ASSIGNMENT – 3 DFC (Distributed File Client) solution


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In this assignment one client DFC (Distributed File Client) is uploading and downloading
files onto and from 4 servers DFS1, DFS2, DFS3 and DFS4. (DFS means Distributed File
Server.) The DFS servers are all running locally on a single machine with different port
numbers, for e.g. from 10001 to 10004.
When DFC want to upload a file to the 4 DFS servers, it first split the file in to 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 group the 4 pieces in to 4 pairs (P1, P2), (P2, P3),
(P3, P4), (P4, P1). At last 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 bellow 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.
Note: when handling MD5 in number format try not overflow
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 need to be 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 show identity to DFS,
so requests can be accepted. Please create your own dfc.conf.
Server DFS1
Server DFS2
Server DFS3
Server DFS4
Username: Alice
Password: SimplePassword
Figure 1. dfc.conf configuration file
Inside the DFC client it should provide 3 functions LIST, GET and PUT
1) LIST command inquires what file in stored on DFS servers, and print file
names stored under Username on DFS servers (e.g., ./DFS1/Alice,
./DFS2/Alice. ./ is your project directory).
LIST command should also be able to identify if file pieces on DFSs 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, two servers which had parts (3,4) and (4,1) are down. In this
case, other two servers would be able to give only parts 1,2 and 3. 4th part is
not available at all. Thus, you cannot retrieve this completely and your list
command must specify this information to the user. A sample output for list
command is as follows:
2.txt [incomplete]
Figure 2. LIST command and output example
2) GET command downloads all available pieces of a file from all available DFS, if
the file is reconstructable then write the file into your working folder. If the
file is not reconstructable, 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 DFS using scheme that we described in the
first page.
PUT 1.txt
Figure 4. PUT command example
Each command must be sent with the username and password info to server in clear
text to be identified. Server must check these credentials and must serve requests
only if username and password matches as per the dfs.conf file available with the
DFC should be able to print error messages send back by DFS servers in case
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 number 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.
After start each DFS server need 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
“Invalid Username/Password. Please try again.”
2) Directory handling
When a valid user request come in. 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.
If piece 2 and 3 of 1.txt is received from Alice at DFS1. Then store them at:
./ 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 DFS server does not
response 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.
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
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 password 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
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 (Mandatory for CSCI5273/ECEN5023 students. Extra credits
for CSCI4273 students – 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 (Mandatory for CSCI5273/ECEN5023 students.
Extra credits for CSCI4273 students – 10 points)
Right now DFS handles all files from same user in one directory. Implement an
extra command “MKDIR” with in 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 – 10 points (Extra credits for all students)
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
2) Please complete the assignment in the directory where the code runs (relative
directory), do not specify absolute directory such as /home/user/Desktop/
3) Include comments in your codes and try to maintain 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 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 file, and NOT the file name.
3. Can I use library or systemcall for md5sum?
Yes, you can use it for computing md5sum.
4.Will there will be only one dfc.conf file ?
There will be one dfc.conf file per User. i.e. Alice will have her own file with her username
and password mentioned in it. And bob will have separate file with his username and
password mentioned in it.
5. Will server have only one dfs.conf file?
Yes, server will only maintain 1 conf file which will list all the users with their passwords.