Round Robin Load Balancing

✅ When is it appropriate

Round Robin is suitable if most of the following apply:

all servers have equal CPU and memory capacity
services are stateless
requests take a similar amount of time to process
no additional configuration or routing rules are needed
traffic arrives at a steady pace without large sudden spikes

Round Robin sends each new request to the next server in the list, one by one, in rotation. No configuration is needed beyond listing the server addresses. It works well when all servers are identical and no request needs to reach a specific server.

❌ When is it NOT appropriate

Round Robin may not be ideal if:

servers differ in CPU or memory capacity
the load is uneven or bursty
session data is stored on individual servers rather than a shared database
services are stateful

If a server goes down, Round Robin continues sending requests to it until it is manually removed from the pool. If session data is stored on individual servers, a user's next request may land on a different server that has no record of that session.

👍 Advantages

simple to implement
each server receives an equal share of requests over time
no performance overhead from computing routing decisions
no routing rules or tuning required

👎 Disadvantages

does not account for differences in server performance
ignores server health or availability
a failed server keeps receiving requests until manually removed from the pool
cannot route to the fastest available server or the one with the fewest active connections

🛠️ Typical use cases

stateless web applications where all servers have equal capacity
stateless microservices
steady traffic without large sudden spikes
development or testing environments
APIs where all endpoints have similar response times

⚠️ Common mistakes (anti-patterns)

using Round Robin when session data is stored on individual servers, causing users to lose their session when routed to a different server
applying it when servers have very different CPU or memory capacity, causing weaker servers to become overloaded
running without health checks, so crashed servers keep receiving requests
using Round Robin during traffic spikes when all servers receive an equal share of the surge regardless of how busy they already are

The most common failure is storing session data on individual servers. A user logs in on Server A, but their next request goes to Server B, which has no record of the login, and the user is logged out. Round Robin has no concept of which server handled previous requests from the same user.

💡 How to build on it wisely

Recommended approach:

Use Round Robin when all servers have equal capacity and requests do not need to reach a specific server.
Combine with health checks to avoid sending requests to unavailable servers.
Track response times per server. If one consistently responds slower, replace it or switch to Weighted Round Robin.
Switch to Weighted Round Robin if servers differ in CPU or memory capacity, or to Least Connections if some requests take significantly longer to process.
Always add health checks from the start. Without them, Round Robin will keep sending requests to a server that has crashed.

If servers start showing uneven load, users report being logged out between requests, or a failed server keeps receiving traffic, Round Robin alone is no longer sufficient. Switch to Weighted Round Robin or Least Connections when these problems appear.

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