Round Robin Algorithm: Debugging and Testing
Asked at:
DoorDash
Paxos
Brex
DESCRIPTION
Implement a traffic router that distributes incoming requests across multiple servers using the round robin algorithm. The router should cycle through available servers sequentially, sending each new request to the next server in line. For example, with servers ['A', 'B', 'C'], requests should route to A → B → C → A → B → C... in order.
Input:
servers = ['server1', 'server2', 'server3'] route() called 5 times
Output:
server1 server2 server3 server1 server2
Explanation: Each call to route() returns the next server in circular order
Constraints:
- At least one server must be available
- Server list remains constant after initialization
- Must handle any number of consecutive route() calls
- Must maintain state between calls to track current position
Understanding the Problem
The core challenge is maintaining state across multiple function calls to track which server should receive the next request. You need a mechanism to cycle through servers sequentially and wrap around to the beginning after reaching the last server. The solution requires storing both the list of servers and the current index position, then incrementing this index with proper modulo arithmetic to achieve circular behavior.
Building Intuition
A naive approach might use a counter that increments indefinitely, but this could eventually overflow with millions of requests. A better approach uses the modulo operator (%) to keep the index within bounds: index % server_count always returns a valid position (0 to n-1). For example, with 3 servers, indices 0, 1, 2, 3, 4, 5 map to positions 0, 1, 2, 0, 1, 2.
Round robin load balancing is fundamental to distributed systems and prevents any single server from being overwhelmed. It's used in DNS servers, reverse proxies (like Nginx), Kubernetes service routing, and database connection pools. Understanding this pattern is essential for building scalable web applications.
Common Pitfalls
Implementation
Router Class Structure
Create a TrafficRouter class that stores the server list and current index as instance variables. The constructor initializes these values, setting the index to 0. This establishes the foundation for maintaining state across multiple routing calls. For example, TrafficRouter(['A', 'B', 'C']) creates a router ready to distribute requests starting from the first server.
class TrafficRouter:def __init__(self, servers):self.servers = serversself.current_index = 0
Round Robin Routing Logic
Implement the route() method that returns the current server and advances the index. Use the current index to select a server from the list, then increment the index with modulo arithmetic ((index + 1) % length) to wrap around. For example, with 3 servers at index 2, the next index becomes (2 + 1) % 3 = 0, cycling back to the start.
def route(self):server = self.servers[self.current_index]self.current_index = (self.current_index + 1) % len(self.servers)return server
What We've Learned
- Pattern: Use modulo operator for circular iteration through fixed-size collections
- State Management: Instance variables preserve position across multiple method calls
- Use Case: Load balancing in web servers, proxies, and distributed systems
- Simplicity: Round robin provides fair distribution with O(1) time complexity and minimal overhead
Problems to Practice
easy
This problem requires maintaining pointers to track positions in an array while performing operations, similar to how round robin maintains a current index position. It reinforces array manipulation and index management skills needed for debugging round robin implementations.
easy
Round robin involves circular/cyclic iteration through a collection, which is conceptually similar to detecting and working with cycles. This problem helps understand circular data structure patterns that underpin round robin scheduling algorithms.
Question Timeline
See when this question was last asked and where, including any notes left by other candidates.
Mid May, 2026
DoorDash
Senior
Mid March, 2026
DoorDash
Mid-level
Asked in HM round
Mid March, 2026
DoorDash
Mid-level
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