Implement a two-user communications handler

Q: Implement a two-user communications handler

This is a System Design interview question from Akuna Capital for Software Engineer roles. View the full question and solution on PracHub.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Question

One-at-a-time Two-User Communications Handler

Context

Design and implement a communications handler that supports exactly one active conversation at a time between two distinct users. Each user is represented by a Caller with a unique name.

Requirements

Define a custom exception type:
- ConnectionException that accepts a single error-message string.
Define an abstract base class CommsHandlerABC with these abstract methods:
- connect(user1: Caller, user2: Caller) -> str
  - Establish a call and return a success message, or raise ConnectionException.
- hangup(user1: Caller, user2: Caller) -> str
  - End the call and return a success message, or raise ConnectionException.
- clear_all() -> None
  - Force-clear any existing call and reset state.
Implement a concrete CommsHandler that enforces:
- Only one active call at any time.
- A user cannot connect with themselves.
- While a call is active, any new connect attempt fails with error: "Connection in use. Please try later".
- hangup succeeds only for the currently connected pair (order-agnostic). Otherwise raise: "{A} and {B} not found in the communication channel." (note trailing period).
- clear_all resets the state immediately.

I/O Behavior

Commands to support (semicolon-separated in a single line or as separate lines):
- connect <user1> <user2>
- hangup <user1> <user2>
- clear_all
Output:
- On success: prefix with "Success: " and include the handler's returned message.
- On error: prefix with "Error: " and include the exception's message.

Sample

Sample Input:
- connect Alice Bob; connect Carol Dave; hangup Alice Bob; connect Carol Dave; hangup Carol Dave; connect Eve Eve; hangup Alice Bob
Expected Output:
- Success: Connection established between Alice and Bob; Error: Connection in use. Please try later; Success: Alice and Bob are disconnected; Success: Connection established between Carol and Dave; Success: Carol and Dave are disconnected; Error: Eve cannot connect with Eve; Error: Alice and Bob not found in the communication channel.