle multiple requests
Internal requests can be queued if necessary
Handling initial requests
Single thread to read web socket
Multithreaded versus Single threaded processing
Using non-blocking (asynchronous) I/O
4/10/2019

34. Handling Complex Applications
Lecture 10: The Web Server
Handling Complex Applications
The web server
Can handle PHP, Servlets, etc.
But these have limited capabilities
These run in limited environments
Don’t want to overwhelm the server
The server has other responsibilities
What if your application is more complex
You need to provide complex services (e.g. machine learning, data mining, search)
Then you might want to have your own server
4/10/2019

35. User Server Organization
Lecture 10: The Web Server
User Server Organization
Based on a client-server model
Client: app code in the web server
Each request is its own client
Can be done via PHP or other server side code
Socket-based communication
Server runs on a host and accepts connections on a port
Client connects to that host-port
Sends command/request
Reads response, processes it to HTML/JSON
Returns it to the browser
Server: self-standing system
Note this is more for CS than design
4/10/2019

36. PHP Language Simple interpreted (scripting) language Untyped
Lecture 10: The Web Server
PHP Language
Simple interpreted (scripting) language
Untyped
Basic data types: string, int (long), float (double)
Complex data types: associative arrays, classes
Lots of built-in functions
Good string support
“hello $var, this is a ${expr}. “
Good documentation (esp. for libraries)
4/10/2019

37. Node.JS Event Example Request comes in
Lecture 10: The Web Server
Node.JS Event Example
Request comes in
JavaScript code creates database query based on parameters
Starts query and registers continuation
When query completes (done asynchronously)
Continuation is invoked. Template file is opened and a new continuation is provided
When file is ready to read (done asynchronously))
A stream from the file to the client is established
The file is output asynchronously
We’ll get into this in detail next week
4/10/2019