AWS Elastic Load Balancing:
- has the task of distributing traffic throughout the cluster of servers to ensure higher responsiveness and availability of applications, websites, or databases
- automatically distributes the incoming traffic across multiple targets, such as:
- EC2 instances
- containers
- IP addresses
- monitors the health of its registered targets, and routes traffic only to the healthy targets
- targets can be added and removed from load balancer as the needs change, without disrupting the overall flow of requests to the application
- scales your load balancer as your incoming traffic changes over time
- supports the following load balancers:
- Application Load Balancers
- Network Load Balancers
- Gateway Load Balancers
- Classic Load Balancers
Load balancer:
- serves as the single point of contact for clients
- distributes incoming application traffic across multiple targets which increases the availability of your application
- has one or more listeners added to it
Listener:
- checks for connection requests from clients, using the protocol and port that you configure
- has rules defined for it
- a default rule must be defined for each listener, additional rules can be defined optionally
- Rules determine how the load balancer routes requests to its registered targets
- Each rule consists of:
- priority
- one or more actions
- one or more conditions; when conditions for a rule are met, then its actions are performed
- specified target groups
- When load balancer receives a request, it evaluates the listener rules in priority order to determine which rule to apply, and then selects a target from the target group for the rule action.
Target groups:
- Each target group routes requests to one or more registered targets, such as EC2 instances, using the protocol and port number that you specify.
- target can be registered with multiple target groups
- Routing is performed independently for each target group, even when a target is registered with multiple target groups
- health checks are configured per target group
- used to monitor the health of the registered targets
- they are performed on all targets registered to a target group
- they can be configured so that the load balancer can send requests only to the healthy targets
- routing algorithm is configured at the target group level
- default: round robin
- least outstanding requests routing algorithm can alternatively be specified
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