PROGRAMMING ASSIGNMENT – 4

1. Objective:

To create a distributed file system for reliable and secure file storage.

2. Background:

A Distributed File System is a client/server-based ​application ​that allows client to store and retrieve files on multiple ​server​s. One of the features of Distributed file systems is that each file can be divided into pieces and stored on different servers and can be retrieved even if one server is not active.

3. Assignment Description:

In this assignment one distributed file system client (​DFC) uploads and downloads to and from 4 distributed file servers (​DFS1​, ​DFS2​, ​DFS3 and ​DFS4)​. DFS stands for distributed file system. The DFS servers are all ​running locally on a single machine with
different port numbers, for e.g. from 10001 to 10004.

When a ​DFC wants to upload a file to the 4 ​DFS servers, it first splits the file into 4 equal length pieces P1, P2, P3, P4 (a small length difference is acceptable if the total length cannot be divided by 4). Then the ​DFC groups the 4 pieces into 4 pairs such as (P1, P2), (P2, P3), (P3, P4), (P4, P1). Finally, the ​DFC uploads them onto 4 ​DFS servers. So now the file has redundancy, 1 failed server will not affect the integrity of the file.

Deciding which pairs to upload on which server:

This depends on the MD5 hash value of the file. Let x = MD5HASH(file) % 4

The Table 1 below shows the upload options based on x

x value
DFS1
DFS2
DFS3
DFS4





0
(1,2)
(2,3)
(3,4)
(4,1)
1
(4,1)
(1,2)
(2,3)
(3,4)





2
(3,4)
(4,1)
(1,2)
(2,3)
3
(2,3)
(3,4)
(4,1)
(1,2)
Table 1. How to determine pieces’ upload locations.

You can use MD5 or MD5SUM system call or your choice of md5 hash library

DFS servers should be able to identify username and password in clear text. And only provide store and retrieve services if the username and password matches.



a. Functions for DFC:

The client needs to run with the following command

# dfc dfc.conf

The configuration file dfc.conf contains the list of DFS server addresses, username and password shown below. The username and password are used to authenticate its identity to ​DFS​.Please create your own dfc.conf.



Server DFS1 127.0.0.1:10001

Server DFS2 127.0.0.1:10002

Server DFS3 127.0.0.1:10003

Server DFS4 127.0.0.1:10004

Username: Alice

Password: SimplePassword

Figure 1. A sample dfc.conf

Inside the ​DFC​,it should provide 3 commands ​list​,​get​and ​put

    1) list ​command inquires what files are stored on ​DFS servers, and print file names under the Username directory on ​DFS​servers.

list ​command ​should also be able to identify if file pieces on ​DFS ​servers ​are enough to reconstruct the original file. If pieces are not enough (means some servers are not available) then “[incomplete]” will be added to the end of the file. For example, suppose that two servers which had parts (3,4) and (4,1) in Table 1 are down. In this case, other two servers would be able to give only parts 1, 2 and 3, not 4. Thus, the DFC cannot reconstruct its file completely. Figure 2 shows that a user types “list” in the DFC and it could retrieve enough number of pieces for the two files text2.txt and image.png while it could not have enough pieces for the file test1.txt.

















Figure 2. LIST command and output example

    2) get​command downloads all available pieces of a file from all available ​DFS servers, if the file can be reconstructed then write the file into your working folder. If not, then print “File is incomplete.”

Here is the ​get​command example


get 1.txt

Figure 3. GET command example


    3) put command uploads file onto ​DFSs using the scheme that we described in the first page.


put 1.txt

Figure 4. PUT command example



DFS servers must check the DFC’s credentials and must serve requests only if the username and password sent from the DFC match those in dfs.conf. Note that dfs.conf is the configuration file of DFS servers.

The​DFC​should be able to print error messages if it fails in authentication.


b. Function for DFS:

The ​DFS​servers need to be run by the follow command:

    • dfs /DFS1 10001 &

    • dfs /DFS2 10002 &

    • dfs /DFS3 10003 &

    • dfs /DFS4 10004 &

In this assignment we will run all 4 servers locally and use port numbers to distinguish them as shown in Table 2.

Server
Port
DFS1
10001
DFS2
10002
DFS3
10003
DFS4
10004

Table. 2 Server ports

Each ​DFS ​server should have its own directory named DFS1/

DFS2/

DFS3/

and
DFS4/ respectively under your project directory.

Each ​DFS ​server needs to read /dfs.conf so that it knows all available users and their password.


Alice SimplePassword

Bob ComplexPassword

Figure 5. Sample dfs.conf file format

    1) USER handling

Each LIST, GET and PUT command should be accommodated by a valid user name and password, otherwise ​DFS ​will send the following error message back

“Invalid Username/Password. Please try again.”

    2) Directory handling
When a valid user request comes in. The ​DFS server will always check if there is a folder named after the username under the DFS’s directory. ​if there is not, create one and use this to handle all file pieces of this user.

For example:

./DFS1/Alice/ is Alice’s folder and ./ is your project directory.

        3) File pieces handling

When file pieces arrive, store it in the user’s folder and rename it in the following way.

Example:
If piece 2 and 3 of 1.txt is received from Alice at ​DFS1​.Then store them at:

./DFS1/Alice/.1.txt.2

./DFS1/Alice/.1.txt.3

./ is your project directory

the “.” prefix identifies this is a piece file not a conventional file. The numbered suffix identifies which piece it is.

    c. Misc.

        1) Time out or server not available

A client must try for 1 second to connect to the server. If a DFS server does not respond in 1 second, we consider that server is not available.

        2) Handle multiple connections


DFS servers should be able to handle simultaneous connections from different DFC​clients, say Alice and Bob, at the same time.

You can use pthead()/fork()/select() and refer your Assignment 2-3.

    d. Evaluation

    1. Config files are correctly parsed.

    2. Files are correctly put as per the mechanism explained earlier.

    3. All the functions (GET, LIST, PUT) are working in a loop seamlessly.

    4. Text files, image files, files with different extensions are working correctly.

    5. Reliability through redundancy

We will check whether the DFC client still shows the files correctly after killing 1 or 2 servers (kill -9 PID of each server). The expected outcome is to show incomplete files as well as complete files with ‘LIST’ and ‘GET’ command.

    6. Privacy through encryption

We will check whether the file is readable or not after changing the password in dfc.conf. Also, we will check how two clients with the same ID but with different passwords operate. Expected outcome is that the client without the valid password cannot read the file content. For example, if we try requests with username Alice, and one with correct password and another with incorrect password, it must serve requests with valid password.


This assignment is quite an extension to the PA-1. However, this application must be more robust. Means, to get full points, your code must not hang when we are running commands after one another, it must not stop if one server fails or it reconnects again etc.

Above-mentioned are the subset of test scenarios for this assignment and should be considered as general guidelines. There may be few additional/different test scenarios during interview grading.


e. Extra Credits

    1) Data encryption (5 points)
You encrypt pieces at ​DFC before sending them to ​DFS servers using the password in dfc.conf. Choose your own choice of encryption algorithm. This can be very simple as XOR encryption. You can store data in encrypted format or in a decrypted format. Both approaches are okay.

    2) Implement subfolder on DFS (5 points)

Right now DFS handles all files from the same user in one directory. Implement an extra command “MKDIR” within DFC, so that you make subfolders on ​DFS​.

MKDIR subfolder
Also try to upgrade ​LIST​, ​GET and ​PUT commands so that they can access, download and upload files in sub folders of that user.


For example: LIST subfolder/ PUT 1.txt subfolder/

GET 1.txt subfolder/

        3) Traffic optimization (5 points)

In the default GET command it gets all available pieces from all available servers it actually consumes twice of the actual data needed. Find a way to make an upgraded GET command so that it can reduce traffic consumption.



    f. Submission requirement

        1) Please submit your DFC and DFS codes with all configuration files (dfs.conf and dfc.conf) and README file together in one tar.gz file under the format your_identy_key_PA2.tar.gz

        2) Please complete the assignment in the directory where the code runs (relative directory), do not specify absolute directory such as /home/user/Desktop/ etc.

        3) Include comments in your codes and try to maintain a clear programming style.

    g. Few questions that may arise while implementing:

1. What port numbers to use?

Any port numbers can be used on the server. The client must choose the correct port number of each server as mentioned in the dfc_conf file.


    2. Should I compute the md5sum of file name or the content of file? md5sum must be computed of the content of the file, and NOT the file name.

    3. Can I use library or system call for md5sum?

Yes, you can use both.

4.Will there will be only one dfc.conf file ?

There will be one dfc.conf file per User.

5. Will servers have only one dfs.conf file?

For simplicity, you can use the same dfs.conf for all four servers. In real systems, each server running on a different machine runs its own dfs.conf while the content of this file may be the same across different servers.