Get to Know the Types of Load Balancers

Learning Objectives

After completing this unit, you will be able to:

  • Differentiate the types of Elastic Load Balancing load balancers.
  • Explain the features of the Application Load Balancer and Network Load Balancer.
  • Choose the right load balancer type based on the application.

As you learned in the previous unit, using the ELB service is the easiest way to balance the traffic to the EC2 Auto Scaling group used for the cat photo application. However, you now need to choose the right type of load balancer for your cat photo app. To do that, you need to understand the features that differentiates the types of ELB.

Application Load Balancer

Service icon for the Application Load Balancer.

Here are some primary features of Application Load Balancer (ALB).

ALB routes traffic based on request data. It makes routing decisions based on the HTTP protocol like the URL path (/upload) and host, HTTP headers and method, as well as the source IP address of the client. This enables granular routing to the target groups.

Send responses directly to the client. ALB has the ability to reply directly to the client with a fixed response like a custom HTML page. It also has the ability to send a redirect to the client which is useful when you need to redirect to a specific website or to redirect the request from HTTP to HTTPS, removing that work from your backend servers.

ALB supports TLS offloading. Speaking of HTTPS and saving work from backend servers, ALB understands HTTPS traffic. To be able to pass HTTPS traffic through ALB, an SSL certificate is provided by either importing a certificate via Identity and Access Management (IAM) or AWS Certificate Manager (ACM) services, or by creating one for free using ACM. This ensures the traffic between the client and ALB is encrypted.

Authenticate users. On the topic of security, ALB has the ability to authenticate the users before they are allowed to pass through the load balancer. ALB uses the OpenID Connect protocol and integrates with other AWS services to support more protocols like SAML, LDAP, Microsoft AD, and more. 

Secure traffic. To prevent traffic from reaching the load balancer, you configure a security group to specify the supported IP address ranges. 

ALB uses the round-robin routing algorithm. ALB ensures each server receives the same number of requests in general. This type of routing works for most applications.

ALB uses the least outstanding request routing algorithm. If the requests to the backend vary in complexity where one request may need a lot more CPU time than another, then the least outstanding request algorithm is more appropriate. It’s also the right routing algorithm to use if the targets vary in processing capabilities. An outstanding request is when a request is sent to the backend server and a response hasn’t been received yet.

For example, if the EC2 instances in a target group aren’t the same size, one server’s CPU utilization will be higher than the other if the same number of requests are sent to each server using the round-robin routing algorithm. That same server will have more outstanding requests as well. Using the least outstanding request routing algorithm would ensure an equal usage across targets.

ALB has sticky sessions. In the case where requests need to be sent to the same backend server because the application is stateful, then use the sticky session feature. This feature uses an HTTP cookie to remember across connections which server to send the traffic to.

Finally, ALB is specifically for HTTP and HTTPS traffic. If your application uses a different protocol, then consider the Network Load Balancer (NLB).

Network Load Balancer

Service icon for the Network Load Balancer.

Here are some primary features of Network Load Balancer (NLB).

Network Load Balancer supports TCP, UDP, and TLS protocols. HTTPS uses TCP and TLS as protocol. However, NLB operates at the connection layer, so it doesn’t understand what a HTTPS request is. That means all features discussed above that are required to understand the HTTP and HTTPS protocol, like routing rules based on that protocol, authentication, and least outstanding request routing algorithm, are not available with NLB.

NLB uses a flow hash routing algorithm. The algorithm is based on:

  • The protocol
  • The source IP address and source port
  • The destination IP address and destination port
  • The TCP sequence number

If all of these parameters are the same, then the packets are sent to the exact same target. If any of them are different in the next packets, then the request may be sent to a different target.

NLB has sticky sessions. Different from ALB, these sessions are based on the source IP address of the client instead of a cookie. 

NLB supports TLS offloading. NLB understands the TLS protocol. It can also offload TLS from the backend servers similar to how ALB works.

NLB handles millions of requests per second. While ALB can also support this number of requests, it needs to scale to reach that number. This takes time. NLB can instantly handle this amount of requests.

NLB supports static and elastic IP addresses. There are some situations where the application client needs to send requests directly to the load balancer IP address instead of using DNS. For example, this is useful if your application can’t use DNS or if the connecting clients require firewall rules based on IP addresses. In this case, NLB is the right type of load balancer to use.

NLP preserves source IP address. NLB preserves the source IP address of the client when sending the traffic to the backend. With ALB, if you look at the source IP address of the requests, you will find the IP address of the load balancer. While with NLB, you would see the real IP address of the client, which is required by the backend application in some cases. 

Select Between ELB Types

Selecting between the ELB service types is done by determining which feature is required for your application. Below you can find a list of the major features that you learned in this unit and the previous. 

Feature Application Load Balancer Network Load Balancer




Connection draining (deregistration delay)

IP addresses as targets

Static IP and Elastic IP address

Preserve Source IP address

Routing based on Source IP address, path, host, HTTP headers, HTTP method, and query string


Fixed response

User authentication

Wrap Up

In this unit, you learned about Application Load Balancer and Network Load Balancer. To select the right type of load balancer, make sure you understand the requirements for your application. In the case of the cat photo application, it uses the HTTP and HTTPS protocols. 

The right option here is to use the Application Load Balancer as long as you don’t absolutely require any features that Network Load Balancer offers, like static IPs. There are also no constraints with using DNS to reach the application, so ALB should be used.


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