	CS 580 Client-Server Programming Fall Semester, 2000 Servers & States
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Contents of Doc 18, Servers & States

References
Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, Addison-Wesley, 1995


Doc 18, Servers & States Slide # 2

Servers & States

Common operations of servers

Managing sockets
Managing threads
Parsing protocol
Handling Client requests
Interacting with databases

Design your server to keep these things independent

Reduce complexity of the code
Promote reuse


Doc 18, Servers & States Slide # 3

Some Context Assume we are writing a server to handle Airline reservation system

Assume there are three possible commands from client to server:

getCities
getLunchMenu
getManifest flightNumber

We have components that handle sockets and threads

We have a class that implement the ServerEngine interface

The processRequest method is called to handle the client request

interface ServerEngine
   {
   public ServerEngine newInstance(ProgramProperties settings);
   public ServerEngine cleanInstance( );
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException;
   }

We have a class ProtocolParser, which will parse the protocol


Doc 18, Servers & States Slide # 4

Using If statements to Handle a Request

class AirlineServer implements ServerEngine {
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException {
      ProtocolParser requestData = new ProtocolParser( in );
      String request = requestData.getCommand();
      if (request.equals( "getCities" ))
         call get cities code
      else if (request.equals( "getLunchMenu" ))
         call get lunch code
      else if (request.equals( "getManifest" ))
         call get manifest code
      else  //illegal request
         deal with illegal request
      etc.
      }
   }

For small number of commands if/switch statements work well

They are fast
Easy to understand


Doc 18, Servers & States Slide # 5

Complex Case - States

An Example
SPOP

Example protocol: Simple Post Office Protocol (SPOP)

Commands have the same functionality as POP but are limited to the following:

USER <username>
PASS <password>
LIST
RETR <message number>
QUIT

SPOP features
The authentication consists of two commands that have to come in sequence:

USER
PASS

The other commands (except for QUIT) cannot be executed before the user has been authenticated.

What does this mean for the client and server?


Doc 18, Servers & States Slide # 6

Parsing
Simple approach:

start:
get command
if  command is "USER" then
   username = argument
   get command
   if   command is "PASS" then
      password = argument
      if valid(username,password) then
         while true
            get command
            if command is "LIST" then
               performLIST
            elsif command is "RETR" then
               performRETR(argument)
            elsif command is "QUIT" then
               return
            end if
         end while
      end if
   end if
end if


Doc 18, Servers & States Slide # 7

Simple Parsing Problems

Major problems with this algorithm:

Hard to read.


Can the protocol be easily extracted from the code?
Are all cases handled correctly?

Hard to modify.


If the protocol changes, the code will most likely have to be rewritten


Doc 18, Servers & States Slide # 8

Finite Automata - State Machines

A better way of looking at all of this is using a picture.

Naming the states

We will use the following names for the states:

0	NoAuth
1	HaveUser
2	Process
3	Invalid
4	Quit


Doc 18, Servers & States Slide # 9

Implementing a State Machine: switch

int state = 0;
while (true) {
   command = next command;
   switch (state) {
      case 0:
         if (command.equals("USER")) {
            username = argument;
            state = 1;
         }
         else if (command.equals("QUIT"))
            state = 4;
         else
            error("Unknown: " + command);
         break;
      case 1:
         if (command.equals("PASS")) {
            if (valid(username, argument))
               state = 2;
            else {
               error("Unauthorized");
               state = 3;
            }
         }
         else
            error("Unknown: " + command);
         break;
...
Doc 18, Servers & States Slide # 10

Switch Method Analysis

Major problems with this algorithm:

Still hard to read.

Need the state machine picture to understand what is going on.

Hard to modify.

If the protocol (and therefore the state machine) changes, the code will most likely have to be rewritten.

Hard to debug.

The code will get very long very quickly.

Advantages:

The code within the while(true) can be put into a function and only called when there is new input.


Doc 18, Servers & States Slide # 11

Implementing a State Machine: Table

Commands	States
Commands	NoAuth	HaveUser	Process	Invalid	Quit
USER
PASS
LIST
RETR
QUIT

Each cell needs:

A function to process request
Next state on success
Next state on failure


Doc 18, Servers & States Slide # 12

The State Table

Commands	States
Commands	NoAuth	HaveUser	Process	Invalid	Quit
USER	actionUser	actionNull	actionNull
	HaveUser	Invalid	Invalid	Quit	Quit
	Invalid	Invalid	Invalid	Quit	Quit
PASS	actionNull	actionPass	actionNull
	Invalid	Process	Invalid	Quit	Quit
	Invalid	Invalid	Invalid	Quit	Quit
LIST	actionNull	actionNull	actionList
	Invalid	Invalid	Process	Quit	Quit
	Invalid	Invalid	Invalid?	Quit	Quit
RETR	actionNull	actionNull	actionRetr
	Invalid	Invalid	Process	Quit	Quit
	Invalid	Invalid	Invalid?	Quit	Quit
QUIT	actionQuit	actionQuit	actionQuit
	Quit	Quit	Quit	Quit	Quit
	Quit	Quit	Quit	Quit	Quit

Key

Function to process request

Next State on success

Next State on failure

How do we implement such a table?
One way is to use function pointers


Doc 18, Servers & States Slide # 13

Function Pointers in C/C++

void quickSort( int* array, int LowBound, int HighBound)
{
   // source code to sort array from LowBound to HighBound
   // using quicksort has been removed to save room on page
}
void mergeSort(int* array, int LowBound, int HighBound)
{   // same here}
void insertionSort(int* array, int LowBound, int HighBound)
{   // ditto }
void main()
{
   void (*sort) (int*, int, int);
   int size;
   int data[100];
   // pretend data and Size are initialized
   if (size < 25)
      sort = insertionSort;
   
   else if (size > 100) 
      sort = quickSort;
   else
      sort = mergeSort;
   sort(data, 0, 99);
}
Doc 18, Servers & States Slide # 14

C/C++ Function Pointers & Airline Example

typedef struct
{
   char *command;
   int (*function)( char*);
} vector;
vector commandTable[] =
{
   {"Cities", getCitiesList},
   {"LunchMenu", getLunchMenu},
   {"Manifest", getFlightManifest}
}; 
void executeCommand( char* inputLine, vector commandTable)
   {
   char* command = getCommand( inputLine );
   for ( int k = 0, k < TABLESIZE; k++ )
      if ( strcmp( command, commandTable[k].command ) == 0 )
          commandTable[k].function( inputLine );
   }

Notes:

Since getFlightManifest requires an argument, all functions given the same argument

Use of hashtables would make all this easier


Doc 18, Servers & States Slide # 15

SPOP State table: C/C++

In C/C++ we can define the following:

struct
{
   int      currentState;
   char      *command;
   int      stateIfSucceed;
   int      stateIfFailed;
   int      (*action)(char **);
} actionTable[] =
{
   {0, "USER", 1, 3, actionUser},
   {0, "QUIT", 4, 4, actionQuit},
   {1, "PASS", 2, 3, actionPass},
   {1, "QUIT", 4, 4, actionQuit},
   {2, "LIST", 2, 2, actionList},
   {2, "RETR", 2, 2, actionList},
   {2, "QUIT", 4, 4, actionList},
   {0, 0, 0, 0, 0}
};

Advantages:

Easy to see what is going on.

Even easier if the states are given names.

Easy to add new commands.


Doc 18, Servers & States Slide # 16

Function Pointers in Java

Use Reflection

Class.getMethod maps strings to methods

public Method getMethod(String name,Class parameterTypes[])
      throws NoSuchMethodException, SecurityException

Returns a Method object that reflects the specified public member method of the class or interface represented by this Class object. The name parameter is a String specifying the simple name the desired method, and the parameterTypes parameter is an array of Class objects that identify the method's formal parameter types, in declared order.

The method to reflect is located by searching all the member methods of the class or interface represented by this Class object for a public method with the specified name and exactly the same formal parameter types.

Throws: NoSuchMethodException 
   if a matching method is not found. 
Throws: SecurityException 
   if access to the information is denied.


Doc 18, Servers & States Slide # 17

Simple Class for Example

class Example
   {
   public void getLunch()
      {
      System.out.println( "Lunch Time!");
      }
   public void getLunch( String day)
      {
      System.out.println( "Lunch Time for " + day);
      }
      
   public void eatOut( String where)
      {
      System.out.println( "MacDonalds? ");
      }
   public void eatOut( int where)
      {
      System.out.println( "PizzaHut? " + where );
      }
   }


Doc 18, Servers & States Slide # 18

Using Class.getMethodSimple Example

import java.lang.reflect.Method;
class Test
   {
   public  static  void  main( String  args[] ) throws Exception
      {
      Example a = new Example();
      Class[] stringType = { Class.forName( "java.lang.String" ) };
      Object[] stringParameter = { "Monday" };
      Method tryMe;
      tryMe = a.getClass().getMethod( "getLunch", stringType );
      tryMe.invoke( a, stringParameter );
      }
   }

Output Lunch Time for Monday


Doc 18, Servers & States Slide # 19

Using Class.getMethod - Some Details

import java.lang.reflect.Method;
class Test
   {
   public  static  void  main( String  args[] ) throws Exception
      {
      Example a = new Example();
      Class[] stringType = { Class.forName( "java.lang.String" ) };
      Class[] intType = {  java.lang.Integer.TYPE  };
      Class[] noType = { };
      Object[] stringParameter = { "Monday" };
      Object[] intParameter = {  new Integer(6) };
      Object[] noParameter = { };
      Method tryMe;
      tryMe = a.getClass().getMethod( "getLunch", stringType );
      tryMe.invoke( a, stringParameter );
      tryMe = a.getClass().getMethod( "getLunch", noType);
      tryMe.invoke( a, noParameter );
      tryMe = a.getClass().getMethod( "eatOut", intType );
      tryMe.invoke( a, intParameter );
      }
   }

Output Lunch Time for Monday
Lunch Time
PizzaHut? 6


Doc 18, Servers & States Slide # 20

Airline Example

class AirlineServer implements ServerEngine 
   {
   public String getCitiesList(  ) { blah }
   public String getLunchMenu(  ) { blah }
   public String getFlightManifest( String usedHere ) { blah }
   public String invoke( String methodName, String argument )
      {
      Class[] argType = { };
      Object[] parameter = { };
      if (  argument != null )
         {
         argType = {  Class.forName( "java.lang.String" ) };
         parameter = { argument };
         }
   
      Method toInvoke;
      toInvoke = this.getClass().getMethod( methodName, argType);
   
      return  toInvoke.invoke( this, parameter );
   }


Doc 18, Servers & States Slide # 21

Java & Reflection

Java's reflection can be very useful

Awkward to use

Slow

Limited compared to:

Smalltalk
Lisp
Perl
Python
Ruby

Using reflection we could implement a state table as on slide 13


Doc 18, Servers & States Slide # 22

Function Pointers in Java #2

Inner Classes

Airline Example

import java.util.HashMap;
public interface Command {
   public String execute(String agrument );
}
public class AirlineServer  implements ServerEngine {
   public String getCitiesList(  ) { blah}
   public String getLunchMenu(  ) {  more blah}
   public String getFlightManifest( String usedHere ) { blah blah}
   class CitiesCommand implements Command {
      public String execute(String argument) {
         return getCitiesList();
      }
   }
   class LunchCommand implements Command {
      public String execute(String argument) {
         return getLunchMenu();
      }
   }
   class FlightManifestCommand implements Command {
      public String execute(String argument) {
         return getFlightManifest(argument);
      }
   }
Doc 18, Servers & States Slide # 23

Still in the AirlineServer Class


   HashMap commands = new HashMap();
   
      {
      commands.put( "getCitiesList", new CitiesCommand());
      commands.put( "getLunchMenu", new LunchCommand());
      commands.put( "getFlightManifest", 
         new FlightManifestCommand());
      }
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException {
      ProtocolParser requestData = new ProtocolParser( in );
      String request = requestData.getCommand();
      String requestParameter = requestData.getParameter;
      Command clientRequest =(Command) commands.get( request);
      clientRequest.execute( requestParameter );
   }
}


Doc 18, Servers & States Slide # 24

Objects as Functions
Using "thin" classes allows us to create objects that are just functions

The object functions can be mapped to strings

Allows us to implement state tables

A bit awkward to use


Doc 18, Servers & States Slide # 25

Objects as States

The Basic Idea

Each method (pass, user, etc.) performs the proper action for the given state and returns the next state

SPopState is abstract state with the default behavior for each method

Server is done with client when we reach the Quit state, so I did not add methods to that state


Doc 18, Servers & States Slide # 26

Strawman Driver Program

class SPopServer implements ServerEngine 
   {
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException 
      {
      SPopState currentState = new NoAuth();
      do
         {
         ProtocolParser requestData = new ProtocolParser( in );
         String request = requestData.getCommand();
         if ( request .startsWith( "PASS" ) )
            currentState = currentState.pass();
         else if ( request .startsWith( "USER" ) )
            currentState = currentState.user();
         etc.
            
            send response to client
         }
      while ( ! currentState instanceof Quit  );
      }
   }
Doc 18, Servers & States Slide # 27

Issues
The methods of SPopState and child classes:

need arguments

need to return two values: next state and response to user

Solution
Pass in arguments all the time

Return an array or

Let one argument be a response object, which state can modify by adding proper response

So create SPopResponse class!

What are the responsibilities of SPopResponse?


Doc 18, Servers & States Slide # 28

CardboardMan Driver Program

class SPopServer implements ServerEngine 
   {
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException 
      {
      ProtocolParser requestData = new ProtocolParser( in );
      String request = requestData.getCommand();
      SPopState currentState = new NoAuth();
      do
         {
         SPopResponse answer = new SPopResponse();
         if ( request .startsWith( "PASS" ) )
            currentState = currentState.pass( answer );
         else if ( request .startsWith( "USER" ) )
            currentState = currentState.user( answer );
         etc.
            
         output.println( answer );
         }
         while ( ! currentState instanceof Quit  );
      }   
   }
Doc 18, Servers & States Slide # 29

CardboardMan SPOPState

public class SPOPState {
   public SPOPState USER( other arguments,
                        SPopResponse serverResponse   )
   {
      return defaultAction( serverResponse );
   }
   public SPOPState PASS( other arguments,
                  SPopResponse serverResponse   )
   {
      return defaultAction( serverResponse );
   }
   public SPOPState LIST( other arguments,
                        SPopResponse serverResponse   )
   {
      return defaultAction( serverResponse );
   }
   private SPOPState defaultAction(      SPopResponse
                      serverResponse )
   {
      serverResponse.error();
      return new Invalid();
   }
}


Doc 18, Servers & States Slide # 30

Issue - The State of the States How do States get past Information?

The NoAuth state gets the user name

The HaveUser state gets the password

The HaveUser state needs the user name to verify password

But the PASS command from client does not contain user name

The Process state needs user name to get mail messages

SoggyCardboardMan NoAuth

public class NoAuth extends SPOPState
{
   public SPOPState user( some other arguments,
                     SPopResponse serverResponse   )
   {
      serverResponse = new SPopResponse.ok();
      String userName = get userName from argument list;
      return new HaveUser( userName );
   }
}

Now HaveUser object has user name!


Doc 18, Servers & States Slide # 31

Problem with State having States?!(Who is on first?)
Each client request can result in creating a new state object

If a large number of clients connect to a concurrent server at the same time, the creation of all the state objects can become a performance issue

Solution is to create one object per state class and reuse it

If the objects have data fields (data members) then two threads can not use the same object

Solution is to not allow the state classes to have any data fields

So where do we store the user name so other state objects can access it?

Solution is to create a class to store the data needed by all states

Call this class SPopData

What are the responsibilities of SPopData?


Doc 18, Servers & States Slide # 32

TinMan NoAuth

public class NoAuth extends SPOPState
   {
   public SPOPState user( some other arguments,SPopData data,
                     SPopResponse serverResponse   )
      {
      serverResponse = new SPopResponse.ok();
      String userName = get userName from argument list;
      data.setUserName( userName );
      return new HaveUser( );
      }
   }


Doc 18, Servers & States Slide # 33

TinMan SPopServer

class SPopServer implements ServerEngine 
   {
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException 
      {
      SPopState currentState = new NoAuth();
      SPopData clientData,
      do
         {
         ProtocolParser requestData = new ProtocolParser( in );
         String request = requestData.getCommand();
         SPopResponse answer = new SPopResponse();
         if ( inputLine.startsWith( "PASS" ) )
            currentState = currentState.pass( clientData, answer );
         else if ( inputLine.startsWith( "USER" ) )
            currentState = currentState.user( clientData, answer );
            etc.
            
         output.println( answer );
         }
         while ( ! currentState instanceof Quit  );
      }   
   }


Doc 18, Servers & States Slide # 34

Issue - How to create Just One Object

// Only one object of this class can be created
public class NoAuth extends SPOPState {
   private static NoAuth _instance = null;
   private NoAuth()   { }
   public static NoAuth getInstance() {
      if (  _instance == null )
          _instance = new NoAuth();
      return _instance;
   }
   public SPOPState user( some other arguments,SPopData data,
                     SPopResponse serverResponse   ) {
      serverResponse = new SPopResponse.ok();
      String userName = get userName from argument list;
      data.setUserName( userName );
      return new HaveUser( );
   }
}
class Program {
   public void aMethod() {
      X = NoAuth.getInstance();
   }
}


Doc 18, Servers & States Slide # 35

The Interesting Questions
Who does the actual work?

checking the password and user name
getting the mail messages

Who does the mapping from strings to functions?

How does all this fit together?

Does anyone understand this?

Special Bonus Question
Can you determine how to eliminate the need for mapping from strings to functions without using if statement (or switch statements)?

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2000 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.

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