Top 22 Load Balancing Interview Questions And Answers

Question 1

What Is Load Balancing?

Accepted Answer

**Load balancing** is simple technique for distributing workloads across multiple machines or clusters. The most common and simple load balancing algorithm is Round Robin. In this type of load balancing the request is divided in circular order ensuring all machines get equal number of requests and no single machine is overloaded or underloaded.  
  
**The Purpose of load balancing is to**  
  
*   Optimize resource usage (avoid overload and under-load of any machines)
*   Achieve Maximum Throughput
*   Minimize response time

**Most common load balancing techniques in web based applications are**

1.  Round robin
2.  Session affinity or sticky session
3.  IP Address affinity

Question 2

What Is Round-Robin Load Balancing?

Accepted Answer

**Round‑robin load balancing** is one of the simplest methods for distributing client requests across a group of servers. Going down the list of servers in the group, the round‑robin load balancer forwards a client request to each server in turn. When it reaches the end of the list, the load balancer loops back and goes down the list again (sends the next request to the first listed server, the one after that to the second server, and so on).
![](https://www.cisco.com/en/US/i/200001-300000/220001-230000/224001-225000/224609.jpg)

Question 3

Name some advantages of Round-Robin Load Balancing

Accepted Answer

The main benefit of round‑robin load balancing is that it is **extremely simple to implement.** However, it does not always result in the most accurate or efficient distribution of traffic, because many round‑robin load balancers assume that all servers are the same: currently up, currently handling the same load, and with the same storage and computing capacity.

Question 4

What Is Sticky Session Load Balancing? What Do You Mean By "Session Affinity"?

Accepted Answer

*Sticky session* or a *session affinity technique* is another popular load balancing technique that requires a user session to be always served by an allocated machine.

In a load balanced server application where user information is stored in session it will be required to keep the session data available to all machines. This can be avoided by always serving a particular user session request from one machine. The machine is associated with a session as soon as the session is created. All the requests in a particular session are always redirected to the associated machine. This ensures the user data is only at one machine and load is also shared.  
  
This is typically done by using SessionId cookie. The cookie is sent to the client for the first request and every subsequent request by client must be containing that same cookie to identify the session.

**What Are The Issues With Sticky Session?**

There are few issues that you may face with this approach

*   The client browser may not support cookies, and your load balancer will not be able to identify if a request belongs to a session. This may cause strange behavior for the users who use no cookie based browsers.
*   In case one of the machine fails or goes down, the user information (served by that machine) will be lost and there will be no way to recover user session.

Question 5

What are some variants of Round-Robin Load Balancing algorithm?

Accepted Answer

The following variants to the round‑robin algorithm take additional factors into account and can result in better load balancing:
* **Simple Round-Robin** -  Simplest methods for distributing client requests. It just forwards a client request to each server in turn. When it reaches the end of the list, the load balancer loops back and goes down the list again.
*   **Weighted Round-Robin** – A weight is assigned to each server based on criteria chosen by the site administrator; the most commonly used criterion is the server’s traffic‑handling capacity. The higher the weight, the larger the proportion of client requests the server receives. If, for example, server A is assigned a weight of 3 and server B a weight of 1, the load balancer forwards 3 requests to server A for each 1 it sends to server B.
*   **Dynamic Round-Robin** – A weight is assigned to each server dynamically, based on real‑time data about the server’s current load and idle capacity.

Question 6

Why should we use Load Balancer (except preventing overloading)?

Accepted Answer

**Load balancers** distribute incoming client requests to computing resources such as application servers and databases. In each case, the load balancer returns the response from the computing resource to the appropriate client. Load balancers are _effective_ at:

* Preventing requests from going to _unhealthy servers_
* Preventing _overloading resources_
* Helping _eliminate single points of failure_

Additional benefits include:

* _SSL termination_ - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
* Removes the need to install _X.509 certificates on each server_
* _Session persistence_ - Issue cookies and route a specific client's requests to same instance if the web apps do not keep track of sessions

Question 7

What Is a TCP Load Balancer?

Accepted Answer

A **TCP load balancer** is a type of load balancer that uses transmission control protocol (TCP), which operates at layer 4 — the transport layer — in the open systems interconnection (OSI) model. TCP load balancers are for applications **that do not use HTTP**. When deployed in front of a database cluster, a TCP load balancer spreads requests across all available server configurations.

Question 8

What is the Difference Between Weighted Load Balancing vs Round Robin Load Balancing?

Accepted Answer

The biggest drawback of using the round robin algorithm in load balancing is that the algorithm assumes that servers are similar enough to handle equivalent loads. If certain servers have more CPU, RAM, or other specifications, the algorithm has no way to distribute more requests to these servers. As a result, servers with less capacity may overload and fail more quickly while capacity on other servers lie idle.

**The weighted round robin load balancing algorithm allows site administrators to assign weights to each server based on criteria like traffic\-handling capacity**. Servers with higher weights receive a higher proportion of client requests. For a simplified example, assume that an enterprise has a cluster of three servers:

* Server A can handle 15 requests per second, on average
* Server B can handle 10 requests per second, on average
* Server C can handle 5 requests per second, on average

Next, assume that the load balancer receives 6 requests.

* 3 requests are sent to Server A
* 2 requests are sent to Server B
* 1 request is sent to Server C.

In this manner, the weighted round robin algorithm distributes the load according to each server’s capacity.

Question 9

What Is Load Balancing Fail Over?

Accepted Answer

*Fail over* means switching to another machine when one of the machine fails. Fail over is a important technique in achieving high availability. Typically a load balancer is configured to fail over to another machine when the main machine fails.  
  
To achieve least down time, most load balancer support a feature of *heart beat* check. This ensures that target machine is responding. As soon as a hear beat signal fails, load balancer stops sending request to that machine and redirects to other machines or cluster.

Question 10

What is the difference between Session Affinity and Sticky Session?

Accepted Answer

**Sticky session** means that when a request comes into a site from a client all further requests go to the same server initial client request accessed. I believe that session affinity is a **synonym** for sticky session, but there are different ways of implementing it:

1.  Send a cookie on the first response and then look for it on subsequent ones. The cookie says which real server to send to.
    **Bad** if you have to support cookie\-less browsers
2.  Partition based on the requester's IP address.
    **Bad** if it isn't static or if many come in through the same proxy.
3.  If you authenticate users, partition based on user name (it has to be an HTTP supported authentication mode to do this).
4.  Don't require state.
    Let clients hit any server (send state to the client and have them send it back)
    This is not a sticky session, it's a way to avoid having to do it.

I would suspect that sticky might refer to the cookie way, and that affinity might refer to #2 and #3 in some contexts, but that's not how I have seen it used (or use it myself).

Top 22 Load Balancing Interview Questions

What Is Load Balancing?

Name some advantages of Round-Robin Load Balancing

What Is Round-Robin Load Balancing?

Explain what is Reverse Proxy Server?

What Is a TCP Load Balancer?

What is the Difference Between Weighted Load Balancing vs Round Robin Load Balancing?

What are some variants of Round-Robin Load Balancing algorithm?

What is the difference between Session Affinity and Sticky Session?

Why should we use Load Balancer (except preventing overloading)?

What Is Load Balancing Fail Over?

What Is Sticky Session Load Balancing? What Do You Mean By "Session Affinity"?

What is the different between Layer7 vs Layer4 load balancing?

What are the Pros and Cons of the "sticky session" load balancing strategy?

What affect does SSL have on the way load balancing works?

Name some metrics for traffic routing

When to choose Round-Robin load‑balancing method?

What Is IP Address Affinity Technique For Load Balancing?

What are some Load Balancing Algorithms you know?

What Are The Issues With Sticky Session?

What Is a UDP Load Balancer?

Explain what is “Power of Two Random Choices” Load Balancing?

Compare UDP Load Balancer vs TCP Load Balancer