What is an Application Load Balancer?

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Do you know how big applications manage multiple user requests simultaneously without compromising speed? The AWS Application Load Balancer (ALB) achieves this by effectively managing traffic, distributing requests across multiple servers, and routing them to different targets.

Understanding AWS Application Load Balancer

The load balancing tool, ALB by AWS, manages and allocates oncoming traffic to various targets like IP addresses, ECZ, or containers. Operable at app layer 7, it efficiently routes HTTP/HTTPS traffic depending on the requested content. 

Along with routing, it verifies the overall health of registered targets and passes them to targets in good condition. The balancer's configuration is mandatory to form groups and file targets within those groups. 

To verify client connection requests, you must create listeners and their rules. The requests will be transmitted to the service tool and allocated to the defined targets or its groups of clients as per the rules. It secures the website by utilizing SSL/TLS ciphers and protocols.

Application Load Balancer


How Does AWS Application Load Balancer Work?

Clients can generate requests while using the app. ALB Listeners can receive those requests after confirming the configured protocol. The listener rules verify the request and transfer it to the desired group if approved.

To offload the TLS encryption and decryption job, utilize the HTTPS listener available by the ALB tool. By using effective load-balancing code and verifying listener rules, both single and multiple target groups receive healthy targets.

AWS Application Load Balancer Key Features

Two-way TLS Support

TLS is a two-way authentication protocol that connects clients to the servers. It helps authenticate and check client certificates. The balancer tool will proxy the details mentioned in the client certificate to the designated targets, which is helpful for apps when making authorization decisions.

Content Routing

As per the requested content, like HTTP headers or URLs, ALB allocates requests to distinct targets. This promising feature is beneficial in a microservice architecture, where multiple services are requested simultaneously, and the balancer transfers them to the desired targets according to their request path.

Sticky Sessions

The sticky session is the mechanism when requests are generated from the same user and passed to the same target. ALB supports both cookies, applications, and duration. 

It is easy to handle sticky sessions by deciding the routing time when ALB continuously sends user requests to the desired target. You can enable the feature at a group level. Different types of sticky cookies can be combined across all groups.

Request Tracing

By using a unique ID, you can trace your request and navigate various services that comprise the majority of traffic for distributed apps and websites. Enabling the trace identifier allows you to explore your application thoroughly and uncover all the timing and performance issues. You can spot problems within every request by monitoring it individually.

User Authentication

The authentication functionality can be offloaded to the ALB through your website. The ALB will authenticate all the users accessing the cloud apps. It will integrate the Amazon Cognito to authenticate users using social identity providers like Facebook, Google, etc. 

For anyone having a customized openID-compatible IDP solution, the ALB will authenticate all the enterprise users by connecting them directly with the identity provider.

Web App Firewall (WAF)

AWS WAF enables the protection of web apps on balancers. The firewall protects the app from common exploits severely affecting its security and availability. You often have no idea that your application uses excessive resources, but a firewall can detect it.

Fixed Response

ALB controls the app-served user requests and responds to them with HTTP error-response codes and messages. The balancer sends the message by itself without forwarding any request to your website.

Health Check

ALB confirms the health of incoming targets and ensures that the traffic must transfer to the healthy target group. It makes the app more secure, reliable, and available.

Support of WebSockets and HTTP/2

The tool supports HTTP/2 and WebSockets and allows all modern web-based applications to maintain connections. It improves the application's overall performance and will enable users to benefit from its improved version.

Enhanced Security Features

There is an option to generate and handle security groups related to load balancing while using Amazon VPC. Configure the balancer's settings to get additional security features. One can also utilize ALB without having a public IP address to behave as an inner balancer.

Comparison of Application Load Balancer (ALB), Classic Load Balancer (CLB), and Network Load Balancer (NLB)

FeatureApplication Load Balancer (ALB)Classic Load Balancer (CLB)Network Load Balancer (NLB)
LayerLayer 7 (HTTP/HTTPS)Layer 4/7 (TCP/SSL, HTTP/HTTPS)Layer 4 (TCP/UDP, TLS)
Use CasesMicroservices, WebSocketSimple web applicationsExtreme performance and low latency
Target TypeInstances, IP addresses, Lambda functionsInstancesInstances, IP addresses
Advanced RoutingPath-based, host-based, HTTP headers, query stringNot supportedNot supported
PerformanceHighMediumExtremely high
Static IP SupportNot supportedNot supportedSupported
TLS TerminationSupportedSupportedSupported
LoggingAccess logs (detailed)Access logsFlow logs
PricingHigher (more features)LowerPay-per-use, low cost for high throughput

Benefits of Using AWS Application Load Balancer 

Improved App Availability

The app remains available to all users when ALB delivers traffic to various targets. While managing load, there is no scope for experiencing failure or slow down while accessing or opening the app.

Better Security

Multiple features of Load Balancer help improve app security, such as SSL termination, WAF, etc. ALB also helps protect your app from unexpected threats and web exploits.

Better Scalability

The tool can easily scale the traffic capacity and handle multiple requests without manual intervention. Even during traffic spikes, ALB can handle the load and allocate requests to the desired target groups.


ALB is a cost-effective solution that reduces the operational overload of handling and maintaining an app's traffic load. It allows you to improve the app's overall performance without doing anything manually.

Integration Ease

The load balancer can seamlessly integrate with AWS services like ECS, EKS, auto-scaling, etc., allowing you to create the cloud infrastructure your app needs to function effectively.

Common Issues and Mistakes to avoid when using AWS Application Load Balancer

PitfallDescriptionHow to Avoid
Improper Health ChecksIncorrectly configured health check paths/settings can lead to false positives/negatives.Verify health check settings and paths regularly.
Inefficient Routing RulesComplex or poorly ordered routing rules can decrease performance.Optimize rule order and specificity for efficient routing.
Insufficient Security GroupsMisconfigured security groups can leave the ALB vulnerable to attacks.Set up security groups to allow necessary traffic only.
Ignoring SSL/TLS Best PracticesUsing outdated SSL certificates and weak encryption protocols.Use up-to-date certificates and strong encryption methods.
Misconfigured Target GroupsIncorrectly set up target groups can lead to targets not being registered or marked as unhealthy.Ensure targets are properly registered and monitor their health.


AWS ALB is undoubtedly a powerful solution for balancing the load on app servers and routing it to various targets. It is easy to control traffic differently, such as through content, host, or path. 

Properly balancing requests enhances the app's performance and makes it secure, reliable, and available. If you run a web app on a larger scale, you will need a balancer to fulfill your load-managing requirements.

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