Final Exam - Practical Part

Distributed Systems and Network Programming - Spring 2023

Task 1 - UDP Socket Programming [8.5 Points]

Write a UDP server and a client using Python socket module to implement a minimalistic DNS service

• The client sends queries to the server over a UDP socket
• The server replies with a Resource Record (RR) from a predefined list or an error if the record is not found

Client (client.py)

• Create a class Query with two string attributes (type, key)
• Once started, the client creates the following three Query instances

  Query(type="A", key="example.com")
  Query(type="PTR", key="1.2.3.4")
  Query(type="CNAME", key="moodle.com")
  

• Serialize the instances using json module and send them (one by one) over UDP to the listening server
• Receive and deserialize the server reply, then print the resulting dictionary to the standard output

Server (server.py)

• Create a class RR with three string attributes (type, key, value)
• Once started, the server listens on all interfaces on port 50000
• The server runs forever, unless a KeyboardInterrupt is received
• The server maintains a list of resource records. The list initially contains the following two records

  RR(type="A", key="example.com", value="1.2.3.4")
  RR(type="PTR", key="1.2.3.4", value="example.com")
  

• Once the server receives a Query from a client, it replies with the requested record or an error
• To create an error reply, add 'value': 'NXDOMAIN' to the received query and send it back to the client

Example Run

$ python3 server.py
Server: listening on 0.0.0.0:50000
Client: {'type': 'A', 'key': 'example.com'}
Server: Record found. Sending answer.
Client: {'type': 'PTR', 'key': '1.2.3.4'}
Server: Record found. Sending answer.
Client: {'type': 'CNAME', 'key': 'moodle.com'}
Server: Record not found. Sending error.
^CServer: Shutting down...

$ python3 client.py
Client: Sending query for example.com
Server: {'type': 'A', 'key': 'example.com', 'value': '1.2.3.4'}
Client: Sending query for 1.2.3.4
Server: {'type': 'PTR', 'key': '1.2.3.4', 'value': 'example.com'}
Client: Sending query for moodle.com
Server: {'type': 'CNAME', 'key': 'moodle.com', 'value': 'NXDOMAIN'}

Task 2 - Remote Procedure Call [8.5 Points]

Write a server and a client to implement a simple remote calculator

• The client calls stub functions to do elementary mathematical operations
• The server executes the corresponding implementations to calculate the required results and send them back to the client
• Communication happens over the local network, with Protocol Buffers as the data serialization format

Server (server.py)

• Run a gRPC server that listens on all interfaces on port 50000
• The server should run forever unless a KeyboardInterrupt is received
• Expose the following functions for RPC. The logic for each function is explained below:
  • Add(a: int, b: int) returns a + b
  • Subtract(a: int, b: int) returns a - b
  • Multiply(a: int, b: int) returns a * b
  • Divide(a: int, b: int) returns a / b or math.nan if b equals zero
• Handle remote calls from the client and return the required results

Client (client.py)

• Once executed, the client calls the following remote functions one by one

  Add(10,2)
  Subtract(10,2)
  Multiply(10,2)
  Divide(10,2)
  Divide(10,0)
  

• The client should print the query and the received results, as shown in the example run below

Schema file (calculator.proto)

• Defines the protocol of communication between the client and the server
  • The calculator service with remote functions that can be called through rpc
  • Request/response message types for the client/server communication
• Make sure you have grpcio and grpcio-tools installed. You may use a virtual environment if needed
• Compile the .proto file using the following command

  python3 -m grpc_tools.protoc --proto_path=. \
  --python_out=. --grpc_python_out=. calculator.proto
  

Example Run

$ python3 server.py
gRPC server is listening on 0.0.0.0:50000
Add(10,2)
Subtract(10,2)
Multiply(10,2)
Divide(10,2)
Divide(10,0)
^CShutting down...

$ python3 client.py
Add(10,2) = 12.0
Subtract(10,2) = 8.0
Multiply(10,2) = 20.0
Divide(10,2) = 5.0
Divide(10,0) = nan