Sticky sessions, also known as session affinity, is a feature of the AWS Application Load Balancer (ALB) that allows you to maintain session state for HTTP and HTTPS traffic. When you enable sticky sessions, the ALB ensures that a user's requests are always directed to the same server in a group of servers. This can be useful in scenarios where you need to maintain session state on a specific server, such as when you're using in-memory caching or have a database connection that needs to be maintained across multiple requests.

The AWS ALB supports two types of sticky sessions: cookie-based stickiness and application-controlled stickiness.

1. Cookie-based stickiness: This type of sticky session uses a cookie to track a user's session and ensure that subsequent requests from that user are sent to the same target server. When you enable cookie-based stickiness, the ALB inserts an HTTP cookie into the response to the user's first request. The cookie contains a unique identifier for the user's session and the ARN of the target group to which the request was routed. The user's browser then sends the cookie back to the ALB with each subsequent request, allowing the ALB to route the request to the same target group.
2. Application-controlled stickiness: This type of sticky session allows you to use your own application-generated cookie or URI parameter to track a user's session. You must configure your application to include the appropriate cookie or URI parameter in the response to the user's first request. The ALB then extracts the cookie or URI parameter from subsequent requests and uses it to route the requests to the same target group.

Sticky sessions can be useful for applications that require session state to be maintained on a specific server. However, it's important to consider the trade-offs before enabling sticky sessions on your ALB. Sticky sessions can lead to uneven distribution of traffic across target groups, as all requests from a particular user will be directed to the same server. This can potentially overload a specific server and affect the overall performance of your application.

Additionally, sticky sessions can also lead to longer response times for users if the target server becomes unavailable or if the user's session is lost for any reason. In such cases, the user will need to start a new session on a different server, which can result in slower response times and potentially degrade the user experience.

In summary, sticky sessions can be a useful feature in some cases, but it's important to consider the trade-offs and carefully evaluate whether it's the right solution for your application. If you decide to use sticky sessions, it's recommended to closely monitor the performance of your application and target servers to ensure that the load is evenly distributed and that response times remain within acceptable limits.