Given a list of backends and an incoming stream of requests, implement a round-robin request router that returns the next healthy server for each request. Handle server add/remove and health changes, ensure fair rotation with correct wrap-around, and avoid skipped or duplicated assignments. You are given failing tests indicating incorrect routing—identify and fix the bug, then write tests for single server, multiple servers, removal during routing, and health flapping. Analyze complexity and thread-safety considerations.

This question evaluates understanding of round-robin routing, health-aware load balancing, dynamic membership management, rotation-index correctness, testing, and concurrency considerations.

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Debug round-robin request router | DoorDash Interview Question

Round-Robin Request Router with Health and Dynamic Membership

You are given a list of backend servers and an incoming stream of requests. Implement a round-robin request router that returns the next healthy server for each request.

Requirements

Round-robin across servers with correct wrap-around.
Only return healthy servers; skip unhealthy ones.
Support server add and remove at runtime.
Support health state changes at runtime.
Ensure fair rotation (no skipped or duplicated assignments when state changes happen).
If no healthy servers exist, the router should signal that state (e.g., by raising an exception or returning a sentinel).

Bug-Fix Prompt

You are given failing tests indicating incorrect routing order after server removal and during health flapping. Identify the likely bug causing skipped or duplicated assignments and fix it.

Deliverables

Implement the router with the following interface (or equivalent in your language):
- add_server(id)
- remove_server(id)
- set_health(id, healthy: bool)
- next() -> id (returns the next healthy server)
Fix the routing bug so that:
- The rotation index is correctly advanced only after returning a server.
- Removal of a server adjusts the rotation index to avoid skipping or duplicating assignments.
Write tests for:
- Single server
- Multiple servers
- Removal during routing
- Health flapping (servers toggling between healthy/unhealthy)
Analyze time/space complexity and discuss thread-safety considerations and mitigations.

Assumptions

Server IDs are unique and comparable (e.g., strings or integers).
If all servers are unhealthy (or there are none), the router returns a defined error or raises an exception.
New servers are appended to the rotation and should be included in subsequent calls to next().

Requirements

Round-robin across servers with correct wrap-around.

Only return healthy servers; skip unhealthy ones.

Support server add and remove at runtime.

Support health state changes at runtime.

Ensure fair rotation (no skipped or duplicated assignments when state changes happen).

If no healthy servers exist, the router should signal that state (e.g., by raising an exception or returning a sentinel).

Deliverables

Implement the router with the following interface (or equivalent in your language):

add_server(id)
remove_server(id)
set_health(id, healthy: bool)
next() -> id (returns the next healthy server)

Fix the routing bug so that:

The rotation index is correctly advanced only after returning a server.
Removal of a server adjusts the rotation index to avoid skipping or duplicating assignments.

Write tests for:

Single server
Multiple servers
Removal during routing
Health flapping (servers toggling between healthy/unhealthy)

Analyze time/space complexity and discuss thread-safety considerations and mitigations.

Debug round-robin request router

Quick Overview

Round-Robin Request Router with Health and Dynamic Membership

Requirements

Bug-Fix Prompt

Deliverables

Assumptions

Solution

Comments (0)

Debug round-robin request router

Quick Overview

Round-Robin Request Router with Health and Dynamic Membership

Requirements

Bug-Fix Prompt

Deliverables

Assumptions

Solution

Comments (0)