Implementation Overview

Disclaimer:
The UDP client is not implemented by me. Only the server and the TCP client (subscriber) were developed as part of this project. For testing, I used this repository.

Server

The server listens on both TCP and UDP protocols, using two separate sockets—one for each protocol.

For I/O multiplexing, poll() is used with three initial pollfd entries:

• Standard input (stdin)
• TCP socket
• UDP socket

The server runs continuously with the following behavior:

1. Standard Input Handling

   • If input is available on stdin, the server checks for the exit command.
   • This is the only accepted server-side command and causes a clean shutdown.

2. TCP Connection Handling

   • When data is available on the TCP socket, a new client connection is accepted.
   • The Nagle algorithm is disabled (TCP_NODELAY is set).
   • The client sends its ID, which is checked against existing active clients.
     • If the ID is unique, it is registered in a mapping of IDs to file descriptors.
     • The server informs the client whether the connection is accepted.
     • If the ID is already in use, the client is instructed to terminate.

3. UDP Message Handling

   • When data is available on the UDP socket, the server receives a message from a UDP client.
   • The message is parsed into an internal structure and transformed into a tcp_message_t structure for TCP delivery.
   • The server extracts the topic and checks all TCP client subscriptions using regular expressions (to support wildcards).
   • If a match is found, the client ID is added to a topic-to-subscribers map.
   • The decoded message is sent to all currently connected TCP clients subscribed to the topic.

4. TCP Client Commands

   • When a connected TCP client sends data, the server processes subscribe/unsubscribe commands.
   • If the client disconnects, it is removed from active tracking.
   • For each command, the server determines whether it's a regular or wildcard-based topic and updates subscriptions accordingly.

Upon receiving the exit command from stdin, the server closes all open sockets and terminates gracefully.

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Subscriber (TCP Client)

Upon startup, the subscriber:

• Opens a TCP socket and disables the Nagle algorithm.
• Connects to the server and sends its ID.
• Awaits confirmation that it can proceed (based on unique ID check).

If approved, the client enters a loop using poll() for I/O multiplexing, handling two main cases:

1. User Commands

   • If input is available from the user (via stdin), the client processes:
     • exit: terminates the client.
     • subscribe <topic> or unsubscribe <topic>:
       • Fills a tcp_client_request structure with the topic and command type (true for subscribe, false for unsubscribe).
       • Sends the request to the server.

2. Server Messages

   • If data is available from the server, it is printed to stdout in the format specified for TCP messages.

At termination, the client closes the socket it originally opened.

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Data Structures

• udp_message_t

  • Holds the topic (string), data type (1 byte), and payload.

• tcp_message_t

  • Contains the following:
    • IP address (as a string)
    • Port number (UDP sender)
    • Data type (1 byte)
    • Data format (e.g., INT, FLOAT, etc.)
    • Topic (string)
    • Payload (formatted for readability)

• tcp_client_request

  • Used for subscription commands.
  • Fields:
    • A boolean flag (true = subscribe, false = unsubscribe)
    • Topic string

• int_tcp_t and float_tcp_t

  • Helper structures used for decoding UDP messages into readable TCP formats.

These structures enable effective message parsing, transformation, and routing from UDP to subscribed TCP clients.

Server-Client Usage Guide

Start the Server

To start the server, use the following command:

./server <PORT>

• <PORT>: The port number on which the server will listen for both TCP and UDP connections.

The server will listen for:

• TCP connections from subscribers (clients)
• UDP datagrams from publishers (predefined clients)

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Start a TCP Client (Subscriber)

To start a TCP client (subscriber), use:

./subscriber <ID_CLIENT> <IP_SERVER> <PORT_SERVER>

• <ID_CLIENT>: A string (max 10 ASCII characters) identifying the client.
• <IP_SERVER>: The IPv4 address of the server (e.g., 127.0.0.1).
• <PORT_SERVER>: The same port number used to start the server.

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TCP Client Commands (from keyboard input)

Once started, the TCP client accepts the following commands via keyboard:

• Subscribe to a topic:

subscribe <TOPIC>

• Unsubscribe from a topic:

unsubscribe <TOPIC>

• Disconnect the client:

exit

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Wildcards Tutorial

TCP clients can subscribe to topics using wildcards to match multiple levels in topic names.

* Wildcard

The * wildcard matches zero or more levels in the topic hierarchy.

Example:

subscribe UPB/precis/*/value

This matches topics like:

• UPB/precis/1/value
• UPB/precis/room1/value
• UPB/precis/sensors/room1/value

But not:

• UPB/precis/room1/value/errors

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+ Wildcard

The + wildcard matches exactly one level in the topic hierarchy.

Example:

subscribe a/+/b

This matches:

• a/x/b
• a/room1/b

But not:

• a/x/y/b
• a/b

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Combined Wildcards

You can use wildcards in combination:

subscribe UPB/+/1/+
subscribe */temperature

These will match:

• UPB/precis/1/temperature
• UPB/acs/temperature
• anything/temperature

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Duplicate Subscriptions

If a client subscribes to both a specific topic and a matching wildcard, it will still receive only one copy of each matching message.

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Server Shutdown Command

The server accepts the following command from keyboard:

exit

This will close:

• All connected TCP clients
• The server process itself