client-server-app

A system to transfer web pages over the network using client/server architecture. The system consists of two main distributed components: server and client, which may run on different hosts in the network. A server is a program that accepts multiple incoming TCP connections from clients and responds to requests sent by clients.

A protocol called Json-based Hypertext Transfer Protocol (JHTTP) which is used by server and clients to communicate. JHTTP has a set of request methods to indicate the desired action to be performed for a given web page. Please note that web pages are text files only.

The system has been designed to use TCP Sockets. Server creates multiple threads and uses them to process clients’ requests. All the communicated messages between the server and clients must be encapsulated in the form of JSON messages.

Server

A server maintains a root folder (directory) which contains all the web pages named as www.

When the server program is executed, the program checks that if www folder exists on the same folder in which the server executable file resides. If this folder does not exists, server program should create it automatically.

Server accepts TCP connection requests from clients and responds to the communicated JHTTP requests. The server must be able to serve multiple clients concurrently. As soon as a client is connected to the server, the server is ready to accept request messages including GET, PUT, DELETE, DISCONNECT requests.

Table 1

Request Type	Meaning
GET	A GET request is used to retrieve a web page content from the server.
PUT	A PUT request replaces or adds the target web page content on the server with the request payload.
DELETE	A DELETE request deletes the specified web page.
DISCONNECT	Terminates the connection with the server

Path to one file should follow this rule: Starts with slash ‘/’ + between 0 to 10 nested folders (the length of folder name ranges from 1 to 20 ACCEPTED CHARACTERS) + End with slash ‘/’ + file name (the length of file name ranges from 1 to 10 ACCEPTED CHARACTERS) + Dot ‘.’ + file extension (the length of file extension ranges from 1 to 5 ACCEPTED CHARACTERS). In what follows, you can find some examples:

• /index.html (ACCEPTED)
• /A1/B/C532vaAZu67/6D/7/A2/1rule.ex3 (ACCEPTED)
• /index123450.html543 (NOT ACCEPTED)

Path to one folder should follow this rule: Starts with slash ‘/’ + between 1 to 10 nested folders (the length of folder name ranges from 1 to 20 ACCEPTED CHARACTERS) + ends with slash ‘/’. In what follows, you can find some examples:

• /A/B/C/ (ACCEPTED)
• /A1/B/C532vaAZu67/6D/7/A2/ (ACCEPTED)
• / (NOT ACCEPTED)
• The List of ACCEPTED CHARACTERS: [a-z, A-Z, , 0-9]

Client

A client is a command-line interface (CLI) that accepts commands from the end user, sends JSON request messages to the server accordingly, and shows server responses on the CLI in the desired format. Table 2 shows lists of commands and their arguments that the end-user can type.

The end-user types a command and its required arguments on the CLI. Commands are executed by Enter (Return) key. According to each command, the client program creates a corresponding request message from Table 1 and sends it to the server. Upon on receiving a request message by the server, a response message is generated by the server and is sent back to the client.

Please note that messages sent between the client and the server are all in JSON format (more details in Section 4). The response message can be either success or error. Client shows appropriate texts on the CLI based on the message received.

• If an end-user enters a command which is not defined, has missing or extra arguments, or it has error in any forms, client program prints nothing on the CLI and goes to the next line waiting for a new command.
• Commands are case-sensitive.
• If the source file in the put command cannot be found on the client system, the CLI shows Source file not found.
• The program is supposed to only accept relative paths, for example, /index.html or /cmd/index.html. A relative path refers to a location that is relative to a root folder i.e. www folder. Please note that the root folder is not part of the target path. If www appears as part of the path, for example, /www/index.html, it means that there is a subfolder called www within the root folder.

Table 2

Request Type	Arguments	Meaning	Example
connect	ip, port	connect to a server with the IP address ip and port address port	connect 192.168.0.10 2000
get	target	retrieve the content of the specified web page from the server	get /index.html
put	source, target	upload the content of the specified source web page on the client file system to the specified target web page on the server	put /test.html /finance/index.html
delete	target	delete the specified web page from the server target can be an empty folder	delete /finance/test.html
disconnect		disconnect from the server	disconnect

Request Messages

All request messages have a key-value pair that shows the type of the request message. type can either of the following: GET, PUT, DELETE, DISCONNECT. If type is GET the format is:

{
  "message": "request",
  "type": "GET" ,
  "target": "/cmd/index.html"
}

If type is PUT the format is:

{
  "message": "request",
  "type": "PUT" ,
  "target": "/cmd/index.html",
  "content": "web page content in utf-8 format"
}

If type is DELETE the format is:

{
  "message": "request",
  "type": "DELETE" ,
  "target": "/index.hmtl"
}

If type is DISCONNECT the format is:

{
  "message": "request",
  "type": "DISCONNECT"
}

Response Messages

JSON format for response messages is as follows:

{
"message":"response",
"code":"a valid JSON number data type" ,
"content":"a valid JSON string data type"
}

The list of “code” values and content values are shown in Table 3.

• Please note that CLI shows the value of the “content” key for all “code” values, for example OK for code 201.

Table 3

Code	Meaning	Content
200	GET request was successfull	The content of the target web page in utf-8 format
201	PUT request was successful, new file created	Ok
202	PUT request was successful, but file is overwritten	Modified
203	DELETE request was successful	Ok
400	Target web page does not exist (GET or DELETE)	Not Found
401	Request format is incorrect, missing or extra agruments and wrong message type and invalid targets	Bad Request
402	Any other erros	Unknown Error

Connect and Disconnet

The end-user enters connect command immediately after running the client program. This command results in a TCP connection establishment with a server, for example: connect 192.168.0.10 2000

The end-user enters disconnect command to close down connection and quits program. This command results in a TCP disconnection with a server, for example: disconnect

Running the program

Run client and server executable as follows: $ java -jar server.jar -p port $ java -jar client.jar